CONCEPTOS EN ASTROGEOLOGÍAheliocentric ideas of Aristarch of Samos as well as a new conception related to the size of the universe: “But Aristarch has brought out a book consisting

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CONCEPTOS EN ASTROGEOLOGÍA

POR

JOSÉ ANTONIO RODRÍGUEZ LOSADA Dpto. Edafología y Geología Universidad de La Laguna

[email protected]

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The planetary perception The dynamics that operated to drive Earth’s evolution and shape its surface have probably worked elsewhere in the rest of the Solar System. Earth scientists can recognize those forces on other planetary bodies and try to explain why they are observable in the Earth in a familiar manner and on other planets in ways that may not be so familiar. The effects in Planetary Geology are to grow insight into planetary scale problems and to escape the limited earthbound view of nature. Earth scientists are concerned that active processes on Earth have masked or removed much of the records of Earth’s own history. However, evidences of the oldest geological processes occurred on our planet are usually preserved on other planetary surfaces. The study of the Earth and other planetary bodies on a grand scale is fully of practical benefits in different fields of knowledge such as technology, natural resources, economy or meteorology among others, all of them derived from better studies of the atmospheres, solid crusts and possible oceans on other planetary bodies. A critical date for the planetary perspective was the decision taken in 1961 by the U.S.A administration of John F. Kennedy to put an astronaut over the Moon surface, decision that was materialized on 21 July 1969 when astronaut Neil Armstrong landed on the Moon and taken the first lunar sample. This represented a very important event for the Planetary Geology. The development in aerospatial technology and in jet propulsion ( 1942-1946) greatly conditioned the arrangement of the first missions to the Moon and other planets from 1959. The scientists can agree with the assumption that planetary geology emerged as an individual science in 1961 when geologist Eugene Shoemaker created the term “Astrogeology” as a branch of the Geological Sciences within the United States Geological Survey, establishing the Field Center in Flagstaff (Arizona) in 1963. A health problem prevented Gene Shoemaker (1928-1997) to be the first astronaut geologist. One of the most significant events for the geological community was that of the Apollo 17 mission in 1972, when the first astronaut geologist Harrison Schmidt walked over the moon surface taking interesting samples from the Taurus Littrow Valley such as the orange soils of pyroclastic particles made of volcanic glass and ilmenite crystals. Geologic processes in the Solar System: a brief description Due to our own life on Earth and the opportunity to make a close up view of its evolution, our planet seems to be the more complex and diverse among the terrestrial planets of the Solar System. Four main geologic processes have worked to shape the planetary surface of the Earth and other

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inner planets and moons with differential intensity depending on the characteristics of them such as orbital location, size, and atmospheric features among others. These are a) volcanism, b) tectonic activity, c) gradation and d) impact cratering. Volcanism consists on the emission of molten material to the surface. On the terrestrial planets the molten material (magma) is composed of melted silicate rocks at temperatures ranging from 600 to 1300 ºC and gases. This kind of volcanism is properly called as magmatic volcanism. On icy satellites the material is mostly liquid water or mushy ices emitted at or bellow 0ºC mixed with some rocky fraction. This is a type of emissions that characterizes the so called cryovolcanism. In both cases molten material can be erupted in the form of lava flows (nonfragmented silicate magma or cryomagma), pyroclasts (fragmented melt) during explosive volcanic eruptions or cryoclasts by a geyser-like activity. Tectonic activity involves lateral or vertical movements of rocks by folding or faulting, resulting in the formation of mountain-ranges accompanied by seismic activity. Both volcanism and tectonic activity are geologic processes driven by the release of the internal planetary energy. Gradation involves erosion, transportation and deposition of surfacial materials. This last phenomenon takes place throughout external agents mainly running water, ice and wind. In this case, gravity plays an important role specially related with the mass wasting transportation, landslides or avalanches among others. Gradation depends of the surface environmental conditions that are controlled by several factors including gravity, temperature and the presence of an atmosphere. The fourth major geologic process is the impact cratering throughout which, materials falling from the space such as asteroids, meteoroids or comets, modifie the planetary surfaces. By studying the landforms originated by each of the previously mentioned processes it is possible to reconstruct a historical evolution of a planetary surface. Geologic processes have operated in a different manner from planet to planet but in general, a process as the impact catering is today quite rare in the Solar System whereas it has played a predominant role in shaping the planetary surfaces during the early history of the Solar System. The Solar System in the Milky Way Galaxy Although the XVI century defines the transition from the Ptolemaic model (geocentric) to the heliocentric one during the Copernican revolution, it was in the third century BC when Aristarch (310 - 230 BC), a Greek astronomer and mathematician, born in Samos, proposed, based on their own observations and philosophical ideas, the heliocentric version of the universe in which, is the Sun, not the Earth the center of the known universe. Archimedes (287 - 212 BC), an Aristarch’s pupil, when wrote to

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Syracuse King Gelon about the concept of the universe, manifested the heliocentric ideas of Aristarch of Samos as well as a new conception related to the size of the universe: “But Aristarch has brought out a book consisting of certain hypotheses, wherein it appears, as a consequence of the assumptions made, that the universe is many times greater than the 'universe' just mentioned. His hypotheses are that the fixed stars and the sun remain unmoved, that the earth revolves about the sun on the circumference of a circle, the sun lying in the middle of the orbit, and that the sphere of fixed stars, situated about the same centre as the sun, is so great that the circle in which he supposes the earth to revolve bears such a proportion to the distance of the fixed stars as the centre of the sphere bears to its surface. ( in The Sand Reckoner)”. Aristarch thus believed that stars were infinitely far away, and saw this as the cause why there was no visible parallax. The stars are much farther away than was assumed in ancient times. These ideas were in disagreement with the explanations given by the geocentric theory and consequently were strongly rejected in his epoch until they were revived and applied to the currently known Solar System by Nicolaus Copernicus nearly 1800 years latter. Latter, William Herschel ( 1738-1822) discovered that there was a homogeneous distribution of stars along the galactic plane in all directions, suggesting the idea of a central position of the Sun in the galaxy. Harlow Shapley ( 1885-1972) proposed in 1915 that the Solar System was located not near the center of the galaxy but in a marginal position. This discovery signified a new step away from the anthropocentric ideas of the universe. Now, it is known that the Solar System is located in the outer sector of the Milky Way Galaxy (Fig. 1). The Sun is about 28000 light-years from the center of the galaxy, which is about 100.000 light-years across, out towards the edge of one of its spiral arms (the Orion arm). It takes roughly 200-250 million years to orbit once around the Milky Way at an estimated speed of about 250 km/seg and has the ecliptic plane oriented nearly perpendicularly to the galactic plane.

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Figure 1: Location of the Sun in the galactic disk The characteristics of the Solar System Before entering the problem concerning the origin of the Solar System, it will be interesting to spend some time for discussion on their features and structure, taking into account that an explanation on its origin and evolution have necessarily to explain all of those features. Many of them exhibit prominent relationships actually not well understood but allow introducing us in new concepts on celestial dynamics. The usual classification of solar-system bodies was recently questioned by new discoveries of probably remote comet-like objects orbiting into regions beyond the Neptune’s orbit. The classification of planets into gas giants, outer ones on one side and inner or terrestrial ones on the other side will be replaced by other one in which, giant or outer planets comprises gas giants ( Jupiter and Saturn) and icy giants ( Uranus, Neptune). This classification constitutes a key feature for several modern theories about the origin of the Solar System. Most of the planets are accompanied by orbiting satellites resembling planetary mini-systems that play an important role for a better comprehension of the celestial dynamic. Planetary orbits and angular momentum distribution As the Sun orbit around an axis perpendicular to the galactic disk, the planets, asteroids and comets of the Solar System turn around the centre of masses of the system, located inside the solar’s volume near its geometrical centre. Similar orbital dynamic is exhibit by satellites orbiting around the planets. The features of the planetary orbits arise from the well known laws of orbital motion deduced by Johannes Kepler (1571-1630), mostly based on detailed observational data from Tycho Brahe (1546-1601). These are:

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1. Planets orbit following elliptical trajectories with the Sun located at one

focus. 2. The radiovector of a planet sweeps out equal areas at same time

intervals. 3. The square of the orbital period is proportional to the cube of the

average distance from the Sun which seems to the semi-major axis. Each orbital system has a characteristic and unique proportionality constant that differs from others of different orbital systems with different centre of masses.

These laws based on observational data did not taken into account the dynamical aspects of planetary motion until Isaac Newton (1642-1727) solved this problem by analyzing the motion of two bodies moving together under an inverse square law of attraction.

Table 1. Orbital parameters of the planets Planet Average distance to the Sun (AU) Eccentricity Angle respect to the ecliptic Mercury 0.39 0.21 7.00º Venus 0.72 <0.01 3.40º Earth 1.00 0.02 Mars 1.52 0.09 1.85º Jupiter 5.20 0.05 1.30º Saturn 9.55 0.06 2.48º Uranus 19.22 0.05 0.77º Neptune 30.11 <0.01 1.77º Pluto 39.54 0.25 17.15º

1 AU= 1.5 x 1011 m (average Sun-Earth distance) All planets move around the Sun along direct orbits (counter clockwise from a north perspective). However, many moons translate around their planets following retrograde orbits. Retrograde orbits can be explained as originated by capture mechanism of other bodies in heliocentric orbit passing near the corresponding planet. Continuous discoveries show a great number of satellites in retrograde orbit, for instance the two satellites of Mars, 44 of around 63 of Jupiter, 14 of around 33 in the Saturn system, 9 of 27 in the Uranus system and 7 of 13 in the Neptune system (Table 1a).

Table 1a. Satellites with retrograde orbit in the Solar System Phobos Mars Deimos Mars Ananke (Jup XII) Jupiter

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Carme (Jup XI) Jupiter Pasiphae (Jup VIII) Jupiter Sinope (Jup IX) Jupiter 1999, J1 Jupiter 2000 J2,J3,J4,J5,J6,J7,J9,J10 Jupiter S2001 J1-J11 Jupiter S2002 J1 Jupiter S2003 J1-J19 Jupiter Phoebe Saturno 2000 S1-S12 Saturno S2003 S1 Saturno 1999 U1,U2,U3 Urano S2001 U1,U2,U3 Urano Caliban Urano Sicorax Urano S2003 U3 Urano Triton Neptuno Nereida Neptuno S2002 N1-N4 Neptuno S2003 N1 Neptuno

One of the most remarkable facts in the Solar System is the regular spacing in the orbital radii of the planets (Table 1). In 1596 Kepler, when only 6 planets were known (from Mercury to Saturn), related this number to the existence of the 5 perfect solids (the 5 regular solids of Pythagoras and Plato: octahedron, icosahedron, dodecahedron, tetrahedron and cube) and decided that the planets were spaced as they were because the planetary orbits were arranged around these 5 geometric figures inscribed each one into the other determining an invisible structure of spheres sustaining the planets and consequently the distances between them. First assigned the cube (the outermost one) to Saturn, the tetrahedron to Jupiter, the dodecahedron to Mars, the icosahedron to Venus and the octahedron (the innermost one) to Mercury, being the Earth the most important part of the universe which separate the outer group (Mars, Jupiter and Saturn) and the inner one (Mercury and Venus). This explanation known as the Cosmic Mystery was incorrect but it represented the first work supporting the Copernican system fifty years after the Copernicus’s death. Latter, in the 18th century, it was developed a simple formula to calculate the semimajor axis of those planets, known as Titius-Bode Law in the form of: a(n)=a(0) + 0.3 x 2n-1 , n= 1,2,3, where a(0) is the mean distance from Mercury to the Sun in AU and n= 1,2,3, corresponds to Venus, the Earth, Mars and so on. As can be remarkable in Table 2, the results obtained from the formula matches well with the observed data. This law was reinforced after the discovery of Uranus by William Herschel in 1781 while the gap existing between Mars

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and Jupiter was soon filled by Ceres, the largest asteroid, found by Giuseppe Piazzi in 1801 and after by the rest of asteroids.

Table 2. Actual semi-major (s-m) axes compared with the Titius-Bode results n 1 2 3 4 5 6 7 8 9 Mercury Venus Earth Mars Ceres Jupiter Saturn Uranus Neptune Plutos-m axis(AU) 0.39 0.72 1.00 1.52 2.80 5.20 9.55 19.22 30.11 39.54a(n) 0.40 0.70 1.00 1.60 2.80 5.20 10.00 19.60 38.80 77.20

In 1846 Neptune was discovered at a distance of 30.11 AU from the Sun and finally Pluto was found at 39.55 AU from the Sun. These discoveries damaged the plausibility of the Titius Bode law due to the great differences between the obtained a(n) values and the real semi-major axes of the two last planets as is exhibit in the Table 2. It is noted the highest eccentricity and inclination shown by the two extreme planets of the system (Mercury and Pluto). In the case of Pluto, its high orbital eccentricity drives the planet to closer positions from the Sun than that of Neptune when passing the perihelion. Nevertheless, due to the special relationship between their respective orbits the two planets does not approach together closer than 18 AU. Recently, due to the continuous increasing in the computers power, a simulated evolution of orbits during long periods of time similar to the age of the Solar System was able to reproduce. Computer simulations seem to indicate that planetary orbits have remained steady during the whole life of the Solar System (4.5 billions years). Other important feature of the Solar System is its angular momentum distribution (Table 3). The sun spins roughly 6º away from the vector representing the angular momentum of the Solar System (Table 4). As can be deduced from the table 3, the angular momentum of the Sun represent less than 0.5% of the whole angular momentum of the system which strongly contrast with the amount of mass, containing the sun slightly more than 99% of the total mass of the system.

Table 3. Angular momentun (Jul.sec) relative to the system centre SUN 1.5287976 x 1041 MERCURY 9.13143 x 1038 VENUS 1.8404217 x 1040 EARTH 2.691 x 1040 MARS 3.538704 x 1039 JUPITER 1.95738 x 1043 SATURN 7.969983 x 1042 URANUS 1.7165568 x 1042 NEPTUNE 2.534292 x 1042 PLUTO 3.56489 x 1038 TOTAL 3.199763411 x 1043

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Orbital resonances Orbital resonances occur when the orbital periods of two or more bodies are related by an integer ratio. This causes that the bodies will be repeatedly in conjunction in similar positions. As the longitude of conjunction precesses, gravitational perturbations are enhanced. This tends to stabilize the conjunction just at the periapse or apoapse. In this case, their eccentricity will be affected and forced to a minimum energy configuration. All the planets exhibit rotation (precession) in their perihelion longitudes. If one of the two bodies is less massive than the other one, then the small one finally will be swept away from its orbit. This is the origin of the well known Kirkwood gaps in the asteroid belt due to gravitational interaction between Jupiter and the asteroids. The most prominent corresponds to 2:1, 3:1; 5:3 and 7:2 resonances. When the simple ratio between orbital periods affects to more than two bodies, then the resonance is known as Laplace Resonance. In the Galilean system there are simple relationships between the mean motions of the giant satellites as follows: n(Io) – 2n(Europa)= 0.7395º/day n(Europa) – 2n(Ganymede)= 0.7395º/day n(Io) – 3n(Europa) + 2n(Ganymede)= 0º/day. In the Saturn system: n(Mimas) – 2n(Tethys)= 0.6º/day n(Enceladus) – 2n(Dione)= - 0.338º/day 3n(Titan) – 4n(Hyperion)= 0.062º/day. In the Uranus system: n(Miranda) – 3n(Ariel) + 2n(Umbriel)= 0.069º/day For Jupiter and Saturn: 5n(Saturn) – 2n(Jupiter)= 0.408º/day For Neptune and Pluto: 2n(Neptune) – 3n(Pluto)= 0.0091º/day (this indicates a 3:2 resonance of Pluto with Neptune in which, the orbital period of Pluto is almost exactly 1.5 times longer than the one of Neptune). Rotations: planets as giant gyroscopes Almost all the planets rotate in the same way as they orbit around the Sun, which mean in a counter clockwise as viewed from a North perspective. All rotation period of a large body of great mass circled by one or more small bodies (e.g. satellites around a planet or planets around the Sun) can be theoretically calculated from the rotation equation (Esmaeil, 2003). This equation establishes a relationship between the orbital period of a major

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body and other factors such as the mass ratio, densities ratio, orbit inclinations and some celestial constants among others in the form:

( )( ){ } ( )( ) ( )( )

++≡

22322

2////cos/4

akrdDmMRrMmimMGakTπ

π

where:

T=Rotation period of the large body (sec). K=Moment of inertia factor of the large body. M= Large body mass or parent Body mass (kg). G=Gravitational constant. R= Polar radius for the large body(meter). D= Density for the large body(Kg/m3). a=semimajor axis for the center of mass of all satellites that orbit large body (meter) =Distance between the large body center and the center of masses of all satellites that orbit this large body. i=Orbit inclination of the center of mass of all satellites that orbit large body (degrees). m=Total mass of all satellites that orbit the large body (kg). d= Mean density for the total satellites that orbit large body(Kg/m3). r =Mean Polar radius of all satellites that orbit the large body (meter). Rotational periods of Mercury or Venus are not available throughout the rotation equation as they are not encircled by satellites. The rotational characteristics of the planets are shown in table 4. It is noted the negative values for the rotation period on Venus, Uranus and Pluto indicating that the planets rotate in the direction opposite to that in which they orbit the Sun. Those are planets with retrograde rotation and this negative value of their rotating periods matches with an axis tilt greater than 90º. The retrograde rotation of Venus, Uranus and Pluto became as direct if viewed from a South perspective. Even if their tilts were hypothetically removed then their became in planets with direct rotation, as are the rest of planets on the Solar System.

Table 4. Rotational features on the planets of the Solar System

Rotation period inclination of axis (degrees)

inclination of axis (degrees)

Observed (hr) Calculated(hr) Mercury 1408 0

Venus 5832(-) 177.4 (North perspective)

2.6 (South perspective)

Earth 24 24 23.45 Mars 25 24 23.98 Jupiter 10 9 3.08

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Saturn 11 11 26.73

Uranus 17.2(-) 1597.92 (North perspective)

82.08 (South perspective)

Neptune 16 12 28.8

Pluto 153.3(-) 151122 (North

perspective)58 (South

perspective) Sun 600 600 7 Due to the rotational dynamic the planets behave as giant gyroscopes and consequently they maintain their spin axes oriented in the same direction even if any torque is applied in order to change the spatial orientation of the angular momentum. When a torque is applied to a rotating body a change of its angular momentum vector and therefore of its spin axis occurs. The magnitude of the torque defines the change of angular momentum in the form: τ = dL/dt The rate of this change defined as the precessional frequency can be expressed in the generic form: ωp = τ/Iω This implies that a high rate of change in the spin axis orientation of a rotating body could drive to a significant change (decrease) of the angular velocity. In the case of a planet, a dramatic change in the orientation of its spin axis by a precessional tilt (e.g. caused by a collision against other massive body) may contribute to a variation (decrease) of its original rotational period. Now it is accepted that early collisions contributed in part to the great tilt of the spin axis on Venus and Uranus. It is well known that angular momentum of an almost spherical body depends on its mass, radius and angular velocity. For a uniform mass distribution the angular momentum can be expressed as: L=0.4mr2ω As the planets have no uniform density distribution, the inertial momentum factor will differ from 0.4. Density increases from surface to the centre and consequently this factor decreases. In the case of the Sun with a central density about 100 times greater than its mean density that factor is about 0.055, while for the terrestrial planets is roughly 0.35 and for the giant planets is about 0.22 (Table 5).

Table 5. Rotational angular momentum of the Sun and planets. Inertial factor Rotational momentum (Jul. Seg) SUN 0.055 1.5E+41 MERCURY 0.35 8.5E+29 VENUS 0.33 -1.8E+31 EARTH 0.33 5.9E+33 MARS 0.36 1.9E+32 JUPITER 0.25 4.3E+38 SATURN 0.22 9.1E+37

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URANUS 0.22 -1.6E+36 NEPTUNE 0.22 1.1E+36 PLUTO 0.29 -5.4E+28

Actually, precession of spin axis is present in the planets of the Solar System. For instance, the Earth show circular motion of its spin axis with respect to the fixed stars caused by the combined gravitational torque of the Sun and the Moon with a period of 25770 years. This periodically changes the spin axis orientation with respect to the fixed stars and to the ecliptic. All planets exhibit not only such type of oscillation in their spin axis but even the oscillating amplitude varies with time. Planetary features Some characteristics of the planets appear listed in table 6. As more massive objects result in stronger gravities then more massive planets get internally more compressed than less massive ones. A higher compression results in a decrease in the planetary size and thus in a greater density. Consequently, objects with similar geochemical composition and different masses will show different densities in the sense of greater density for the most massive. This can make to conclude that the two objects would have different geochemical compositions. The parameter known as uncompressed density was defined as free of this mass dependence and then provides a more precise estimation of the true composition of a planetary body (Table 6).

Table 6. Features of planetary bodies

Diameter (km)

Mass (Earth=1)

Density (gr/cm3)

Uncompressed density (gr/cm3)

Mercury 4900 0.05 5.4 5.3 Venus 12100 0.8 5.2 3.95 Earth 12800 1 5.5 4.03 Mars 6800 0.1 3.9 3.71 Jupiter 142900 318 1.3 Saturn 120500 95 0.7 Uranus 51100 14 1.3 Neptune 48400 17 1.8 Pluto 2290 0.002 2

Mass of Earth=5.97x1024 kg

Except for Pluto, planets were initially considered to be of two types. An inner group of four planets, known as “terrestrial planets”, being the Earth the largest member. These are planets of slow rotation, closely spaced orbits, solid surfaces, weak or absent magnetic fields, relative small size and high density due to rocky materials (mainly formed by silicate minerals) and cores consisting of iron with small proportion of nickel and

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probably traces of other elements such as sulphur. This group exhibits an almost total absence of satellites except for the two external members which are orbited by the Moon in the case of the Earth and by Phobos and Deimos in the case of Mars. Despite the fact that the Moon is a satellite, due to its large size relative to the Earth, it is possible to consider the Moon as the fifth terrestrial body in the inner Solar System in the discussions of other physical properties of the inner planets. Following this line of reasoning, a common feature of them is the high number of scars due to impact cratering in the form of craters of a great variety in sizes and large depressions such us the Caloris basin on Mercury or the Imbrium basin and South Pole-Aitken among others on the Moon or the Chryse Basin on Mars. Another group of four planets defines the outer group. These four external planets are quite different from the ones of the inner group. These are of great sizes and relatively low mean density similar to that of the water. They show widely spaced orbits, fast rotation, and strong magnetic fields. There is no evidence of solid surface on them and thus no record of collisional history exists. In general a great number of satellites and rings orbit each of these planets, known as “Giant planets”. Due to differential features the outer group was separated into “gas giant planets” which comprises Jupiter and Saturn and “ice giant planets” which includes Uranus and Neptune. The internal structure of the giant planets is quite different from the terrestrial ones. The gas giant planets have compositions similar to that of the Sun, mostly of hydrogen and helium meanwhile the icy giant planets are mainly of icy compounds such as water methane and ammonia with minor proportion of gas compounds (hydrogen, helium). Theoretically, it is suggested that at the highest depths of Jupiter and Saturn hydrogen becomes in an ionized phase known as metallic hydrogen. Uranus and Neptune, with minor content of hydrogen, less size and thus less internal compression, are not capable to induce the change of the hydrogen to its metallic phase. The rocky-metal (and perhaps some ice) cores on Jupiter and Saturn could account from 10 to 20 terrestrial masses. The case of Pluto must be considered apart from the terrestrial o giant planets classification. Pluto is an oddball planet and does not fit either category. Previously to its discovery in 1930, it was estimated the existence of a ninth massive planet of about 6 terrestrial masses. This estimation was decreasing until the discovery of its satellite Charon in 1978 which finally gave an estimated mass of about 0.002 terrestrial masses. Given its low density, obviously can not be considered as a terrestrial planet. It seems to be evident that this last planet was originated by some processes different to the ones that formed the rest of planets. The discoveries of trans-Neptunian objects suggest that Pluto is another member of such group and

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only the traditional heritage of an ancient discovery of the so called last member of the planetary family maintain its category even with clear evidences against its actual classification. The satellite systems and rings Most of the planets are orbited by smaller bodies called satellites. Many of them are large bodies and similar in size to Mercury. Generally, their sizes are too much smaller than the orbited planet except for the case of the system Earth-Moon and Pluto-Charon in which the satellites are of a great size relative to the one of their respective planet. They can be grouped into two categories: regular and irregular satellites. Regular satellites are generally large bodies moving along near circular orbits concentrated almost in the equatorial plane and it is most likely that their origin are linked to the processes of planetary construction. Irregular satellites are small bodies that orbit in regular orbits and generally show irregular shapes contrasting with the predominant spherical shapes of the formers. Nevertheless, one of the largest irregular satellites is Triton, which orbit Neptune in an almost circular bur retrograde orbit. Irregular satellites are often related to capture mechanisms. The Earth-Moon pair The Moon is the fifth more massive satellite in the Solar System with a mass of 7.35x1022 kg and 3475 km in diameter. Some relevant features appear listed in table 7.

Table 7. Characteristics of the Moon Mass (kg) 7.35E+22Diameter (km) 3475Mean density (gr/cm3) 3.34Uncompressed density (gr/cm3) 3.3Mean distance from Earth (km) 384400Rotational period (days) 27.32Orbital period (days) 27.32Orbital eccentricity 0.055Tilt of spin axis (degrees) 1.54Orbital inclination (degrees) 5.15surface gravity (Earth=1) 0.17

The relationship between the Moon and the Earth was the subject of observations and studies many centuries ago, specially their tidal interactions. Although Sun induces tidal effects on the Earth are the lunar tides, exceeding that of the Sun by more than 2 times, the most significant and of clear visible effects on the Earth surface.

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Tides are originated in an extended body by the differences in the gravitational field produced under the influence of other neighboring body at its surface and at its centre. In this case mutual tidal effects affect the two linked bodies. James Jeans in 1929 gave a good explanation for the tidal effects. Given two bodies of masses M and m, the accelerations due to the mass m at the points A, B, D, E can be expressed as follows (Fig. 2): a(A) = (2Gmr)/R3 a(B) = (-2Gmr)/R3 a(D) = (-Gmr)/R3

a(E) = (Gmr)/R3

Figure 2: Tidal effects on the Earth-Moon System

This implies the appearance on Earth of two daily high tides caused by bulges on the opposite faces of the Earth. Each day oceanic level rises and falls by two times. These variations depend on the relative positions of the Moon and the Sun with respect to the one of the Earth on its orbit. Differences in the mid oceanic sea level can reach 1m, but coastal geometry may amplify or mask the tidal effects at the shoreline. The highest tides (spring tides) occur as the Moon, Earth and the Sun are aligned but when Earth-Sun and Earth-Moon are perpendicularly disposed, then tidal are reduced giving the so-called neap tides. As the Earth can not suddenly react to the tidal stress, then the tidal bulge does not appear beneath the Moon. Because of the terrestrial angular velocity of spin is greater than the orbital angular velocity of the Moon around the Earth, the maximum tides appear few degrees ahead of the Moon. The nearest bulge produces a tangential force on the Moon greater than the one located at the far side of the Earth and consequently, orbital angular momentum of the Moon progressively increases as the spin rate of

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Earth decreases. Theoretically, it is expected an equal duration of the lunar month and the terrestrial day, around 50 present days in the future. Nevertheless, the same tendency occurred on the Moon since its origin, but as it is less massive than Earth tidal effects has synchronized its spin period to its orbital period with the consequence that now it always shows the same face to the Earth. This kind of synchronization is found for all the satellites in the Solar System. Another important consequence of the mutual interactions on the Earth-System pair, already previously mentioned, is the precession of the spin axis of the Earth caused by the Moon on its orbit due to the differential pull on the closer and distant areas of the equatorial protuberance. This applies a torque that originate the known spin-axis precession period of 25770 years. Satellites on Mars and Pluto Mars is encircled by two satellites known as Phobos and Deimos. Both are small, of irregular shape and orbiting very close to the planet (table 8). Their appearance similar to that of the asteroids, is consistent with the idea of they are captured bodies. Although they exhibit low orbital inclinations and eccentricities typical on the regular satellites, their retrograde orbits support the suggested idea of their capture. Pluto is accompanied by satellite Charon which represents the largest one and more massive relative to its planet in the Solar System (table 8). Charon exhibit a synchronized orbital and rotational period, both similar to the spin period of Pluto. Consequently, Pluto and Charon behave as a rigid rotating system of two coupled bodies. Curiously, the planets devoid of satellites, Mercury and Venus, exhibit the greatest spin periods on the Solar System. Particularly, the very slow spin rate of Venus remains as an unsolved problem and probably a combined series of events involving catastrophic collisions could account not only for its actual spin rate from a primitive fast spin but also for the spectacular tilt of its spin axis.

Table 8: Satellite systems for Mars and Pluto

Planet SatelliteSemi-major axis (km)

Average diameter (km)

Mass ratio (S/P)

Density (g/cm3)

Mars Phobos 9270 23 1.81E-08 1.95 Deimos 23400 12 3.01E-09 1.7 Pluto Charon 19600 1186 0.122 1.7

Satellites system on Jupiter

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Some relevant data of the Jovian satellites are shown in table 9. One of the most interesting subjects to discuss is the relationships between the orbital phenomena of the Galilean satellites. As it is mentioned in previous paragraphs, it is well known the relationship between orbital angular velocities of the Galilean satellites as n(Io)- 3n(Europa)+ 2n (Ganymede) = 0º. Furthermore, their respective orbital longitudes match with L(Io)-3L(Europa)+2L(Ganymede)=180º That means that the three satellites can not align at the same side of Jupiter. Because Galilean satellites seem to be like a small replication of the planetary system, regular orbital distances between satellites can be observed in a similar way of that exhibit by the planets. Thus, Titius-Bode type equations can be deduced for the Jovian system to calculate the approximate orbital radii for the Jovian satellites. Despite this match, there are major differences between the Galilean system and the Solar System. These differences point to the magnitude of the orbital angular momentum of the orbiting bodies with respect to the spin angular momentum of the central mass. As it was mentioned before, the angular momentum of the giant planets on the Solar System raises up 200 times greater than the one due to the spin of the Sun. By Contrast, the rotational momentum of Jupiter is about 100 times that of the orbital momentum of its satellites, mainly due to the proportionally greater distances between giant planets respect to the Sun radius and by the faster rotation of Jupiter compared to that of the Sun. Jupiter rotates almost 70 times faster than the Sun. Only the four Galilean satellites exhibit spherical shape. The next largest satellite, Amalthea exhibit irregular shaped surface. Two groups of outer satellites with respectively direct and retrograde orbits and with orbital inclinations differing more than 90º characterize the external satellites system which most likely corresponds to captured asteroids when they approached to Jupiter.

Table 9: Jupiter satellite data Name a (km) i (deg) e Period

(days) Size (km)

Small Inner Regulars and Rings Metis 128100 0.021 0.001 0.3 44 Adrastea 128900 0.027 0.002 0.3 16 Amalthea 181400 0.389 0.003 0.5 168 Thebe 221900 1.07 0.018 0.68 98

Galileans Io 421800 0.036 0 1.77 3643 Europa 671100 0.467 0 3.55 3122 Ganymede 1070400 0.172 0.001 7.16 5262 Callisto 1882700 0.307 0.007 16.69 4821

Prograde Irregular

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Themisto 7507000 43.08 0.242 130 9 Leda 11165000 27.46 0.164 240.9 18 Himalia 11461000 27.5 0.162 250.6 184 Lysithea 11717000 28.3 0.112 259.2 38 Elara 11741000 26.63 0.217 259.6 78

Retrograde Irregular Groups Euporie 19302000 145.8 0.144 550.7 2 Orthosie 20721000 145.9 0.281 622.6 2 Euanthe 20799000 148.9 0.232 620.6 3 Thyone 20940000 148.5 0.229 627.3 4 Harpalyke 21105000 148.6 0.226 623.3 4 Hermippe 21131000 150.7 0.21 633.9 4 Praxidike 21147000 149 0.23 625.3 7 Iocaste 21269000 149.4 0.216 631.5 5 Ananke 21276000 148.9 0.244 610.5 28 Pasithee 23096000 165.1 0.267 719.5 2 Chaldene 23179000 165.2 0.251 723.8 4 Kale 23217000 165 0.26 729.5 2 Isonoe 23217000 165.2 0.246 725.5 4 Aitne 23231000 165.1 0.264 730.2 3 Erinome 23279000 164.9 0.266 728.3 3 Taygete 23360000 165.2 0.252 732.2 5 Carme 23404000 164.9 0.253 702.3 46 Kalyke 23583000 165.2 0.245 743 5 Eurydome 22865000 150.3 0.276 717.3 3 Autonoe 23039000 152.9 0.334 762.7 4 Sponde 23487000 151 0.312 748.3 2 Pasiphae 23624000 151.4 0.409 708 58 Megaclite 23806000 152.8 0.421 752.8 6 Sinope 23939000 158.1 0.25 724.5 38 Callirrhoe 24102000 147.1 0.283 758.8 7

satellites discovered in 2003 S/2003 J1 24557295 163.4 0.345 781.6 4 S/2003 J2 28570410 151.8 0.38 982.5 2 S/2003 J3 18339885 143.7 0.241 504 2 S/2003 J4 23257920 144.9 0.204 723.2 2 S/2003 J5 24084180 165 0.21 759.7 4 S/2003 J6 20979105 156.1 0.157 617.3 4 S/2003 J7 23807655 159.4 0.405 748.8 4 S/2003 J8 24514095 152.6 0.264 781.6 3 S/2003 J9 22441680 164.5 0.269 683 1 S/2003 J10 24249600 164.1 0.214 767 2 S/2003 J11 22395390 163.9 0.223 683 2 S/2003 J12 19002480 145.8 0.376 533.3 1 S/2003 J13 24000000 141 0.412 737.8 2 S/2003 J14 25000000 140.9 0.222 807.8 2 S/2003 J15 22000000 140.8 0.11 668.4 2 S/2003 J16 21000000 148.6 0.27 595.4 2 S/2003 J17 22000000 163.7 0.19 690.3 2 S/2003 J18 20700000 146.5 0.119 606.3 2

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S/2003 J19 22800000 162.9 0.334 701.3 2 S/2003 J20 17100000 55.1 0.295 456.5 3 S/2003 J21 20600000 148 0.208 599 2 S/2003 J22 20700000 151.1 0.233 601 2 S/2003 J23 24055500 149.2 0.309 759.7 2

a:The mean semi-major axis. i:The mean inclination. e:The mean eccentricity. Period:The time of one revolution around Jupiter. Size:The diameter of the object. (based on data from http://www.ifa.hawaii.edu/~sheppard/satellites) The Jupiter’s ring was discovered by Voyager 1. The ring is composed of three main components. It is about 7000 km wide and has an external boundary located around 130000 km from the centre of the planet. The existence of this ring allow to suspect that ring system were a normal feature in major planets as is found in all the major planets of the Solar System. A significant asteroidal system constrained in clouds of small bodies lie in the same orbit as Jupiter. These are located around pints located 60º ahead or behind the planet position called Lagrangian points and they constitute the so called Jupiter’s Trojans system. It is known the impossibility to find general solutions to the three body’s problem. However, Joseph-Louis Lagrange (1736-1813), an Italian-French mathematician, found that some special configurations of three bodies did satisfy the equations of motion of them. Such configurations involve collinear and equilateral arrangement of the bodies. Considering the mass of one of the three bodies as negligible with respect to the one of the other bodies, Lagrange found 5 possible positions for the smallest body of certain stability. Three of them correspond to a collinear configuration of the three bodies known as L1, L2 and L3 points and the other two correspond to a equilateral positions called L4 and L5 (Fig. 3). For a Three body system formed by the Sun, Jupiter and a comparatively very small asteroid, L4 and L5 constitute the apex of two equilateral triangles having the largest masses at their vertices (that means the positions of the Sun and Jupiter). Only L4 and L5 are stable points and constrain the locations where the so-called Trojans orbit around the Sun in a 1:1 orbital resonance with Jupiter. There are around 1700 identified Trojans of less than 300 km in diameter, a thousand of them orbiting at the L4 point and almost 700 confined at L5 (http://cfa-www.harvard.edu/iau/lists/JupiterTrojans.html).

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Figure 3. The five Lagrangian points.

The Saturn orbital satellites and rings system With 33 identified members Saturn has the second most heavily populated satellite system (table 10).

Table 10: Saturn satellite system Name a (km) i (deg) e Period (days) Size (km)

Regular Satellites Start of Inner most Ring (D) 66000 km

Pan 133600 0 0 0.575 20 Atlas 137700 0 0 0.602 32 Prometheus 139400 0 0.002 0.613 100 Pandora 141700 0 0.004 0.629 84 Epimetheus 151400 0.335 0.021 0.69 119 Janus 151500 0.165 0.007 0.7 178 Mimas 185600 1.566 0.021 0.94 397 Enceladus 238100 0.01 0 1.37 499 Telesto 294700 1.158 0.001 1.89 24 Tethys 294700 0.168 0 1.89 1060 Calypso 294700 1.473 0.001 1.89 19 Dione 377400 0.002 0 2.74 1118

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Helene 377400 0.212 0 2.74 32 End of Outer most Ring (E) 480000 km

Rhea 527100 0.327 0.001 4.518 1528 Titan 1221900 1.634 0.029 15.95 5150 Hyperion 1464100 0.568 0.018 21.28 266 Iapetus 3560800 7.57 0.028 79.33 1436

Irregular Groups Kiviuq 11365000 46.16 0.334 449.2 16 Ijiraq 11440000 46.74 0.322 451.5 12 Phoebe 12944300 174.8 0.164 548.2 120 Paaliaq 15199000 45.13 0.364 686.9 22 Skadi 15647000 152.7 0.27 728.9 8 Albiorix 16404000 33.98 0.478 783.5 32 Erriapo 17616000 34.45 0.474 871.9 10 Siarnaq 18160000 45.56 0.295 893.1 40 Tarvos 18247000 33.51 0.536 925.6 15 Mundilfari 18709000 167.5 0.208 951.4 7 Suttung 19463000 175.8 0.114 1016.3 7 Thrym 20382000 175.8 0.47 1086.9 7 Ymir 23096000 173.1 0.333 1312.4 18 a:The mean semi-major axis. i:The mean inclination. e:The mean eccentricity. Period:The time of one revolution around Saturn. Size:The diameter of the object. (based on data from http://www.ifa.hawaii.edu/~sheppard/satellites) Titan is the largest satellite. With 5150 km in diameter it is the second largest satellite in the Solar System, being Ganymede the largest one. Even slightly larger than Mercury, it matches well with the Galileans or giant Jupiter’s satellites. Despite the great size of Titan, some of the Saturnian’s satellites have diameters greater then 1000 km as is the case of Tethys, Dione, Rhea and Iapetus. There are some resonances phenomena between the satellites as the case of Enceladus-Dione and Mimas-Tethys having 2:1 resonances or the 4:3 resonance of Titan-Hyperion. Similar to the Jupiter’s Trojans, there are some 1:1 resonances between satellites in the Saturnian’s system such as those of Tethys-Calypso-Telesto being Calypso and Telesto located near L4 and L5 lagrangian points that means 60º ahead and behind Tethys on its orbit and the 1:1 resonances of Dione and Helene with similar triangular configuration in the system Saturn-Dione-Helene being the last one the less massive of the system. Between the most outermost satellites of Saturn, Phoebe moves along a retrograde orbit suggesting that it is a captured asteroid. Also, some peculiar satellites are Rea an extremely craterized object, Iapetus that has a leading hemisphere much darker than the trailing one, with differences in the respective albedoes of 10 times between the two sides or Enceladus that appears as the most bright object in the Solar System.

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The ring system on Saturn is one of the most spectacular structures in the Solar System. It geometry was deduced in 1659 by Christiaan Huygens. The ring system appears as a disk-like feature that extend from an innermost part around 65000 km above the planet surface to 480000 km at the external edge. Saturn's ring system is composed of 7 primary divisions designed as D, C, B, A, F, G and E from the innermost ring to the outermost ring. Each major division is further subdivided into thousands of individual strands. Between the A and B rings is a gap called the Cassini division named after Giovanni Domenico Cassini (1625-1712) who discovered the Gap in 1676. Between the A and F rings lies the Encke gap. There are complex tidal interactions as a result of orbital resonances between some Saturn’ moons and the ring system. Some of the moons, the so-called shepherd satellites play a relevant role in keeping the constrained rings. Mimas, seems to be responsible on the existence of the Cassini division as this corresponds to a 0.5 period of Mimas, 0.3 period of Enceladus and 0.25 period of Tethys. Another important division known as Encke division corresponds to 0.6 period of Mimas. Some rings show complex internal structures such as the braided feature of the F-ring which seems to be constrained by the shepherd moons Prometheus and Pandora. The shepherd effect over a particle orbiting between two major objects can be explained as follows. When a particle moving inside the orbit of the outermost object overtakes its position, then the particle is perturbed and displaced to a more external orbit just outside the external object. Then, the particle will be overtaken by the external object and will be perturbed into a more internal orbit now inside the orbit of the external object and so on. Similar effects will exist from the innermost object to the particles on its external vicinity. The combined effects from the innermost and the outermost major objects tend to confine the intermediate orbiting particles within a constrained intermediate region. An intriguing discoveries made by the Voyager spacecraft was the existence of dark radial features up to 20,000 kilometers long that move in curious patterns on the B ring (Fig. 4). These features commonly known as spokes had scientists perplexed because they seemed to defy gravity. Spokes are thought to be microscopic dust particles that have levitated away from the ring plane. They rotate at the same rate as Saturn's magnetic field indicating they are affected by electromagnetic forces. Their origin is unknown and it could be possible that meteorites passing through the rings or another similar event triggered such structures.

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Figure 4: Saturn's A and B rings separated by the Cassini Division. Spokes (Darker streaks) extend across the B-ring. The satellite system of Uranus Uranus spins slower than Jupiter and Saturn and one of the most intriguing features is that its equator appears tilted at 98 º respects to its orbital plane and consequently making its rotation retrograde. Additionally, all of the 18 satellites pertaining to the regular group orbit near the equatorial plane. The rest of the 9 satellites of the irregular group orbit in planes highly inclined with respect to the equatorial plane of the planet (Table 11). One of its most striking satellites of Uranus and of the entire Solar System is Miranda (470 kilometers across), which appears to be formed by three dark oval to square shaped regions known as coronae showing tectonic and volcanic features following concentric patterns. Four of the outermost satellites have over 1000 km in diameter (Ariel, Umbriel, Titania and Oberon). The Uranus rings system was discovered in 1977 during the Voyager encounters. They are quite different from those of Jupiter and Saturn. Some of the moons as are the case of Cordelia and Ophelia act as shepherd satellites for the outermost ring known as Epsilon ring. The rings are too narrow varying from few to hundred kilometers in width. Neptune satellite system Previously to the Voyagers encounters the only known satellites of Neptune were Triton and Nereid being Triton the unique giant satellite with

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and almost circular but retrograde orbit in the Solar System. Both are considered as irregular satellites. Actually, it is known the existence of 13 satellites orbiting Neptune (table 11). The six outermost of them, including Triton, exhibit high orbital inclinations with respect to the equatorial plane of their planet. Nereid orbit along the highest eccentric orbit between all the satellites in the solar system (e = 0.751). Neptune has also a ring system. Earth-based observations showed faded arcs instead of complete rings. Images from the Voyager 2 confirmed that the rings are complete with bright clumps. One of them exhibits a curious twisted structure. The existence of ring systems in the giant planets is partly explained by the presence of dusty, rocky or icy particles orbiting within the Roche limit. This concept was first established by the French astronomer Edouard Roche (1820-1883) and is defined as the minimum distance to which a large satellite can approach to its planet without being cracked by tidal forces. The Roche limit extends roughly about 2.5 times the radius of a planet depending on the relative masses and densities of the planet and satellite. Usually, small satellites can be found inside the Roche limit but a combination of their cohesive forces and small sizes contribute to avoid the tidal breakup. Concerning the ring structures, it is likelihood that they could be produced by resonant perturbations triggered by the inner satellites as is described above.

Table 11: Satellite systems on Uranus and Neptune Name a (km) i (deg) e Period (days) Size (km)

Uranus system Regular Satellites Cordelia 49800 0.085 0 0.335 40 Ophelia 53800 0.104 0.01 0.376 42 Bianca 59200 0.193 0.001 0.435 51 Cressida 61800 0.006 0 0.464 80 Desdemona 62700 0.113 0 0.474 64 Juliet 64400 0.065 0.001 0.493 93 Portia 66100 0.059 0 0.513 135 Rosalind 69900 0.279 0 0.558 72 Belinda 75300 0.031 0 0.624 80 Puck 86000 0.319 0 0.762 162 Miranda 129900 4.338 0.001 1.41 471 Ariel 190900 0.041 0.001 2.52 1158 Umbriel 266000 0.128 0.004 4.14 1169 Titania 436300 0.079 0.001 8.71 1578 Oberon 583500 0.068 0.001 13.46 1522 Irregular Groups Caliban 7231000 140.9 0.159 579.7 72 Stephano 8004000 144.1 0.229 677.4 32

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Trinculo 8504000 167.1 0.22 759 18 Sycorax 12179000 159.4 0.522 1288.3 150 Prospero 16256000 152 0.445 1977.3 50 Setebos 17418000 158.2 0.591 2234.8 47

Neptune system Regular Satellites

Naiad 48200 4.74 0 0.294 58 Thalassa 50100 0.205 0 0.311 80 Despina 52500 0.065 0 0.335 148 Galatea 62000 0.054 0 0.429 158 Larissa 73500 0.201 0.001 0.555 192 Proteus 117600 0.039 0 1.122 416 Irregular Groups Triton 354800 156.8 0 5.88 2706 Nereid 5513400 7.23 0.751 360.1 340 S/2002N1 15686000 134.1 0.572 1874.8 61 S/2002N2 22337190 52.74 0.137 2925.6 40 S/2002N3 22613200 39.56 0.416 2980.4 40 S/2003N1 46738000 137.3 0.45 9136.1 38 S/2002N4 47279670 139.3 0.605 9007.1 60

a:The mean semi-major axis. i:The mean inclination. e:The mean eccentricity. Period:The time of one revolution around Uranus/Neptune. Size:The diameter of the object. (based on data from http://www.ifa.hawaii.edu/~sheppard/satellites)

The asteroid belt Asteroids are primordial residual objects from the formation of the Solar System. It was suggested that they are the remains of a protoplanet destroyed in a massive collision; recently the prevailing idea is that asteroids are leftover rocky matter that never coalesced into a planet mainly due to the gravitational interference from Jupiter's colossal mass that prevented protoplanetary bodies from growing larger than about 1,000 km. Most asteroids are rocky bodies that orbit the Sun between Mars and Jupiter in a "Main Asteroid Belt" that is centered around 2.7 times the Earth-Sun distance. Some of them move inside the orbit of Earth (many of the Near Earth Objects) but others such as the called “Centaurs” are located beyond the Saturn's orbit. In fact, many of them cross the Earth's path (e.g. the Near-Earth Asteroid 4179 Toutatis). Even though most asteroids may be only the size of gravel, there are 16 with diameters ranging 200-300 km being Ceres, the largest one, with an estimated diameter of nearly 1000 km. Asteroids orbit around the Sun in prograde motion and most of them have eccentricies less than 0.3 and orbital inclinations less than 25º (table 12). It is believed that most NEOs are fragments hit from the main belt by a combination of asteroid collisions and the gravitational perturbations from

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Jupiter. NEOs are grouped into three categories, named for famous members of each: 1221 Amor, 1862 Apollo, and 2062 Aten. Amors are asteroids which cross Mars' orbit but do not quite reach the orbit of Earth. Eros (target of the NEAR mission) is a typical amor. Apollos are asteroids which cross Earth's orbit with a period greater than 1 year. Geographos represents the Apollos. Atens are asteroids which cross Earth's orbit with a period less than 1 year. Ra-Shalom is a typical Aten.

Table 12: Characteristics of the most largest asteroids

Name eccentricity inclination(deg) Diameter (km) Mean Distance from Sun (in AU)

Ceres 0.079 10.6 1003 2.766 Pallas 0.237 34.9 608 2.773 Vesta 0.089 7.1 538 2.361 Hygiea 0.1 3.8 450 3.136 Davida 0.18 15.9 323 3.170 Europa 0.101 7.5 289 3.099 Hektor 0.024 18.2 283 5.203 Psyche 0.14 3.1 250 2.919 Undina 0.072 9.9 250 3.189 Bamberga 0.34 11.2 246 2.682 Juno 0.257 13 240 2.667 Themis 0.13 0.76 234 3.129 Arethusa 0.15 13 136 3.073 Eros 0.223 10.8 33x13x13 1.46 Hidalgo 0.657 42.5 15 5.81 Icarus 0.827 6.9 2 1.08

It is known than several tens of other asteroids (with diameters between 150-350 km) are located near or further out than the orbit or Neptune, These objects are Kuiper-belt objects and it is thought that they are related to comets instead of to asteroids. As it was mentioned above, in general asteroids exhibit prograde orbits and relative low eccentricities and low orbital planes. By contrast, comets usually show retrograde orbits, a broad range of orbital inclinations and eccentricities and have high content of volatile compounds. One of the most striking events in the asteroids research was the first landing on the NEO 433 Eros on 13 February 2001 by the NEAR-Shoemaker spacecraft, which sent very spectacular close-up views of its surface.

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Concerning the distribution of asteroids, there exist important gaps that were first explained in 1866 by Daniel Kirkwood (1814-1895) as due to resonance phenomenon between the orbital period of Jupiter and that of the corresponding asteroidal orbit in a similar way to the formation of gaps in the Saturn rings system (e.g. the gaps due to the influence of the satellites Mimas and Enceladus). Nevertheless, the complexity of the resonance phenomenon is evident by the fact of the existence of several gaps in the 1:2, 1:3, 2:5 and 3:7 among others resonances and slight concentrations of asteroids in the 2:3 orbital resonance (Hilda group) and specially the already mentioned and particular 1:1 resonance which defines the Trojan group. Despite the explanation given for the existence of the Trojan group (see satellite system on Jupiter), the reason of why some resonances produce gaps and others produce asteroidal concentrations it is not yet well understood. The comets The first reference to the heliocentric nature of the comets was made by Edmund Halley when deduced that the comet seen in 1682 was the same to the one seen in 1607 and 1531. Through the knowledge and application of the Newton laws, Halley suggested that comets were bodies orbiting the Sun. In the basis of the wide range in the orbital periods of the comets, they were divided into two main categories: “short period” for comets with periods less than 200 years and therefore moving in a similar orbital range than that of the planets and “long period” for comets with periods longer than 200 years. The current ideas on the composition of the comets came from Fred Whipple in 1950, when he defined these bodies in his “dirty snowball theory”. At the center of the comet's head appears the nucleus, usually of several kilometers in diameter. The nucleus is composed by a mixture of ices of water, ammonia, carbon dioxide and methane and dust. Surrounding the nucleus develops the coma which may be over a million km across. The coma is quite bright due to reflected sunlight by a dense cloud of water, carbon dioxide and other neutral gases sublimed from the nucleus. A huge cloud (millions of km in diameter) of neutral hydrogen surrounds the comet. Many of them have two tails, an ionized gas tail (of around several hundred million kilometers) and a dust tail (up to 10 million kilometers) composed of smoke sized particles. Both always point away from the Sun. The dust tail usually curves slightly behind the comet. The ion tail, brighter than the dust tail, is a wispy bluish colored and is propelled straight back of the Sun. Different comet spacecraft encounters have shown detailed photographs from cometary nuclei as was the case of the E.S.A. Giotto mission in 1986.

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As it passed around 600 km apart from the comet Halley, detailed pictures from the nucleus and measurements of the ionized gas density and dust compositions were provided. Carbonaceous-chondrite like compositions in Silicate particles and H,C, N and O rich particles were found, suggesting the presence of organic compounds. High rates of loss from icy materials during the perihelion passage suggests that lifetime of comets is too short compared to the age of the Solar System and consequently, comets should have been disappeared if any reservoir, somewhere located, did not renewed their existence. One year latter of the proposed existence of the Oort cloud by Jan Oort, a possible source of comets was found by Gerard Kuiper in 1951. He argued that outside the orbit of Neptune there must exists a region of comet-like debris orbiting near the plane of the Solar System in the assumption that the Solar System did not suddenly end beyond Neptune and Pluto. Kuiper belt objects have been observed at distances from 30 to 50 AU from the Sun. It was suggested that the mass of the Kuiper belt objects at this region could be similar or slightly higher than that of Mercury. Nevertheless, a possible extent of the Kuiper belt to around 1000 AU was estimated. The amount of Kuiper belt objects greater than 100 km in diameter was calculated in more than 35000. Occasional gravitational perturbation of Neptune can transfer small bodies from the Kuiper belt to the inner regions of the Solar System, becoming them in short period comets. It was in the early 1990s when the direct detection of Kuiper belt objects took place with the discovery of 1992/QB1 at about 40 AU from the Sun. During the years after that discovery more than 500 Kuiper Belt Objects were found, one of them, the 2002 LM60 (Quaoar), is about half the size of Pluto and 43 AU distant from the Sun. That population increases almost weekly. Origin of the Solar System Several topics must be taken into account in order to develop a theory for the origin of the Solar System. A good theory must solve and explain all features of the Solar System from its origin to the actual state. These are: 1. The distribution of angular momentum between the Sun and the planets.

As it is described above, most of the angular momentum of the Solar System is concentrated in the planets and especially in the gas giants. Although the Sun contains more than 99% of the mass of the Solar System, it contains less than 0.5% of the angular momentum due to a very slow spinning that must be satisfactory explained by any theory in an evolutionary context.

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2. It is needed a model to account for a planetary growth in a time scale similar or less than the life time of the circumstellar discs estimated on the range of 107 years.

3. The planets were formed from cold matter, which means that is not derived from a hot star. The presence on Earth and perhaps on the other planets of high abundances of light elements such as lithium, beryllium and boron that are unstable under the nuclear reactions inside the stars, is consistent with the idea by which, the planet forming matter came from regions outside the star.

4. Planets encircle the Sun moving along direct and almost coplanar orbits. 5. The division of the planets into terrestrial, gas giants and icy giants. It

seems clear that planets formed close to the Sun would be less capable to retain a significant atmosphere than those formed further away. However, the existence of outer icy giant planets, as well as the intermediate gas giants must be explained in terms of its evolution into a global context that also explains the observed features of other planetary systems.

6. The existence of regular and irregular satellites. The division of regular and irregular satellites suggests that regular satellites can be related with the planetary formation process by it self while irregular ones are captured bodies. Of especial interest are the Earth-Moon and Pluto-Charon systems.

7. The tilt of the solar spin axis of 6º. Despite the distribution of angular momentum on the Solar System it is noted that the direction on the angular momentum of the Sun is tilted around 6º with respect to the mean plane of the system. If the Sun and planets were formed from a rotating dusty and gaseous disk, what was the mechanism that tilted the solar spin axis far from the perpendicular direction to the mean plane?

8. The features of other planetary systems. From 1992 it is evident that many stars are accompanied by planetary systems which usually show features quite different from the ones exhibit by the Solar System and must be explained as well, perhaps in a general model of star and planetary systems formation.

9. The variability in the orientation of the planetary spin axes. All the planets exhibit some tilt on their spin axes. In the case of Venus and Uranus the tilt is large enough to become them in retrograde spins.

10. The possible significance of the Titius-Bode law which relates orbital order and orbital radii.

11. The existence and compositions of the comets and asteroids. 12. The existence and geochemistry of the meteorites and its possible

relation with asteroids or other parental bodies in the Solar System.

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One of the earliest theories for the origin of the Solar System was the one from Georges, comte de Buffon (1707-1788), a French scientist which suggested an origin by a catastrophic collision of a comet against the Sun surface. Pierre Simon Marquis de Laplace (1749-1827) refused the Buffon’s idea and established the basis of its theory known as the “Laplace nebular theory” described in his “Exposition du Systeme du Monde” on 1796. This theory was preceded by previous works of scientists such as Rene Descartes (1596-1650), Immanuel Kant (1724-1804) and William Herschel (1738-1822). The theory is based in the collapse of an initial slowly spinning spherical cloud of gas and dust that accelerate its rotation and flattens along the spin axis as it contracts. Several annular rings left behind during contraction evolved into condensed spherical bodies leading to a planetary system. This theory was hardly criticized by James Clark Maxwell (1831-1879) who argued that progressive condensation of annular rings to form the planets could not be possible under self gravitation as differential rotation in the outer and inner parts of the rings would destroy any initial condensation. Another major difficulty for the theory is based on the distribution of angular momentum in the Solar System as there was not a known mechanism to explain why the planets have the most part of the angular momentum. Another classic theory was given by Thomas Chamberlain (1843-1928) and Forest Moulton (1872-1952). They suggested that the Solar System derived from matter ejected from an active early Sun under the tidal influence of a massive passing star that prevented the erupted matter from return to the parent star. Derived from this theory is the James Jeans (1877-1946) Tidal theory in which, proto-planets form in a similar scenario than that of Chamberlain and Moulton. First Otto Schmidt (1891-1956) in 1944 and latter Ray Lyttleton (1911-1995) in 1961 developed, from the idea that a star passing through a interstellar cloud of gas and dust would be partially captured forming an envelope of gas and dust that eventually would form the planets, the so called Schmidt-Lyttleton accretion theory. In 1944, Carl Von Weizsäcker (1912 - ) elaborated its vortex theory later revised and revamped by Gerard Kuiper (1905-1973) in 1951. The Weizsäcker theory suggests that the protoplanetary disk is formed by a pattern of clockwise rotating eddies within an anti-clockwise rotating system in pentagonal symmetry (from a north point of view). Material would collide at the boundary between vortices. In these regions material would coalesce to give condensations that finally will form planets. For the pentagonal symmetry of eddies distribution, the orbital radii match well with the Bode’s law. All of those theories explain some aspects of the Solar System formation but will fail in others. One of the main unsolved problems in the early ones is the low solar angular momentum contrasting with the high angular

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momentum of the planets and other major problem is the formation of planets. These are aspects taken into account in more detail in the most recent theories. Since 1960s years new ideas or developments of the old ones have raised in the context of the Solar System origin and planetary formation such as the Protoplanet theory ( McCrea 1960,1988), the Capture theory (Woolfson, 1964), the Solar Nebula theory (Cameron, 1973) and the Modern Laplacian theory (Prentice, 1974). Only the second one consider the idea of planets formed from captured and fissioned filaments (blobs) by an almost condensed Sun from other less massive diffuse proto-star in a primitive stellar cluster scenery. The Modern Laplacian theory suggests that the planetary system as well as the Galilean satellite system formed from particles condensation in concentric families of gas rings. The ring developed in the equatorial plane of the of the rotating protostar cloud. Mass loss occurs in the rings leading to the formation of turbulent stress and viscosity from convective motions of the gas at supersonic speeds. The theory assumes that the generated turbulence is in a non-dissipative state, the existence of a supersonic convection that can produce and maintain jumps of density as higher as 100 times and the rings maintain their stability for a long time. However, different numerical simulations has showed that the mentioned motion is highly dissipative and consequently the turbulent eddies are disrupted (Monaghan, 1991). Recently, a new scenery of planet formation has been proposed, contradicting the traditional belief that our planets emerged from a calm nebula similar to a nearby one located in the Taurus constellation. The new hypothesis suggests that our Solar System formed inside a cosmic Orion-like nebular maelstrom similar to the distant Orion nebula (Boss, 1998; Boss, 2002; Boss et al., 2002). One of the main goals in the Boss’ idea called “disk instability” based on the traditional gas accretion hypothesis is to explain the extremely rapid formation of the gas giant planets. The model offers an answer to an old question of how Jupiter and Saturn formed fast enough to explain the great thickness of their atmospheres of about thousands of miles deep. While the terrestrial planets grew by the collisional accumulation of planetesimals (Chambers, 2004) and final giant impacts between planetary sized bodies in around 50 millions years, traditionally it was believed that giant planets formed by core accretion where around 10 Earth-mass solid cores formed first and then accretion of a gaseous envelope from the solar nebula in roughly 8 millions years occurred. That mechanism exhibit severe difficulty specially related to the formation of the ice giant planets due to the short-lived stable cosmic structure of the nebular gas after which the sun switched on its "solar wind" and blew away the gas.

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In Boss' view, gas giant planets form rapidly (around 1000 years), which is fast enough to prevent the gas loss of the protoplanetary disks. Hence, the thick atmospheres in the gas giant planets no longer seemed so mysterious. However, an additional mechanism to explain the thin atmospheres in the icy giant planets (Uranus and Neptune) was found in the EUV emission of a massive nearby star. Its EUV radiation was intense enough to heat the nebular gas throughout the Solar System's protoplanetary disk of gas and dust. The proto-Uranus and proto-Neptune planets are far from the sun, at a point where gravity is comparatively weak. Heated gas in their vicinity escapes more quickly into deep space with the result of comparatively thin atmospheres for the icy giants. Terrestrial planets formation is still plausible in this scenery even when gas giants formed faster by the so-called disk instability mechanism. The planets in the context of the Planetary Geology: evolution processes on the planetary surfaces Despite the broad spectrum of processes affecting the morphological features of the planetary bodies, there are two of them of special significance in Planetary Geology such as the volcanic activity in its different styles and the impact cratering. Both have modified the surface of the terrestrial planets from the beginning as well as the solid surfaces of the satellites and minor solid bodies orbiting at the outer region of the Solar System and therefore they will be specially focused on this work. Volcanic activity in the Solar System The volcanism consists on the superficial emission of fragmented or non fragmented liquid mixed with variable amount of gas, crystals and rocky fragments which emerge from the planetary interior. These liquids are produced by the partial melting of the solid interior and are known as magmas. As the magma rises to the surface, mainly due to buoyancy forces, it forms lava flows or pyroclastic deposits depending on the eruptive mechanisms (effusive or explosive style). Volcanism causes degassing of the planetary mantles almost from the final growth of the planetary bodies. Accretional heating caused partition of the volatile compounds from the most refractory elements, degassed the mantle and originated the primitive atmosphere. Volcanism plays an essential role by continuously modifying the structure and morphology of the planetary surfaces. Broadly, the Solar System exhibits a gradual transition between two distinct types of volcanism:

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a) Igneous Volcanism, dominant along the inner Solar System and featured by the emission of melted silicate compounds heated in the range of 600º-1200º C (magma).

b) Cryovolcanism, dominant in the outer solar System. This type is characterized by the emission of a mixture of water, “brine” and other liquid or vapor-phase volatiles such as carbon dioxide, methane, ammonia, hydrogen, together with gas-driven solid fragments, onto the surface at temperatures around 0º C if pure water is the main magmatic component or -100ºC to induce the melting of water-ammoniac ice varying the melting temperature depending on the relative proportion of the two volatile components. In this case, the eruptive processes may be triggered by internal or external heating-induced. The ascending material is known as cryomagma and at the surface originates the so-called “cryolavaflows” (equivalent to the igneous lava flows) or “cryoclastic deposits” (equivalent to the pyroclastic ones).

The morphological features of the volcanic activity strongly depend on the magma viscosity and gas concentration. Magmas of low gas content and viscosity develop flat lava flows which may be of high extension (hundred thousand or millions of square kilometers) in the case of the lava plains or lava plateaus. As the viscosity increases, the volcanism produces shield volcanoes, the largest conic structure, characterized by very low dipping flanks (around 10º or less). At intermediate viscosity, may be produced more steeped cones dipping at 25-35º and for high viscosity magmas lava domes of nearly vertical flanks can be formed. When gas concentration increases and the ascending magma gradually depressurize a vesiculation level appears and magma plus exolved gas may become in a magmatic foam. In the case of viscous liquids exolved gas bubbles pressurizes and finally may cause the magmatic foam to fragment and then explosive volcanic eruptions are triggered. Low power explosions drive to the formation of scoria or cinder cones. High-energy eruptions linked to high power volcanic explosions may cover large areas with pyroclastic fall and flow deposits expelled from a vent. It is well known that eruptive mechanisms (effusive or explosive) are strongly related to the type of volcanic structures (e.g. on Earth, lava plateaus, shield volcanoes, stratovolcanoes, cinder and scoria cones, tuff rings, tuff cones and domes). Nevertheless, not only volcanic activity is constrained to a relationship with the growth of planar or conic structures. Volcanic structures must be distinguished into two major types: a) Construction structures (formed by accumulation of lavas, pyroclasts or

both them around the eruptive vent. They are associated to the general term of volcanoes in a great variety of morphologies and sizes).

b) Destruction structures (formed by catastrophic mass wasting erosive processes induced by the eruptive activity, total or partial collapse of

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another volcanic structure or land slide processes. On Earth, most of these processes originate calderas or volcanotectonic depressions).

Consequently, the volcanic activity conditions the growth of construction structures as well as the development of destruction structures in a continuous balance with the gravity force at the planetary surface and the mechanical properties of the rocks forming the volcanic edifices. Some volcanoes have a single history marked by eruptive processes constrained to a short time interval and with no changes in the chemical compositions of the magma during their active history. These are the monogenic volcanoes built up by one kind of eruptions (e.g. cinder cones). Elsewhere, the gas content, chemical composition and magma viscosity may vary during single eruptions or throughout the volcano’s history driving to the formation of complex polygenic volcanic edifices built up by different types of eruptions (generally larger then the monogenic ones, as for instance the stratovolcanoes). Constrains in the volcanic activity of planetary bodies A most relevant factor that conditions the volcanic activity is the planetary size and hence the volume/surface ratio. It is known that after the accretion period the planetary interior is heated mainly by radioactive decay of long lived elements (K, U, Th) which produces a maximum in temperature after few hundred millions years in planetary bodies. After a maximum value on the temperature interior is reached a gradual cooling of the planet starts leading to a gradual decrease in the volcanic activity until its own extinction. The heat interior is the result of balance equilibrium between heat production and heat loss. Heat production inside the planet’s interior strongly depends on the planetary volume while heat loss is most efficient as the volume/surface ratio decreases, which means as the planetary radius decreases. Thus, small planetary bodies with low volume/surface has a relative high surface with respect to the volume and then a most efficient heat loss lead to an early cooling and consequently to an early extinction of the volcanic activity in roughly 1 to 1.5 billions years If no additional external heating sources operate to maintain the volcanism in active state. This is the case of Mercury, the Moon and the asteroids. On the other hand, bodies with higher volume/surface ratio exhibit most ineffective heat loss and high internal temperature will be preserved for a long periods of time as is the case of Venus and Earth (still actives today). An intermediate situation is the one exhibit by Mars which probably ceased it active volcanism after 4 billions years after accretional period. However, other heating sources such as the tidal heating induced explain the intense volcanic activity of small bodies as the case of Io, one of the most

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interesting objects in the Solar System from the volcanic point of view, to be analyzed later. Volcanism in the inner Solar System Mercury. Most of its surface appears saturated with craters. From the volcanic point of view Mercury can be considered as a dead world. Information concerning the morphological features on its surface was obtained from the Mariner 10 in 1974. There are Intercrater plains and Smooth plains both probably of volcanic origin. “Smooth plains” has been identified resembling the lunar maria plains. These structures are extended inside and around the Caloris Basin. Estimations on relative chronology of craters suggest that lava flows on the smooth plains flooded in the same epoch of the formation of the Caloris Basin, to the end of the heavy bombardment around 3.5 billions years ago. It is believed that these relatively dark smooth areas are slightly younger than the Moon-like crater-saturated terrains which date back to the heavy bombardment around 3.8 billions years ago or more. The dynamics of the mercurian mantle during ancient times is not well known. It is accepted that periods of volcanic activity are not tectonically related. The geographical and geochronological correspondence between volcanic structures and large impact crater allow supporting a genetic relationship between them. That means that the great excavations shaped during large impacts craters causes decompression in the mantle triggering partial melting within the depressurized mantle rocks. Magma generated by partial melting associated to decompression phenomena would acted as the main source of the ancient volcanism in Mercury. Venus. The Venusian’s surface appears covered by low concentrations of impact craters suggesting a geologically young age and seems to be almost completely renewed from 300-500 millions years ago. Around 85% of the Venusian’s surface is covered by volcanic materials where large flow fields spread out along hundreds of kilometers. More than one hundred thousand of small volcanoes, few hundreds of large volcanoes, immense lava plains with long lava channels several hundreds or even thousands of kilometers long, enormous caldera-like structures of more than one hundred kilometers wide as well as dome-like volcanoes appear broadly spread along the planet either in the highlands or on plains. Neutral buoyancy zones (NBZ) on Venus evolve in a different manner than that on Earth Fig. 5). On Earth, NBZ forms in upper several km of substrate and volcanic edifice growth is initiated. As NBZ migrates upwards rapidly, vents get focused with rift zones, dike intrusion and higher steeper-sided edifice built. By contrast on Venus, NBZ formation is inhibited at low elevation

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due to the high atmospheric pressure. This pressure and gradient cause the NBZ and magmatic chambers to migrate upwards slowly, enhancing lateral growth of magmatic reservoir. Consequently, the reservoir remains in the substrate and not in the volcanic edifice and vents appear more dispersed, the substrate intrusion dominates over the surface extrusion and larger calderas as well as low shield volcanoes will form (Head and Wilson, 1992). Geochemical data provided by the soviet landers suggest that soil compositions ranges between mafic (lavas from channels in lava flows with silica content around 45% weight) to felsic (silica content around 60% weight), this last most likely typical of the steep sided domes, usually known as “pancakes”. One of the most striking volcanic related structures are the “coronae”, “arachnoids” and “novas” of which there are not equivalent features on Earth or any other planet (Fig. 6). Most of these volcanic structures seem to be developed in volcanic rocks of intermediate to mafic compositions (silica content around 50% or slightly lower). “Coronae” (around 300 known) are large circular to oval features of hundreds of kilometers across, bordered by a deep trench and a concentric cliff system with a central lifted plain. They are thought to be the surface manifestation of internal dynamics of ascending mantle plumes. “Arachnoids” (around 250 mapped on Venus) are circular to elongated structures similar but being smaller than the coronae which appear affected by radial fracture systems and radial dike intrusions. They seem to be associated to intrusion of molten rock which produces systems of radiating dikes and faults. “Novas” (around 50 mapped) are volcano-tectonic structures with a starburst pattern of faults and a broad dome-like uplift of around 200 km of average size. Pyroclastic deposits associated to explosive volcanic eruptions on Venus are scarce or quite unusual. Extremely high atmospheric pressure on Venus inhibits gas exolution from ascending magmas and hence most of eruptive processes are of effusive style. However, it is possible the existence of explosive activity due to pressurized gas under a coherent rocky cover (Fagents and Wilson, 1995). Most of Pyroclastic deposits probably are of ash flow type and may be associated to dome collapse mechanisms or collapses of little eruptive plumes due to gas exolutions in felsic magmas of extremely high gas concentrations. The volcanism appears with a broad global distribution contrasting with the one exhibit on Earth which shows a distribution along plate boundaries. The volcanic distribution on Venus seems to confirm a “mantle plume convection” dynamics that still produce superficial motions and tectonic features such as the “Tesseras” which indicate recent tectonic activity.

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Figure 5: Comparison of reservoir evolution on Venus and Earth (modified from Head and Wilson, 1992). As a conclusion, landers suggest that main lava types are basalts and additionally may occur trachyte-like and rhyolite-like compositions. Minor appearance of Komatiites, carbonatites and may be Sulphur lavas could complete the volcanic scenery over the planet surface.

Figure 6: This radar images shows a chain of pancake domes east of Alpha Regio. Each dome is roughly 25 km wide, and the highest are about 750 m tall

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(left). At the right, Arachnoid structures on Venus. Lighter areas indicate rough topography or slopes caused by faults and impact craters (courtesy of NASA).

The Earth. The terrestrial volcanism constitutes the main reference to the comparative studies on planetary volcanism. From the detailed description of the eruptive events occurred in the Vesuvius volcano in 79 AD by Pliny the Young, terrestrial volcanism has been the main source of knowledge in volcanic activity. It was during the 1970s that planetary volcanism increases its relevance when Linda Morabito working as part of the Voyager navigation team discovered the first active eruptive plumes on Io, in addition, the first extraterrestrial explosive volcanic eruption observed by the man. It is well known that the Earth is divided into three major levels: an outer crust with an average of 8-35 km thick, a rocky mantle extended from the base of the crust to 2900 km deep and a metal dominant core from 2900 km to the centre of the Earth. The Earth interior exhibits a geothermal gradient of around 30ºC/km which decreases with deep and it is estimated a central temperature of more than 5000ºC. This heat is a combination of residual heat produced during planetary accretion and by natural radioactive decay of unstable elements (K, U, Th), despite the high temperatures of the interior, high pressures created by the weight of overlying rocks elevate the solidus threshold of the heated rocks and then they remain in solid state. However, incipient melting is reached at 70-150 km deep. Consequently, two separate solid materials can be distinguished above and bellow that limit. An upper rigid layer comprising the crust and a part of the upper mantle which is known as lithosphere and the rest of the mantle which cools by heat transfer from the mantle-core boundary throughout enormous convective cells. Driven by these convective systems, the solid mantle flows very slowly inducing horizontal motion of the lithosphere. As the lithosphere is broken in a mosaic of pieces called plate tectonics, they interact along boundaries against each other. These boundaries are lineal structures of intense geologic activity, such as earthquakes, volcanoes, and mountain building. Plate tectonics is a combination of two earlier ideas, continental drift and sea-floor spreading. Continental drift is the movement of continents over the Earth's surface and in their change in position relative to each other. Sea-floor spreading is the creation of new oceanic crust at mid-ocean ridges and movement of the crust away from them. Volcanism is mainly constrained at the plates boundary where upwards and downwards motions induce partial melting in the mantle. This volcanism is called interplate and is distinguished from another one called intraplate which is located over punctual mantle plumes and induces partial melting by decompression of an ascending mantle plume at superficial levels and consequently volcanism far from the plate boundaries. In general, volcanic structures may vary in sizes and dip slopes

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depending on the volcanic styles, the monogenic or polygenic character of the volcano, magma viscosity and ejected volume of magma (Fig. 7)

Figure 7: Comparison of terrestrial volcanic structures from the largest volcanic shields (top) to the much smaller cinder cones (bottom). The geographical distribution of volcanoes in our planet, almost entirety grouped along volcanic lines, is the most relevant feature which strongly contrasts with the broad distribution in our neighboring Venus. A first major group of distribution is located on the islands or orogenic belts close to the oceanic trenches around the Pacific Ocean, Indonesia and West Indies. Most of this volcanism is andesitic (silica content around weight 55%), rhyolitic (silica-oversaturated content equal or higher than weight 60%) and basaltic (silica content around weight 47-50%). It is characteristic of the well known calcoalkaline volcanic association (e.g. Andes Mountain Range). Other major group lies under the oceans surface, at the summit of the oceanic ridges. This is a volcanism called tholeitic and it comprises mainly tholeitic basalts (silica content around weight 50%) such as the Mid-Atlantic Ridge basalts, and finally a third minor group is the one located far away from the plate boundaries (intraplate volcanism) formed by isolate volcanoes or volcanic islands spread over ocean floors or continents, called “hot spots”, where a dominant alkaline-like volcanism occurs. It comprises alkali basalts (silica contents around weight 45% or slightly higher), trachybasalts (silica content roughly weight 50-55%), trachytes (silica-saturated content roughly of weight 55-60%) and phonolites (silica-undersaturated content around weight 50-55%) such as the Canary Islands volcanism.

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The Moon. Most of the lunar surface is characterized by an intensely cratered anorthositic crust known as “terrae” and by a smaller outcrops of younger flat basaltic plains filling up the older excavated basins appearing as dark rounded areas on the Moon’s face. These lava plains are known as “maria” and lava flows inside of them are of very large extension and exhibit very low slope angles, suggesting extremely low viscosity. Many of the lava flows exhibit channel-like structures known as “Sinuous Rilles”, resembling the terrestrial collapsed lava tubes. Few cinder cones, small shield volcanoes and domes rise up over the lava flows, most of them located at the near side, where most of the maria are located. It is believed that lunar volcanism developed between 3.9-3.5 billions years, just after the heavy bombardment, then its intensity decreased and most likely extinguished 3 billions years ago. Apollo missions have provided many lunar samples. Analysis of samples shows a great similitude of lunar basalts to that of terrestrial ones. However, the lunar basalts are roughly featured by a higher vesicularity, high Fe, Mg content, bimodal Ti content, low silica and alkali elements content, similar mineral composition to the terrestrial basalts but with no water content and an age between 3.9 to 3.1 Billions years. One of the best evidences on the occurrence of explosive volcanism on the Moon was provided by the samples taken by astronomer and geologist Harrison Schmidt in the last Apollo mission to the moon, Apollo 17 in 1972 that landed near the southeastern edge of Mare Serenitatis in the Valley of Taurus-Littrow. In addition to the maria lava flows there exist several dark areas that have diffuse boundaries and appear more unconsolidated than the maria. These fine-grained pyroclastic-like deposits are called Dark Mantle Deposits (DMDs) and they were probably originated by explosive volcanic eruptions. One of these pyroclastic deposits from the Taurus-Littrow valley is the “Orange and Green Glass Beads” that seem to be pyroclasts composed mainly of ilmenite crystals and glass fragments. Similar in its origin, there exists Dark Halo Craters (DHCs) that have broad low rims ejected uniformly from volcanic vents in strobolian-like eruptions. Volcanism in the Moon seems to be no tectonic-related. Roughly, is more abundant in the near side than on the far side. Despite the great time interval between large impacts and maria flooding, it seems to be a geographical relationship between volcanism and large impact basins suggesting that giant impact phenomena would have triggered huge magmatic systems that finally has driven to the effusion of enormous volumes of basaltic lava flows infilling the impacted basins. Mars. One of the most intriguing features on the Mars surface and not well understood is the one related to its crustal dichotomy. This is defined by a

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Northern younger hemisphere, composed of dominant primary basalts with a relatively low density of craters, known as Northern Plains and a Southern hemisphere, known as Cratered Uplands with a high density of craters distribution, most elevated than the Northern Plains, where despite the occurrence of basaltic magmas, it is likely that primary andesitic magmas and even komatiites may occur. Another relevant feature on the surface of Mars is the existence of two broad topographic bulges elevated few kilometers over the mean altitude: the Elysium Planitia and the Tharsis region. The first one extends roughly 1700 per 2400 km and includes giant volcanoes such as the Hecates Tholus, Elysium Mons and Albor Tholus. Tharsis Region is the largest one with roughly 4000 km wide, standing about 10 km high. This region includes the four largest shield volcanoes in the Solar System which are the Olympus Mons, Ascraeus Mons, Pavonis Mons and Arsia Mons. Olympus Mons is the largest one (Fig. 8). It rises 26 km above the surrounding landscape and extends almost 600 km at it basis. These giant shield volcanoes posses a summit caldera around 90 x 60 km wide and roughly 1300 m depth with six giant craters indicating that the summit has undergone multiple collapse episodes. Olympus caldera is a clearest example of tectonic processes associated to shield volcanism on Mars. It was estimated that the top of the magma chamber is around 16 km depth, at a level much shallower than the source depth of Martian magmas (140-200 km) and thus, positioned within the Olympus Mons edifice (Zuber and Mouginis-Mark, 1992). Other smaller shield volcanoes have growth in other locations such us the mentioned volcanoes on the Elysium Planitia among others. Low relief shields with large irregular craters at their summit and many radial flows along their flanks are called “paterae”. Cinder cones and other dome-like volcanic structures called “tholi” form part of the volcanic scenery on Mars.

Figure 8: 3D view of the Olympus Mons, the greatest shield volcano of the Solar System (courtesy of NASA).

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Little evidences of explosive volcanic eruptions leading to the emission of pyroclastic deposits were found in some places, such as Apollinaris patera or Tyrrhena patera where weak eroded deposits can be recognized of possible pyroclastic origin. Geochemical data provided by the Viking landers and the Mars Pathfinder confirmed the existence of basaltic and andesitic rocks. In addition, the Martian meteorites or SNCs also exhibit mainly basaltic compositions. Some ultramafic compositions on the SNCs such as dunites and pyroxenites are related with Komatiites rocks, the only ultramafic rock of volcanic origin and actually, extremely unusual on Earth. Volcanism has developed from the earlier times, the Noachian (4.6 to 3.8 billions years) to the earlier times, the Amazonian (since 3.5 billions years ago). The intermediate period, the Hesperian and a part of the early Amazonian correspond to the most intense volcanic activity. The most recent volcanoes (Olympus Mons and Arsia Mons) are estimated to be roughly 500 millions years. It is a conventional belief that mantle convection on Mars induces superficial motions and tectonic structures different from the plate tectonic of Earth. Although it seems that volcanic activity have ceased around 500 millions years ago, the mantle convection systems were active in the past with dynamics resembling the “hot spot” convective mechanism instead of the most dominant plate tectonic convection of the Earth. However, the identification of andesites (indicative of continental drift in the context of the terrestrial plate tectonics) by the Mars Pathfinder in 1997, allow suggesting that the previous assumption was not entirely correct and that must not be discarded some kind of ancient plate tectonic dynamics on the early Mars. Volcanism in the asteroid belt Only about 8% of the meteorites correspond to achondrites with typical features of the volcanic rocks which have their origin in the asteroid belt and came from differentiated asteroids. The most significant of these rocks are grouped under the term HEDs (Howardite, Eucrite, Diogenite) and also the so called Aubrites and Ureilites. Thus, there are asteroids with spectral signatures typical of the volcanic rocks and consequently they have an earlier period of volcanic activity. This is the case of asteroid Vesta which is supposed to be the diogenite and eucrite parent body while Nysa shows spectral similarities with the aubrites suggesting that this 43 km asteroid could be one of the aubrite parent bodies. In addition, some NEOs such as 3103 Eger of equal spectral class than Nysa is suspected to be the parent body of the aubrites. Howardites are composed of diogenite and eucrite breccias. Eucrite (Ca-rich members) are the oldest known basalts of the

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Solar System. Most of their rocks forming minerals are pyroxene and plagioclase. Diogenites (Ca-depleted members) are mainly composed of ortopyroxene and less olivine. Concerning the other achondrites, aubrites have similar composition to the enstatite chondrites and consequently it constitutes an evolved enstatite achondrite and ureilites are C-rich achondrites. Volcanism in the outer Solar System As was mentioned above, a different kind of volcanism is expected to occur in the outer region of the Solar System. That is the cryovolcanism in which, magmas (the so-called cryomagmas) raises to the planetary surface at temperatures of around 0ºc or below. As the major component of the bodies in the outer Solar System is water ice and other volatile components such as N2, NH3, CH4 … the eruptive materials are mainly of water ice mixed with other low densities compound in the form of brine-like mixtures. Eruptive mechanisms of cryolavaflows, explosive eruptions and geyser-like ejections are expected to be common events in the icy bodies. This category of bodies includes the planet Pluto and almost all giant satellites of the giant planets. All of them exhibit densities of 2 g cm-3 of below, except for the two internal Galilean satellites Io and Europa that have densities slightly higher than 3 g cm-3 which means that they have a high proportion of rocky components. Volcanism on Io. From the volcanological point of view, Io is the most interesting body of the Solar System and also the most active one. Although is a satellite similar to the Moon in size and density, it represents the best example of tidal-induced volcanic activity which is absent in the Moon. As a result of the orbital resonance with Europe and Ganymede Io exhibit a tidally induced heat generation probably one hundred times the rate of a radiogenic heating, which is enough to partially melt some parts of the Io’s interior. Its surface displays a great variety of colors that it is assumed as due to sulphur compounds and resembles the colors of some volcanoes on Earth (e.g. the Kawa Ijen volcano in Indonesia) where small lava flows of sulphur produce similar colors than that on Io (Fig. 9).

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Figure 9: Visual features of sulphur flows. Left: orange and yellow colors in Tupan Patera on Jupiter's moon Io (Galileo spacecraft, NASA) compared with similar colors of sulphur flows at the Kawa Ijen volcano (Indonesia).

Volcanic activity on Io is intense enough to replenish its surface with a 100 m thick layer each millions years and thus, no impact craters has been seen on Io. Active volcanic plumes rising up to 300 km height were first recognized by the Voyager at the end of the 1970s and latter by the Galileo Mission in 1997. It is believed that these plumes are driven by SO2 or Sulphur gas. Despite the presence of Sulphur compounds over the surface of Io, it is likely that measured temperatures up to 1500ºC are related to molten silicates. Volcanic activity on Io supplies a very thin atmosphere (roughly 10-7-10-12 atm) which partially escapes to form part of a plasma torus around Jupiter. Although there is some evidence of effusive volcanism, the most striking activity is the explosive volcanic eruptions seen on Io by spectacular volcanic jets through the Voyager and Galileo images. Concerning the volcanic jets, three types of volcanic activity has been identified in Io (Kieffer, 1984): 1. Prometheus type: with umbrella-like plumes of 50-120 km height and

vent encircled by fall deposits from 200 to 600 km in diameter. Ejection speeds of 500 m/sec and associated temperatures of 175ºC were estimated. The thermodynamic conditions of the jets seem to be of balanced equilibrium pressure.

2. Loki type: with diffuse or irregular shaped plumes. Similar dimensions to the previous style but probably emitting material under the condition of under-expanded volcanic jets.

3. Pelé type: with emission of large plumes roughly 300 km height and vent encircled by 1000-1500 km in diameter of fall deposits. Ejection speeds of 1000 m/sec and associated temperatures of 325ºC were

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estimated to exist at the vent. This type of eruptive process can be of similar conditions than that of the Prometheus type.

Volcanism in Europa. The Voyager and Galileo images revealed active resurfacing processes throughout cryovolcanic ridges by which, liquid water or ice-crystal mush rises up to the surface made of frozen water. Models based on the density or Europa suggest the existence of an internal rocky sphere 3000 km in diameter surrounded by a 100-150 km thick water layer not entirely frozen but covered by an ice field that seems to be affected by a set of cracks and the mentioned cryovolcanic ridges. The rising of water on the surface throughout the ridges is probably triggered by the warming of water at the water-internal rocky sphere boundary by volcanic phenomena of similar origin than that on Io. In this sense, it is not expected explosive volcanic activity at the water-rock boundary as the hydrostatic pressure may be high enough to prevent gas exolution processes. In addition, there is likelihood that superficial resurfacing by cryovolcanic processes are of effusive style. Cryovolcanism in other icy bodies. There are many evidences of cryovolcanism in numerous satellites of the outer Solar System. Many of them over 500 km in diameter have been imaged in detail by the Voyager and Galileo spacecrafts. It is characteristic the existence of fractures and vents but typical morphologies of the silicate volcanism are absent. Many of them exhibit a surface affected by a series of ridges and furrows of not well known significance. Evidences of active cryovolcanism are visible in satellites such as Triton (Neptune’s satellite) or Enceladus (Saturn’s satellite) or of recent activity such as Ganymede (Jupiter’ satellite) or even ancient cryovolcanic activity such as could be the case of Callisto (Jupiter’s satellite). Differences between Ganymede and Callisto related with the temporal persistence of cryovolcanic phenomena may be due to the tidal heating in Ganymede intense enough to produce a more persistent volcanism than that of Callisto the most external giant satellite of Jupiter and less affected by tidal heating. The case of Triton, the largest satellite of Neptune (2720 km in diameter) may be of special complexity but very interesting as now is the only icy body with current cryovolcanic activity imaged by spacecrafts. Its surface appears poorly cratered suggesting a young age. The older terrains appear overlain with younger smooth plains that seem to have been originated by large effusive flows. In 1989 active plumes of 8 km height were seen at the southern part of Triton. Each of them sent jets of dark cryoclasts upwards through a geyser-like eruptive mechanism producing dark cryoclastic deposits downwind from the vents. Despite the occurrence of tidal and radiogenic heating, it is possible that active plumes could have an external

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origin such as a seasonal heating. As the polar cap is made of transparent nitrogen ice covering a darker substrate, when the sunlight heat the underlying dark layer during the southern springtime it causes the temperature of the dark layer to increase a few degrees centigrade over a superficial temperature of around 235 ºC. This slight thermal increase may vaporize the overlying nitrogen ice layer inducing the eruption of nitrogen gas through the icy crust. Impact cratering phenomena Impact craters are geologic structures formed when a large meteoroid, asteroid or comet smashes into a planetary body. All planets and satellites in the Solar System have been heavily bombarded by such bodies throughout their history (Fig. 10). The surfaces of planets and satellites where geological processes stopped hundred millions or billions years ago clearly show the evidences of this early bombardment (e.g. Moon, Mars, Mercury) while the surfaces of planets with active exogenous or endogenous processes appears continuously renewed by resurfacing volcanic processes among others that erase or simply mask old craters on the planetary surface (e.g. Earth, Venus, Io, Europe among others). On the basis of morphological criteria, impact craters can be differentiated as simple or complex craters. The simple ones are small and exhibit depth-to-diameter ratios roughly 1:5 to 1:7 and a smooth basin shape. The complex craters are larger. In these, gravity triggers the initially steep crater walls to collapse downward and inward, forming a complex structure with a central peak or peak ring and a shallower depth compared to diameter (1:10 to 1:20). As impact structures increase in size, they become increasingly complex, changing from craters with central peaks or groups of peaks to features surrounded by two or more rings. Depending on the surface gravity of the planetary body, the diameter at which craters become complex may vary: At greater gravity, smaller diameter will produce a complex structure. On Earth, this transition diameter is 2 to 4 kilometers depending on target rock properties; on the Moon, with a surface gravity one-sixth than that on Earth, the transition diameter is 15 to 20 kilometers (http://www.solarviews.com/eng/tercrate.htm).

. In general for giant impact structures it is used the term impact basin. For example structures larger than 300 km in the Moon or larger than 150 km on Earth, are termed impact basins. Such large impacts could also produce faulting and ductile deformations over large areas of the planetary surface as well as they may trigger the partial melting of large volumes of the planetary mantle inducing subsequent igneous activity. The central peak or peak ring of the complex structures is formed as the floor of the transient cavity rebounds after the compressional shock wave

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of impact is replaced by upwelling rarefaction waves. Slumping of the rim further modifies and increases the final crater’s diameter.

Figure 10: Cratered terrain on the far side of the Moon (left) with a large crater of 80 km in diameter and central peak. Barringer crater, Arizona (right) a simple crater of around 1.1 km in diameter (courtesy of NASA). In general, material ejected from impact structures are useful markers to study the geologic history of a target by analyzing the relationships between ejecta deposits and other buried or overlying structures. In massive events caused by large impactors, tremendous pressures and temperatures can vaporize the meteorite altogether or can completely melt and mix it with melted target rocks. In some cases, nonterrestrial relative abundance of siderophile elements can be detected in the impact melt rocks within large craters which represent the chemical signature of the meteorite impactor. On Earth, a long-established matter was the distinction between craters of volcanic or of impact origin. This was of special relevance during the controversy about the origin of the Barringer meteorite crater. Although there were evidences of meteorites and absence of volcanic rocks, the Barringer Crater, also known as “Meteor Crater” was previously interpreted as of volcanic origin. It was Daniel Moreau Barringer who assumed an origin by impact phenomenon in 1906. Since the 1960s, numerous studies have discovered other physical markers of impact structures such as the shock metamorphism. Certain shock metamorphic effects have been shown to be exclusively associated to meteorite impact craters; no other geological phenomenon, including volcanism, produces extremely high pressures to cause them. They include shatter cones, multiple sets of microscopic planar features in quartz and feldspar grains, diaplectic glass, and high-pressure mineral phases such as stishovite. All known terrestrial impact structures exhibit some or all of these shock effects.

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One of the most important facts about impacts at hypervelocity condition (several kilometers per second) provided by the Meteor Crater controversy was that the diameter of the generated crater during an impact is several times greater than the impactor diameter. In general terms, the final diameter of the crater may be roughly calculated as about 20 to 30 times the diameter of the impactor. More detailed calculations are given by Collins et al. These authors correlates the final diameter of an impact crater on Earth with the diameter of the transient crater, the target and impactor densities, the size and speed of the projectile at the impact and the Earth’s surface gravity (http://www.lpl.arizona.edu/impacteffects/). Almost all impact craters are surrounded by ejected debris following different morphologies depending on several variables such as gravity, size and speed of the impactor, target properties or atmospheric conditions (pressure, composition and density) among others. The fragmented material dispersed from the impact area and felt back to the planetary surface surrounding the crater is known as ejecta blanket. In general, the average radius of ejecta blanket depends on the crater radius in the form of: Reb = (2.3 ± 0.5)Rc 1.006 (Melosh, 1996). Beyond the continuous ejecta blanket, develops the discontinuous ejecta of thin and patchy deposits with also the presence of secondary impact craters. The ejecta morphology may vary between Ballistic type, Rampart type, Flow type and Radial type. Ballistic ejecta form the cone-shaped ejecta curtain which dominates under vacuum conditions. A change of emplacement styles occurs with increasing atmospheric pressure: from ejecta rampart (at lower pressures), ejecta flow lobes (intermediate pressures) and radial patterns (for relatively higher pressures). A wide spectrum of ejecta morphologies can be found associated to crater excavations including fluid-like ejecta styles, even in the absence of water or other volatiles within the target substrate. That is of special interest for Mars for which, typical ejecta flows around Martian craters could not necessarily indicate greater amounts of crater excavated water-rich substrates but the degree of ejecta entrainment in the dynamic atmospheric resulted from the crater size and ejecta size (Schultz, 1992). Other striking structures associated to impact craters are the ejecta rays, one of the most prominent features visible at the surface of the Moon. Lunar rays appear as narrow filamentous of high-albedo features extending many crater radii from their parent craters. The lunar rays appear bright by a compositional contrast with the surrounding terrain, the presence of immature material, or both. “Immaturity” rays are bright because of the presence of fresh, high-albedo material. Compositional rays can persist far longer than 1.1 billions years, which is the currently accepted age of the Copernican–Eratosthenian boundary. Consequently, the presence of rays is not a unique indication of crater age. The optical maturity parameter should

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be used to define the Copernican–Eratosthenian boundary. The time required for an immature surface to reach the optical maturity index saturation point could be defined as the Copernican Period ( Hawke et al., 2004). There are estimations which correlate time between impacts on Earth with the size of impactors showing an increase of time interval between impacts as the object size increases. (Fig. 11 ).

Figure 11: Impact rates versus impactor sizes on Earth.

Several calculations on impact rates in different parts of the Solar System show differences between the inner and outer Solar System. As example, for comets of absolute magnitude brighter than 9, impacts rates estimations range from 2x10-8 to 60x10-8 comets/year for the terrestrial planets and 1.5x10-4 to 6.5x10-4 comets/year for the giant planets (Levison et al., 2000). Meteorites It was until the early 19th century that was believed that no small objects could exist in the interplanetary space. A change in these conventional idea took place at the end of the 18th century, when a German naturalist Peter Pallas found and latter examined a huge iron mass near the town of Krasnojarsk - a mass that the Tartars said it had fallen from the sky. The 700kg iron mass was partly covered with a black crust, and there were many translucent olivine crystals within the iron matrix. The Pallas' subsequent description encouraged a German physicist, Ernst Florens Chladni (1756-1827), to publish a risky hypothesis in which he assessed that the iron mass and other finds represent rocks that came from the space. In his work entitled "On the Origin of the Pallas Iron and Other Similar to

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it, and on Some Associated Natural Phenomena"; published in 1794, he compiled many data about other meteorites related to finds and falls, many of them, responsible of the phenomenon known as fireballs. By his controversial work, Chladni is considered as the father of the science on meteoritics. Actually, it is estimated that a few hundred tons of meteoritic material in the form of fine dust or larger chunks strikes the Earth every day. Some of them survive the transition through Earth’s atmosphere and reach the Earth’s surface. At this point, it is important to distinguish the difference between the term meteoroid and meteorite. Meteoroid is any natural body coming from the space before it enters the Earth’s atmosphere and meteorite term is applied to those meteoroids or part of them that survived the atmospheric transition, normally causing a meteor phenomenon to finally rest on the Earth’s surface. The recovered meteorites may be defined as falls, when the meteorites were witnessed by someone falling from the sky or finds, when they were not witnessed but latter found them. About 33% of the meteorites are falls and it is estimated a total amount of recovered meteoritic material around 450 tons. The largest one was found in 1920 in (Hobe) Namibia, an iron meteorite of 60 tons weight (Fig. 12).

Figure 12: View of the largest meteorite found in Namibia (left) and a pallasite (right) made of a mixture of iron and stony material.

Roughly, they are grouped as: 1. Stony meteorites (roughly the 93% of total falls and 72% of finds),

made of silicate minerals such as olivine, pyroxene or plagioclase, most or them with similar compositions to the existing at the Earth’s mantle. Depending on the existence of chondrules (spherical aggregates of coarse crystals formed from a rapid cooling and solidification of a melt heated at almost 1500ºC) in the stony meteorites, these are subgrouped onto:

• Chondrites (chondrules present): these are: a. Carbonaceous chondrites: A type containing large

amounts of the magnesium-rich minerals olivine and serpentine and a variety of organic compounds, including

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amino acids. A lighter colored inclusions of high temperature condensated known as CAIs (calcium-aluminium-rich inclusions) are also present.

b. Ordinary chondrites. c. Rumuruti chondrites. d. Enstatite chondrites.

• Achondrites (chondrules absent). a. Martian meteorites. Comprises the so called SNC or

snicks. They are 1.3 billions years or lower age and exhibit high content of volatile. There are basaltic or lherzolitic shergottites, nakhlites (clinopiroxenitic composition) and chassignites (dunitic like composition). A special case of Martian meteorite is ALH84001 made of dominant ortopiroxene. This is the oldest known Martian meteorite (4.5 billions years) and its study has relevant implications in astrobiology.

b. Aubrites. Possibly from the near earth asteroid 3103 (Eger) these are similar to the enstatite chondrites (with no chondrules).

c. Ureilites. Carbon rich achondrites formed in differentiated C-class asteroids.

d. Angrites. Are vesiculated basaltic rocks made of augite and minor olivine and troilite.

e. HED group (Howardites, Eucrites, and Diogenites) from the asteroid Vesta. Howardites are breccias made of eucrites and diogenites. Diogenites are calcium depleted terms, made of ortopiroxene and minor olivine. Eucrites are basalt like rocks made of dominant piroxene and plagioclase.

f. Lunar meteorites. Originated in the lunar highlands or in impact basins. There are anorthosite type, gabbro type, basalt type and norite type.

2. Iron meteorites (around 5% of falls and 20% of finds) are made of nickel-iron metal. Minor amount of accessory minerals such as iron-sulphide, troilite or graphite, often surrounded by iron-phosphate and the iron-carbide are also present. These meteorites are similar to the M type or metallic asteroids. Nickel-iron metal occurs forming two distinct alloys. The most common alloy is kamacite (4 to 7.5% nickel). The other one is known as taenite (27 to 65% nickel). Depending on the distribution of the nickel-iron alloys, iron meteorites may display, after slight acid induced corrosion over fresh cut and polished slices, characteristic features that are used to structurally classify iron meteorites into:

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• Octahedrites ( Ni content higher than 7%). These display a pattern of intercepting bands, called "Widmanstätten figures" by their discoverer, Alois von Widmanstätten. Such pattern shows an intergrowth of larger kamacite and taenite plates. As its spatial geometry is the one exhibit by an octahedron, these iron meteorites are called octahedrites.

• Hexahedrites ( Ni content less than 7%). Are less common than octahedrites and consist of kamacite with a crystal structure similar to the one exhibit by the hexahedron. Hexahedrites do not display Widmanstätten figures, but they often exhibit fine, parallel lines called "Neumann lines" by their discoverer, Franz Ernst Neumann (1848). These lines represent a shock-induced, structural deformation of the kamacite plates.

• Ataxites (Ni content from 16 to 60%). Despite the Hoba meteorite, the largest known, belongs to this type, they are the most unusual ones. They consist of nickel-rich taenite and minor kamacite with no internal structure upon etching, representing the most nickel-rich recognized meteorites.

3. Stony Iron meteorites. With 1% of falls and 2% of finds are less abundant than stony and iron ones are. They consist of roughly equal content of nickel-iron metal and different types of stony components similar to the one of the S type asteroids or asteroids of stony composition (the 17% of all known asteroids). They may be classified into two groups:

• Pallasites. These contain abundant silicate inclusions (mainly of large olivine crystals) in a nickel-iron matrix, making them as the most beautiful known meteorites. They are related to the HED achondrites plus added metal and represent samples of the core/mantle boundary material from differentiated asteroids.

• Mesosiderites. They consist of approximately equal content of nickel-iron metal and silicates. Texturally, the mesosiderites exhibit a complex mixture of a nickel-iron metal portion and a heavily brecciated silicate portion, consisting of mostly pyroxene and plagioclase. Surprisingly, the silicates are evolved igneous rocks, representing the crust of an achondritic parent body. It was suggested that they were formed by the collision of two differentiated asteroids, allowing the still liquid core of one asteroid to mix with the solidified crust of the other.

Studies of isotopic anomalies in meteorites are useful to characterize their geochemical features in a more detailed level. Isotopic anomalies such as 18O16O, 17O16O, 24Mg25Mg and 26Mg, 20Ne21Ne and 22Ne, 28Si29Si and 30Si, 12C13C, 14N15N contain information about the conditions in the early Solar System. Interesting isotopic anomalies were found in the preSolar silicon

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carbide grains (SiC) found in chondrites. These anomalies involve ratios such as 14N/15N versus 12C/13C separating different families with respect to the solar ratios. Other geochemical features are useful to characterize different families of meteorites such as iron plus magnesium oxides versus aluminium oxide, iron versus manganese or major elements ratios such as Calcium/Silicon versus Iron/Silicon, Magnesium/Silicon versus Aluminium/Silicon or Sodium/Silicon versus Iron/Manganese (Fig. 13). Some comparisons of gas compositions in the Martian atmosphere from the Viking landers data with other possible Martian meteorites are useful to confirm the parental body of some meteorites. In this sense is interesting to determine the content of different compounds such as 132Xe, 84Kr, 20Ne, 36Ar, 40Ar, N2, CO2. These are series of geochemical tests that allow a more precise characterization of the meteorites.

Figure 13: Main geochemical features of Terrestrial and Martian igneous rocks based on major elements ratios.

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Head, J.W., Wilson, L. (1992): Magma reservoirs and Neutral Buoyancy Zones on Venus: Implications for the formation and evolution of volcanic landforms. Journal of Geophysical Research 97 (E3): 3877-3903.

Kieffer, S.W. (1984): Factors governing the structure of volcanic jets. In F.R. Boyd (Editor), Explosive volcanism: inception, evolution, and hazards. Studies in Geophysics, National Academy Press, Washington, 143-157.

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Monaghan, J.J. (1991): Flaws in the Modern Laplacian Theory of the Solar System. Proc. ASA 9(2): 240-241.

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Schultz, P.H. (1992): Atmospheric effects on ejecta emplacement. Journal of Geophysical Research, 97 (E7): 11623-11662.

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http://cfa-www.harvard.edu/iau/lists/JupiterTrojans.html http://www.solarviews.com/eng/tercrate.htm http://www.ifa.hawaii.edu/~sheppard/satellites