Solar System - General Questions Q1. What are the properties which distinguish the inner planets from the outer planets? A1. Inner Planets Outer Planets Close to Sun Far from Sun Orbits closely spaced Orbits widely spaced Small Large High density Low density Rocky in composition Gaseous composition A2. All the planets in the solar system orbit the sun in the same direction . The orbits of the planets all lie in approximately the same plane. The orbital motion of the planets and most of the moons and the rotation of most of the planets all occur in the same direction. The planets are divided into 2 groups: small, dense terrestrial planets close to the sun and gas giant planets far from the sun. Most of the mass of the solar system is contained in the sun but most of the motion is contained in the planets. Q2. What are the systematic properties of the solar system? A3. The solar system is flat (all planets orbit in about the same plane) and the motion generally follows a common direction (most planets, moons, and asteroids orbit and revolve in the same direction). There are significant differences between the inner planets (small and dense) and the outer planets (large with low densities). Q3. How is the solar system organized? That is, what general properties does it have? A4. Venus rotates backwards, but very slowly. Uranus rotates sideways. Several small moons orbit their planets in the backward direction. Comets orbit the sun with fairly random directions and orientations. Q4. Describe the exceptions to the pattern of systematic motion in the solar system. A5. The solar system consists of the sun and all objects whose motion is controlled by the sun’s gravity. Those objects include the 9 major planets, moons at most of the planets, asteroids (or minor planets), comets, and assorted debris that floats amongst the larger objects. Q5. How is the term "solar system" defined? What sorts of objects exist in the solar system?
Planets, Astronomy, Physics and Solar System Questions and Answers
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Solar System - General Questions
Q1. What are the properties which distinguish the inner planets from the outer
planets?
A1.
Inner Planets Outer Planets
Close to Sun Far from Sun
Orbits closely spaced Orbits widely spaced
Small Large
High density Low density
Rocky in composition Gaseous composition
A2. All the planets in the solar system orbit the sun in the same direction . The orbits of the planets
all lie in approximately the same plane. The orbital motion of the planets and most of the moons and
the rotation of most of the planets all occur in the same direction. The planets are divided into 2
groups: small, dense terrestrial planets close to the sun and gas giant planets far from the sun. Most
of the mass of the solar system is contained in the sun but most of the motion is contained in the
planets.
Q2. What are the systematic properties of the solar system?
A3. The solar system is flat (all planets orbit in about the same plane) and the motion generally
follows a common direction (most planets, moons, and asteroids orbit and revolve in the same
direction). There are significant differences between the inner planets (small and dense) and the
outer planets (large with low densities).
Q3. How is the solar system organized? That is, what general properties does it have?
A4. Venus rotates backwards, but very slowly. Uranus rotates sideways. Several small moons orbit
their planets in the backward direction. Comets orbit the sun with fairly random directions and
orientations.
Q4. Describe the exceptions to the pattern of systematic motion in the solar system.
A5. The solar system consists of the sun and all objects whose motion is controlled by the sun’s
gravity. Those objects include the 9 major planets, moons at most of the planets, asteroids (or minor
planets), comets, and assorted debris that floats amongst the larger objects.
Q5. How is the term "solar system" defined? What sorts of objects exist in the solar
system?
A6. Terrestrial planets are small, dense, and close to the sun. Giant planets are large, have low
density, and are far from the sun.
Q6. Compare the properties of the terrestrial and giant planets.
A7. Density is defined as the mass of an object divided by its volume. It is a measure of how tightly
packed material is in the object. In our experience, less dense objects float in water while denser
objects sink.
Q7. Define and explain the concepts of density and temperature.
Temperature is a measure of how fast the atoms in an object are moving. The slower the atoms
move, the lower the temperature. At absolute zero, all atomic motion has stopped.
Q8. Define the term "solar system". How is it different from the universe?
A8. The solar system consists of the sun and all objects whose motion is controlled by the sun. The
sun is an ordinary star, just like all the other stars visible in the night sky. Hence, the solar system is
but a very tiny object in our galaxy of 100 billion stars, which is but one of billions of galaxies.
Q9. In general, how can we tell if a newly discovered object is a part of the solar
system?
A9. If the motion of the newly discovered object is controlled by the sun, it is a member of the
solar system. To determine if it is a member of the solar system, we must study its motion to
determine if it goes around the sun or not.
Solar System - Terrestrial Planets Questions
Q1. Compare volcanism as it occurred on Mars to that which is observed on Earth
and the Moon. How do we know that all the volcanoes on Mars are extinct?
A1. Volcanoes on Mars built mountains just as they did on Earth, as opposed to volcanism
on the Moon which erupted from many cracks and fissures to fill low lying areas without
building mountains. Volcanoes on Mars are much bigger than Earth’s volcanoes, because on
Earth a single hot spot makes many volcanoes as the crust moves past the hot spot. We know
that all of Mars volcanoes are extinct because all of them have impact craters on their flanks.
Q2. How can we determine the relative age of a planetary surface from remote
observation? How old are the surfaces of Mercury, Venus, and Mars?
A2. The relative age of a surface is determined from the density of craters in it. The greater
the number of craters per square mile, the older the surface. Mercury has a very old, cratered
surface. Venus has a very young surface with few craters. Mars is split into two regions, one
very old and one relatively young.
Q3. Why are Venus’ and Mars’ atmospheres so very different from ours?
A3. Venus has always been too hot to have liquid water, due to its proximity to the sun.
When carbon dioxide is released into its atmosphere from volcanoes, it merely builds up in
the atmosphere. On Earth carbon dioxide dissolves in the oceans and does not accumulate in
the atmosphere. The extra greenhouse effect on Venus’ atmosphere has run away to produce
a very hot environment.
Mars is a smaller planet than Earth, and its weak gravity is not able to hold its atmosphere
permanently. Now that Mars’ volcanoes are all extinct (also because it is a small planet and
has lost its internal heat), the atmosphere is slowly dissipating. Hence, it is very thin today.
Q4. Describe the four processes that shape the surfaces of solid planets. Give an
example of a planet or moon that has been significantly affected by each process and
describe the effect it has had on the planet or moon.
A4. The four processes which shape the surfaces of solid planets are: cratering (impacts not
only create the craters but they also pulverize the rocks and spread them around the new
craters), volcanism (the outflow from volcanoes covers large areas of the surface with new
rocky material), tectonism (horizontal plate motion creates new surface material at spreading
centers and recycles old surface material where plates collide), and erosion (the action of
wind and water grinds away at the surface features and generally smooths them out).
Q5. Describe the physical properties (temperature, pressure, composition) of the
atmospheres of Venus, Earth, and Mars.
A5. The atmospheres of both Venus and Mars are mostly CO2 while Earth's is N2 (78%)
and O2 (22%). Surface temperatures range from 860 oF at Venus to about 40
oF at Earth and
about 0 oF at Mars. Surface pressures on Venus are 90 times those on Earth, while the
pressure at Mars' surface is only 0.8% of Earth's surface pressure.
Q6. If we find one part of a planet heavily cratered and another part lightly cratered,
what can we conclude about the two parts of the planet?
A6. Assuming that equal numbers of meteorites land on all parts of a planet, the lightly
cratered surface must be much younger than the heavily cratered surface. Without additional
evidence we cannot say what may have formed the young surface.
Q7. For each of the terrestrial planets (plus the Moon), compare the relative size of the
core.
A7. Compared to its size, Mercury's core is the biggest in the solar system. The Moon's is
the smallest and Mars' is also relatively small. Earth's and Venus' cores are in between these
extremes in relative size.
Q8. Pick any single, large surface feature on Mercury, Venus, or Mars: Name it and
describe its characteristics and origin.
A8. Examples: Valles Marineris is a large rift canyon on Mars longer than the entire US is
wide. Olympus Mons is a huge volcano on Mars, much larger than any mountain on Earth.
The Caloris Basin is a large impact crater on Mercury which is almost 1,000 miles across.
Maxwell Montes is a large mountain range on Venus that may be volcanic or may be related
to plate motion.
Q9. How have the surfaces of each of the terrestrial planets (plus the Moon) been
affected by each of the four major surface-forming processes?
A9.
Mercury Venus Earth Moon Mars
Cratering lots very little very
little
lots lots on half of
surface
Volcanism maybe lots spotty lots in selected
regions
lots on other half
Tectonics no some, maybe
some horizontal
motion
lots no one region, but no
horizontal motion
Erosion no no lots no some, mostly from
blowing dust
Q10. What two factors determine whether or not a planet will be able to retain an
atmosphere? Explain how they compete with each other.
A10. The competing factors which determine whether or not a planet can retain an
atmosphere are its gravity and the temperature of the atmosphere. For higher temperatures,
the atoms in the atmosphere move faster and can more easily escape the bonds of gravity.
Q11. Describe volcanic activity as it has occurred on each of the terrestrial planets and
the Moon. What are the similarities and what are the differences?
A11. On the Moon and Mercury, volcanism occurred only early in the history of the planet
as general oozing from cracks in the crust. The lava released this way filled low lying basins
and did not form volcanic mountains. On Mars there was some of this general volcanism, but
there are also very large volcanic mountains. These mountains built up to huge sizes because
the crust remained stationary over hot spots. Volcanism on Earth is concentrated at the
convergent boundaries between plates where subduction returns material to the mantle and
hot spots develop. Isolated hot spots in the middle of plates do not build volcanoes to the
same degree as on Mars because of the motion of the plates over the hot spot. A few volcanic
plains exist on Earth as well. Volcanism on Venus has been global, recovering the entire
surface within the last few hundred million years. Volcanic mountains of various sizes and
types as well as volcanic plains exist across the planet.
Q12. How have the surfaces of the small terrestrial planets evolved differently from
those of the larger terrestrial planets?
A12. Small planets cooled quickly and formed a single very thick plate for the crust. This
crust was dominated by vertical motion and heat loss by conduction, and had no horizontal
motion. Larger planets cooled more slowly and their crust separated into multiple plates
which moved horizontally. Heat was lost primarily through this tectonic motion.
Q13. For each of the 4 terrestrial planets (including the Moon) that we have studied,
identify the major geologic process which has shaped its surface and describe what
effect it has had on the surface.
A13. The most significant geologic process on each of the planets is: Mercury — cratering;
Venus — volcanism; Earth — plate tectonics; and the Moon — cratering.
Q14. For each of the processes mentioned (cratering, volcanism, tectonics & erosion),
describe a planet which has been strongly affected by the process.
A14. The Moon and Mercury (and part of Mars) have been strongly affected by cratering.
Their surfaces are covered by a large number of impact sites. Volcanism has been important
on the Moon, Venus, and Mars (and possibly Mercury). The outpouring of molten material
from the interior has covered large parts of the surface of these bodies with fresh material.
The Earth has been strongly affected by tectonics. Where plates meet, mountain ranges and
trenches are formed; at spreading centers, ridges of new crustal material are seen. Erosion has
also been important on Earth and Mars, gradually wearing away the sharp surface features
from other processes.
Q15. Why is the core of Mercury so large and the core of Mars so small?
A15. A planet forms from whatever material can condense into solid particles at its position
in the solar cloud. Mercury formed close to the sun where the temperature was high. Many of
the common rocky materials could not condense as solid particles at those temperatures. Thus,
iron (which could condense there) became a more prevalent part of Mercury. Mars, on the
other hand, formed farther from the sun where it was cooler. That allowed a greater variety of
rocky material to condense to participate in its formation. Thus, iron was a smaller fraction of
all the material going into Mars.
Q16. Compare the interior structure of Mercury, Earth, and Mars.
A16. Mercury has a large (for its size) iron core. Earth has a much smaller core which is
partly solid and partly liquid. Mars core is even smaller -- the planet is not completely
differentiated. For each planet, the lighter silicate rocks float in the mantle above the core.
Q17. Describe the physical properties (composition, temperature and pressure) in the
atmospheres of the terrestrial planets.
A17. Venus atmosphere is mostly CO2, has a surface temperature of 860oF, and 90 times
Earth pressure. Mars is also mostly CO2, but has a surface temperature of only -20oF and a
surface pressure of only 0.8% Earth's. Earth's atmosphere is composed mostly of N2 and O2,
and has an average temperature of about 60oF.
Q18. Compare the role of volcanism on the surface of the terrestrial planets.
A18. Venus has had such extensive volcanism that the entire surface has been recoated
within the last few hundred million years. Mars had extensive volcanism in the past, but
predominantly in the northern hemisphere. Volcanism on Earth occurs primarily at plate
boundaries and at isolated hot spots.
Q19. What are the two possible sources of the atmospheres of terrestrial planets?
A19. New atmospheric gases are released on terrestrial planets by volcanic activity and, in
their very early history of the solar system, by numerous comet impacts.
Q20. How is the age of a rock determined?
A20. Since all radioactive material decays with a unique half-life (the length of time
required for half a sample of the material to decay), the ratio of daughter elements (the decay
product) to the original material tells you how many half-lives have past since the rock
formed.
Solar System - Giant Planets Questions
Q1. Why are the rock/ice cores of the gas giant planets so much larger than any of the
terrestrial planets?
A1. Planet formation begins as small solid particles stick together to form progressively larger and
larger particles. Farther from the sun where the temperature was cooler, more kinds of material
could exist in solid form. As a result, the outer planets had more raw material to grow from and
became much larger than the inner planets.
Q2. Describe in very general terms the surface appearance of each of the gas giant
planets.
A2. All the gas giant planets have banded atmospheres, with wind patterns parallel to their
equators. Those on Jupiter are the most obvious, but faint impressions of bands can be seen on all
the gas giants. Oval cyclonic storms also are ubiquitous to the gas giant planets. Some, such as the
Great Red Spot on Jupiter, seem very stable while others come and go rather quickly. Uranus and
Neptune have a characteristic blue color from methane clouds high in their atmospheres.
Q3. Compare and contrast the interior structure of the gas giant planets.
A3. All the gas giant planets have cores of rock and ice that are about the same size. Jupiter and
Saturn have extensive layers of metallic atomic hydrogen outside the cores (Jupiter’s is much larger
than Saturn’s). All of the gas giant planets have molecular hydrogen layers on the outside, although
the layers for Uranus and Neptune are rather small compared to those on Jupiter and Saturn.
Q4. What satellite missions have been sent to the gas giant planets?
A4. Pioneer 10 & 11 flew past Jupiter and Saturn. Voyager I and II also flew past Jupiter and Saturn.
Voyager II went on to Uranus and Neptune. Galileo is on the way to Jupiter now.
Q5. Describe the major constituents of the solar system, and place them in the broader
context of the universe at large.
A5. The solar system consists of the sun, and all objects whose motion is controlled by the sun.
That includes the nine major planets, about 50 satellites of the planets, asteroids or minor planets,
and many, many comets. The solar system is a very tiny part of the overall universe. In a scale model
where the solar system is about a mile across, the nearest star is about 2000 miles away. Our galaxy
alone contains more than 100 billion stars.
Q6. Explain why the giant planets radiate more energy than they receive from the sun.
What observation of Saturn confirms a part of the explanation?
A6. Both planets are still shrinking slowly. As matter falls inward, energy is released. On Saturn,
helium condenses and falls as rain in the upper interior. This falling motion also generates heat. The
atmosphere of Saturn is depleted of about half its helium (in comparison with Jupiter), an
observations which supports the second process.
Solar System Formation Questions
Q1. What is chemical differentiation and what effect has it had on the planets?
A1. Chemical differentiation occurs in a liquid or gas when heavy material sinks and less dense
material rises. In the early history of the terrestrial planets, when they were entirely molten, this
allow the heavy, dense materials (such as iron and nickel) to sink to the center to form dense cores
while lighter materials rose to the surface to form a less dense mantle.
Q2. Why did the terrestrial planets chemically differentiate after they formed? What
are the consequences of this event?
A2. Once the terrestrial planets had formed, they quickly melted as result of large scale impacts
and radioactive heating. When the entire planet was molten, heavier material sank toward the
center and lighter material rose to the surface. As a result, most of Earth's iron is in the core and our
surface is composed predominantly of silicate rocks.
Q3. Why does a collapsing cloud of gas become flattened? Be sure to describe
completely the reasons why this happens.
A3. If the cloud is rotating, the centrifugal force of rotation will impede the collapse of material
near the equator more than it will for material near the axis of rotation. Centrifugal force is the force
"created" because matter "wants" to move in a straight line instead of rotating. Since it always
points away from the axis of rotation, it is exactly opposite gravity for matter near the equator, but
in a different direction from gravity (and weaker also) for matter near the axis of rotation. Thus
matter near the axis will collapse faster and the cloud will become flattened at the poles.
Q4. Why are planets that formed close to the sun made of rocks and iron while those
that formed far from the sun mostly gaseous?
A4. Planets form from tiny solid particles that collide and stick together. Close to the sun, the
temperature was high and only a few materials like iron and rock could remain solid. Further from
the sun, the temperature was much lower and various ices could also condense to participate in
planet formation. Hence, the planets close to the sun are made of dense rock and iron while those
further away contain large quantities of ices, most of which have melted and turned to gases.
Q5. What role does rotation play in explaining the properties of planets that form in a
solar system?
A5. As a collapsing cloud gets smaller, it will begin to spin faster and faster. The increasing rate of
spin will cause the outward force of rotation to become stronger. Because this force is strongest at
the equator (where the material spins fastest), the outward force there will work against gravity and
slow down the collapse. Near the axis of rotation, the outward force will be less effective in slowing
the collapse. These regions will collapse faster, and the cloud will become flattened. Eventually,
material will collect in a thin disk surrounding the central concentration of matter. The central
concentration becomes the star while planets form in the disk. This explains why the orbits of the
planets all lie in about the same plane and move in the same direction.
Q6. What role does temperature play in explaining why planets close to the sun are
small and dense?
A6. Planets form as small solid particles stick together. Temperature determines what kind of
material is solid. Close to the sun, where the temperature is high, only a few materials could be solid
– mostly silicate rocks and iron. Further from the sun, where the temperature is much lower, water
could also freeze. Thus, the inner planets are made of dense and relatively scarce material while the
outer planets are both larger and less dense.
Q7. In general, how do planets form from the material that surrounds a forming
star?
A7. As described above, planets form as small solid dust particles gently bump into each other and
stick together. Repeated collisions cause these particles to gradually become larger and larger,
building up the planets once piece at a time.
Q8. The giant planets undergo a second process that allows them to become even
bigger. Describe this process and explain how it affects the nature of the planet.
A8. Because the outer planets form from the very common ices, they become larger than the
inner planets. Because they are bigger, they also have stronger gravities. This strong force of gravity
is enough to pull in individual atoms from the surrounding gas in the disk. This makes them even
bigger, and gives them an overall gaseous composition.
Q9. Why do planets melt soon after they form? What affect does this have on the
planet?
A9. New planets become quite hot as a result of both the numerous collisions with other objects
they experience and the relatively high level of radioactivity they possess. After a planet has melted,
its heavier material will sink to the center while lighter material will float to the outside.
Q10. What is the role of temperature in planet formation? How did the temperature
depend upon distance from the forming sun during planet formation?
A10. Temperature determines which materials can form solid particles in a gas. Since only solid
particles participate in planet formation (see next question), the value of the temperature at a given
location determines which material, and how much material, is available for planet formation at that
location. Close to the forming sun, the temperature was very high and only a few materials could
form solids. The temperature rapidly dropped with distance from the sun until at the distance of
Jupiter the hydrides such as water could freeze.
Q11. What role did gas play in the formation of giant planets?
A11. The giant planets had much more raw material to work with because the hydrides are much
more plentiful than are the silicates and metals. They were able to grow large enough for their
gravity to be strong enough to begin to attract and hold individual atoms of gas from the
surrounding cloud. This allowed them to become much larger, since there is so much more gas
present than solids. This process was much more effective for Jupiter and Saturn, where the overall
density of material was higher, and less important for Uranus and Neptune, which are further from
the sun.
Q12. Why are all the planets differentiated? How did differentiation occur during the
formation of the solar system; that is, what caused it?
A12. As the planets formed, a great deal of heat built up inside the planets. This heat came both
from the violent collisions which were forming the planets and from the radioactive material which
was incorporated into the planets. This internal heat was enough to melt the planet. In the liquid
interior of the planet, heavy material (metals) sank to the center while lighter material (rocks)
floated to the surface. This resulted in a differentiated planet.
Q13. What event stops planet formation?
A13. When the sun has completed its formation, a wind of matter is blown away from the sun.
This wind of matter strips away any remaining gas and dust from which planets might form. Without
additional raw material to work with, planet formation is stopped.
Q14. Why is the biggest planet in the middle of the solar system?
A14. The largest planet in the solar system had to form far enough from the sun for ices to be in
solid form so that its solid core could grow to large size and close enough to the sun for the density
of gas to be high so that a lot of gas could be attracted by the gravity of the large core.
Q15. Why are the gas giant planets so much larger than the terrestrial planets? Why
are Uranus and Neptune not as large as Jupiter and Saturn?
A15. The temperature in the gas cloud which surrounded the sun as it formed became
progressively cooler at greater distances from the sun. Beyond the so-called frost line water could be
solid, thereby dramatically increasing the amount of material in solid form. The gas giant planets
formed in this region, while the terrestrial planets formed closer to the sun where only the rarer
rocky materials could participate in the formation of the planets. Still further from the sun the
overall density was much less, so the growth of the planets was slower. By the time Uranus and
Neptune became big enough for their gravity to attract gas atoms, very little time was left before the
forming sun blew the remaining gas away. Hence, they did not grow as large as Jupiter or Saturn.
Q16. Why did the planets chemically differentiate soon after their formation? What is
the consequence of this event for planets today?
A16. After their formation, the planets were hot enough to completely melt. During this molten
phase, the heavier materials sank to the center while the lighter, less dense materials floated to the
surface. As a result the planets today have dense cores, usually rich in iron, and lighter mantles.
Q17. Explain why temperature is so crucial to the formation of solids from a gas. How
does this effect explain the difference in composition between Mercury and Earth?
A17. Any material changes from a liquid to a solid at some exactly specified temperature. For
example, water freezes at exactly 32 oF. Planet formation begins as small solid particles stick
together and gradually become larger and larger. Since Mercury formed closer to the sun than did
Earth, where the temperature was higher, only iron and few of the rocky materials were solid there.
At Earth’s distance from the sun, many more rocky compounds had solidified.
Q18. Describe the processes which occur to create planetesimals (small planets) in the
disk surrounding a forming star.
A18. Planet formation begins when microscopic particles condense from the gas as the
temperature cools. These small particles collide and stick together, gradually growing through a
process known as accretion. Still later, still larger objects collide and aggregate as the planets begin
to grow.
Q19. Why is Jupiter the biggest of the planets (Hint: There are two reasons)?
A19. Jupiter was able to grow larger because it is the first planet (in distance from the sun) to form
where water ice could condense into small particles. since there is much more ice than the metallic
and rocky materials in the solar cloud, that provided much more material to make Jupiter than the
inner planets had available. Even further from the sun, the overall density of the cloud was smaller,
and thus the planets are smaller also.
Once the protoplanet Jupiter became large enough, its gravity began to attract gas atoms to the
planet, making it much bigger still. This second process of growth resulted in the final giant planet
we see today. Again, the drop in overall cloud density prevented more distant planets from growing
equally large.
Q20. Which is the first common material to form solid grains in a cooling cloud of gas?
Why is the formation of ice so much more important?
A20. Iron is the common material with the highest freezing temperature. It condenses first in a
cooling cloud. Because oxygen is a thousand times more abundant than iron or any other heavy
material, when it reaches its freezing temperature, a great deal more material can freeze than for
any other substance.
Q21. How do dust grains grow to form small planets?
A21. Small planets form as tiny microscopic dust grains stick together when they gently bump into
each other as they orbit the still forming sun. These particles gradually grow through more and more
collisions. This process continues on an ever increasing scale until a small planet has formed.
Q22. In what two ways does the existence of planets like Earth depend upon the
occurrence of supernovae explosions?
A22. The incredible temperatures that occur during a supernova explosion allow nuclear reactions
that consume energy to occur. These nuclear reactions produce heavy elements that would never be
produced in normal stars. The explosion distributes these products into space, where they are much
later incorporated into new generations of stars and planets.
The rapidly expanding cloud of debris from the explosion can also stimulate star and planet
formation if it passes through a nearby giant molecular cloud. The interaction causes pieces of the
giant molecular cloud to be compressed, which initiates the collapse that leads to the formation of a
star, along with its attendant planets.
Space Exploration Questions
Q1. What are some of the reasons given for spending huge sums of money to explore
the solar system?
A1. Among some of the reasons offered to justify the space program are: (1) to generate or
maintain national prestige, for example, we can do something you cannot do, (2) to satisfy the
challenge of exploration, as also motivates the exploration of remote locations on Earth, (3) to gain
scientific knowledge of other places in th solar system, and (4) to develop applications and products
which are useful for everyday life and ordinary people.
Q2. If a new, nearby planet were discovered, describe the overall strategy that should
be employed to explore it.
A2. First, we should send a satellite to fly by the planet to study its general properties. Such a
mission gives us enough general information to determine what aspects require more detailed
investigation during the next phase of exploration. Such explorations are generally undertaken by
orbiting spacecraft, short term landers, and atmospheric probes. Next comes intensive study with
long term landers, often equipped with some kind of roving vehicle for studying other locations.
Finally will come the utilization of the resources or properties of the planet for other purposes.
Q3. Describe briefly one of the major space missions to explore the planets (name of
mission, place visited, type of mission, etc.).
A3. Many missions could be mentioned. Three examples are the Viking mission to Mars that sent 2
orbiters and 2 landers to map the surface, study the atmosphere, and analyze the surface; the
Voyager mission that sent 2 fly-by spacecraft to visit Jupiter, Saturn, Uranus, and Neptune; and the
Magellan mission that sent an orbiter to Venus to map its surface.
Q4. Why was the Apollo program to the Moon undertaken? From a scientific
perspective, what was wrong with it?
A4. The primary motivation for the Apollo program was to maintain national prestige -- to beat the
Russians to the Moon. But the program probably sent humans to the Moon before there was
adequate scientific justification, and we stopped the scientific exploration of the Moon too soon to
gain full benefit from the huge effort and expense of the Apollo program.
Q5. In general terms describe the state of exploration of the solar system.
A5. We have performed fly-by reconnaissance missions of all planets but Pluto. More detailed
investigations have occurred at Venus, the Moon, and Mars. Only the Moon has been visited by
humans.
Q6. Describe in general terms the spacecraft exploration of the Moon.
A6. Many spacecraft have been sent to the Moon, with many failure especially in the early days.
These missions have included fly-bys, hard and soft landers, and ultimately humans. Once the Apollo
mission ended, no spacecraft visited the Moon for nearly 20 years, until rather recently we began to
study the Moon again.
Q7. Spacecraft study of a planet can be done with either a fly-by, orbiter, or lander
type mission. What are the relative advantages and disadvantages of each type of
mission?
A7. Fly-by spacecraft are simple and cheap, but provide very limited opportunity to observe their
target. Orbiting spacecraft provide long-term observations of their target, but still are capable only
of remote observations. Spacecraft which land on their target can take much more complete
observations of their target, but can observe only a limited region of the target.
Q8. In what way have satellites fundamentally changed the study of the solar system,
compared to other branches of astronomy? What are some of the reasons, both
scientific and non-scientific, for exploring the solar system?
A8. Astronomy has traditionally been an observational science. We have been required to study
astronomical bodies from a great distance, unable to probe and test them directly. Satellite
exploration of the solar system has now changed that! We have visited all planets but one, along
with several other smaller bodies in the solar system, and have returned samples of the Moon to
Earth-based laboratories for further study. The original reasons for sending satellites to explore
other bodies in the solar system had more to do with maintaining national pride than with scientific
discovery. The development of technology for other uses, and the potential for exploitation of
resources on other planets, have also sometimes motivated our exploration.
Q9. If you were designing the ideal program to explore a planet by spacecraft, what
types of missions would you suggest for a logical development of our knowledge?
A9. The first mission to explore a planet should be a brief reconnaissance by a fly-by spacecraft to
determine the general properties of the planet so that further studies are properly directed. This
mission can be followed by an orbiter, which conducts a long term, more detailed study of the
planet. Then may come a lander or atmospheric probe to directly sample and measure the material
of the planet. Rovers or long term exploration of the planet may follow, with utilization of the
planet’s resources in the distant future.
Q10. Describe generally the 4 logical steps for exploring a planet. How far have we
gotten through this process for any bodies in the solar system?
A10. 1. Fly-by the planet to see what it's like.
2. Orbit the planet for long term observations
3. Explore (e.g., with a roving vehicle on the surface) to gather detailed information
4. Utilize the planet for human gain
All planets but Pluto have been visited at least once, but we have not reached step 4 for any planets
and have done only a little of step 3 at the Moon.
Q11. Discuss the role that politics plays in decisions regarding the American space
program. Give an example of how politics affected the Apollo program to the Moon.
A11. Decisions about which space missions to pursue are often made more on the basis of politics
than on scientific merit (e.g., "no Buck Rogers, no bucks"). Funding for each year of the development
of a spacecraft must be approved by Congress, and a mission can be canceled at any stage by that
process if the political system loses interest in the project. For example, the Apollo mission to the
Moon was initiated so we could "beat the Russians" to the Moon. Once we got there (before they
did), we quickly lost interest in the project and later scientific missions were canceled.
Venus Questions
Q1. Why can the surface of Venus not be photographed from space? How is the surface mapped?
A1. The atmosphere of Venus is 100% cloudy – no images of the surface can be obtained from space.
However, radio waves do penetrate through the atmosphere, so radar techniques can be used to
map the surface. A radar unit sends out radio waves and receives the reflected beam. By timing how
long it took the waves to return, a map of surface features can be made.
Q2. How do we know that the entire surface of Venus is very young geologically? What is the
cause?
A2. We know the surface of Venus is very young (geologically) because there are very few craters on
the surface. The older a surface is the more craters it will have. Venus’ surface has been recently
covered by volcanic outflows, which have covered the entire surface.
Q3. Why are small craters absent from the surface of Venus? How are large craters on Venus
different from similar sized craters on other planets?
A3. Small meteors will burn up entirely in Venus’ dense atmosphere. Thus, small craters will not
form on Venus. All large craters on Venus have been flooded by lava, which indicates that molten
material is available just under the surface.
Q4. What single process is responsible for the formation of Venus’ surface as we see it today?
How do we know that its surface is relatively young?
A4. Virtually all of the surface of Venus has been covered recently by volcanic outflows. We know
the surface is relatively young because very few craters are seen on the surface.
Q5. Of the four processes which can shape the surface of a planet, three have had a negligible role
on Venus. Describe the observations which support that statement.
A5. Craters are very rare on the surface, as shown in radar maps. There is no evidence of folding
where plates collide or ridges where plates separate, so plate tectonics has not had a major effect on
the surface. Photos from the surface show sharp cornered rocks which show little erosion, which
suggests that erosion has not played a major role in shaping the surface.
Q6. Explain how radar is used to map the surface of Venus. Why must we resort to that technique
to "see" the surface of Venus?
A6. Radar operates by sending out a beam of radio waves and recording the return beam which
bounces off an object. By measuring the length of time required for the reflection to return and the
wavelength of the returning waves, we can determine which point on the surface of the planet a
given reflection came from. Since Venus’ surface is always cloud covered, ordinary photographs do
not reveal its surface. Fortunately, radio waves are not affected by clouds.
Earth Questions
Q1. What observations give us information about the structure and material of the
interior of Earth?
A1. We learn about the interior of Earth by studying the arrival times of earthquake waves at
many different places around the surface of Earth. When, and even whether, the waves arrive
at a particular location tell us how the waves traveled through the interior of Earth. For
example, if only P (or pressure) waves are observed at one place (meaning the S or shear
waves were absorbed somewhere in between the earthquake and the station), that tells us that
liquid material must exist on the path the waves followed.
Q2. Describe the structure and material of the interior of Earth.
A2. Earth contains a core of dense material, probably iron, that occupies about half the
radius of the planet. This core is solid on the inside and liquid on the outside because of the
increasing pressure at the center of Earth. A less dense mantle of silicate rocks surrounds the
core.
Q3. What is the difference between an S and a P wave? What does their arrival (or lack
thereof) tell us about the interior of Earth?
A3. Pressure or P waves are waves that oscillate in the direction the wave is traveling in,
while shear or S waves oscillate perpendicular to the direction of travel. S waves cannot
travel through a liquid. Since there is always a shadow zone in which S waves are not
received from earthquakes, this observation suggests that there is a liquid portion to the core
of Earth. The size of the shadow zone tells us how large this liquid core is.
Q4. When referring to the average density of Earth, what does the concept of
uncompressed density refer to? What does its value tell us about Earth?
A4. The uncompressed density of a planet is the density it would have if its interior were not
compressed by the force of gravity. It is, therefore, a better measure of the properties of the
material from which the object is made than the observed density of the planet. The
uncompressed density of Earth is about 4.5 times the density of liquid water, significantly
higher than the average density of rocks found at the surface of Earth. This result tells us that
Earth must contain significant amounts of materials that are denser than rocks.
Q5. How can we tell that the outer core of the Earth is liquid? Be sure to explain any
terminology or concepts you use in your answer. Why do we believe that the inner core
is solid?
A5. S-type earthquake waves (waves whose vibrations are perpendicular to the direction of
motion of the wave) can not penetrate through a liquid medium. This type of earthquake
wave is not received by stations on the far side of Earth — which indicates that a liquid
region is located somewhere between the station and the site of the earthquake. By mapping
precisely those regions that do not receive S-type waves, geologists can pinpoint the size of
the liquid core of Earth.
While we do not have direct observations to prove that the center of Earth’s core is solid, our
models and computations indicate that it is. As one approaches the center of the Earth, both
the temperature and the pressure increases. With increasing pressure, the minimum
temperature required to melt the iron core also increases — at a faster rate than does the
actual temperature. At some distance from the center, the actual temperature becomes less
than the minimum temperature to melt iron and the core freezes into solid form.
Q6. Why do we suspect that the very center of Earth is solid even though the outer
layers of the core are liquid?
A6. Deeper layers of Earth are subjected ever greater temperature and pressure. We suspect
the inner core is solid because of the greater pressures that exist there. The properties of iron
indicate that iron subjected to such conditions of high pressure and high temperature returns
to the solid state in spite of the high temperature.
Q7. How can earthquake waves tell us whether a planet has a molten core?
A7. Of the two types of earthquake waves, only P (pressure) waves can pass through a liquid.
The other type, S or shear waves, cannot pass through liquids. Hence if only one type can
penetrate through the centre of a planet we know that there must be liquid somewhere inside
the planet.
Q8. Describe the contents and properties of the interior of Earth. Mention such
properties as size, mass, and density of different regions, what they are made of, and
whether they are solid or liquid.
A8. Earth’s interior consists of a dense iron core and a less dense silicate mantle. The radius
of the core is about half the radius of Earth, but accounts for only 17% of its volume and 33 %
of its mass. The core consists of two sections – a solid inner core and a liquid outer core –
both composed mostly of iron.
Q9. Explain in general why convection occurs? Describe the process of convection in
the context of Earth’s mantle.
A9. Convection occurs when heat is added to a fluid faster than it can move through the fluid.
Eventually a hot bubble of fluid begins to rise through its cooler surroundings and deposits its
heat at the top of the fluid. In Earth’s mantle, heat from the core is added to the base of the
mantle. Hot bubbles of rock rise to the top of the mantle, spread out horizontally, and then
fall back toward the center of Earth.
Q10. How do earthquakes help us learn about the interior of Earth? What is the basic
structure of Earth’s interior?
A10. Earthquakes cause vibrations in the Earth which can pass through the interior and be
detected on the opposite side of Earth. By studying exactly when different kinds of waves
arrive at different locations on Earth, we can learn about the material they passed through on
their way. For example, if there is a significant delay in the arrival of waves at one station
compared to a nearby station, it tells us that some material in the path of those waves slowed
down their passage but did not exist along the path the other waves took. The analysis of
many earthquakes allows us to gradually piece together a three-dimensional picture of Earth’s
interior. It consists of a two part iron core that is solid on the inside and liquid on the outside,
a large mantle of less dense rocky material surrounding the core, and a very thin crust on the
top.
Q11. What are the sources of energy which keep the centre of Earth warm?
A11. The interior of Earth is warm because of the energy trapped there during the formation
of the planet and because of the release of energy by radioactive materials which are
concentrated there.
Q12. Describe the structure of the core and mantle of Earth.
A12. The core of Earth is composed of nearly pure iron. It occupies about half the radius of
Earth, but only 17% of the volume. Because it is very dense, it accounts for about one third of
the mass of Earth. The inner portion of the core is solid while the outer portion of the core is
molten. The less dense, rocky mantle which surrounds the core accounts for most of the rest
of the material of Earth.
Q13. Why is it important to have a network of seismic stations to study the structure of
Earth’s interior?
A13. When an earthquake occurs, its waves travel through Earth in all directions. These
waves can be recorded at a network of seismic stations around Earth. The arrival times and
characteristics of the waves tell us how fast the waves traveled through different parts of the
interior. Without a network of stations we would not be able to deduce where the waves
originated or what path they followed through the interior of Earth.
Q14. The observed density of Earth is 5.5, while the uncompressed density is only 4.5.
What is an uncompressed density? Why is it less than the observed density?
A14. For any large object, gravity squeezes the material of the object to a smaller size than
the material would naturally have. This compression increases the observed density of the
object. If you have a good model of the interior of the object, it is possible to predict what the
density of the object would be without the natural compression due to gravity. This predicted
density is the uncompressed density. It is smaller than the observed density because the
uncompressed object (without the effects of gravity)is larger than the observed object.
Q15. What are the two types of seismic waves that travel through the Earth? How can
we use them to discover that the interior of Earth contains a liquid region?
A15. All waves, including seismic waves, are one of two types: transverse waves which
oscillate in a direction perpendicular to the direction of the wave motion and longitudinal
waves which oscillate in the same direction as the wave motion. For seismic waves,
transverse waves are referred to as S waves while longitudinal waves are called P waves. S
waves are not able to move through the body of a liquid. Since S waves are not observed to
penetrate the central core of Earth even though P waves do, we can conclude some part of the
core is liquid.
Q16. Describe the interior structure of the Earth. Compare this structure to that of
Mercury and the Moon.
A16. The interior of Earth consists of a solid inner core and a liquid outer core, both mostly iron,
surrounded by a mantle of silicate rocks. Mercury's core is a much larger fraction of the total planet
than is Earth's, but the Moon has little if any differentiated core. The cores of both Mercury and the
Moon are probably solid because the small bodies have lost their heat content.
Earth - Surface Questions
Q1. How is crust created and destroyed on Earth?
A1. New crust is created on Earth where two plates of old crust separate. Molten material
from the interior of Earth wells up to fill the vacancy left by the separating plates, cools, and
becomes new crust. When two plates of crust collide, one is pushed down into the interior of
Earth while the other is crumpled as it rides up on top of the other plate.
Q2. How do we believe the Hawaiian Islands were produced? In what way do they
provide evidence in support of the concept of plate tectonics?
A2. The Hawaiian Islands, and the undersea mountains to their west, represent a long chain
of volcanic mountains that we believe have been created by a single hot spot in the mantle of
Earth. Different mountains are produced as the crust of Earth moves over this stationary hot
spot. This idea is supported by the observation that each island in the chain is older than the
preceding one as you move from the eastern end of the chain toward the west.
Q3. Describe and explain the variety of geologic processes which occur when two plates
collide.
A3. When two plates collide, one is usually driven downward under the other, its material to
be re-absorbed into the mantle. The other plate is crumpled at the point of collision, forming
folded ranges of mountains along the boundary. Cracks and weak spots in this folded plate
allow hot material from inside Earth to rise to the surface in volcanoes. Motion inside Earth
as the plates slip past each other cause earthquakes.
Q4. How are mountain ranges formed on Earth? Why are they often associated with
earthquake zones?
A4. Mountain ranges are formed on Earth when two plates collide. One plate is subducted
while the other rides on top and is buckled to form mountains. As the two plates slide against
each other, stress and pressure are occasionally released by jerky motion of the plates -- what
we experience as an earthquake.
Q5. What are the two different (although related) processes that create mountains on
Earth? How do they build mountains? Long after the mountain is formed and its shape
altered by other processes, how could we tell which process was responsible for its
creation?
A5. Mountains can be created either by volcanic processes or by the collision of two tectonic
plates. When a volcano erupts, molten rock form the interior flows onto the surface. It may
build up in a pile forming a mountain. When plates collide, one side rides on top of the other.
The buckling of this plate creates tall ridges that become mountain ranges. We can tell the
difference between these types of mountains from the type of rock present. The cooled lava is
a distinctive type of rock, easily recognized long after the end of the volcano.
Q6. For each of the four processes that can shape the surface of a planet, describe how
it has affected the current appearance of Earth.
A6. Cratering has had only a minimal effect on Earth. Volcanism has covered only relatively
localized regions of Earth, often in association with tectonic activity. Plate tectonics through
both the creation of new crust at spreading centers and the destruction of crust during plate
collisions has had a profound effect on Earth’s surface. Erosion by air and water has also had
a profound effect on changing Earth’s surface appearance.
Q7. Why are the continental plates higher than those which make up the ocean floor?
A7. The continental plates are both thicker and less dense than the plates which make up
ocean floors. Since all the plates are floating on the top of the mantle, the less dense ones
float higher just like a light piece of wood floats higher in the water compared to a denser
object.
Q8. Why is there horizontal plate motion on the surface of Earth?
A8. Horizontal plate motion occurs because of the convective currents in the Earth’s mantle.
When hot rising mantle material reaches the top of the mantle, it spreads out and moves
horizontally before sinking back into the deep mantle. Pieces of the crust are carried along on
this horizontal motion of the mantle.
Q9. What is the evidence for horizontal plate motion on the surface of Earth?
A9. There are many lines of evidence which support the concept of horizontal plate motion.
Among them are the sequential ages of the islands in the Hawaiian island chain (and several
similar chains in the Pacific Ocean) as a result of a plate moving over a single hot spot
volcano and the similarities of rock strata and fossils between the tips of south America,
Africa, and Antarctica from the time that those regions were joined together in Pangaea.
Q10. How is Earth's crust being recycled? Discuss both the destruction and the
creation of the crust.
A10. Because of plate motion of the crust, crustal material is being subducted back down to
the mantle where plates collide and one plate is pushed down under the other. Where plates
are separating, mantle material rises to the surface to create new crust.
Earth - Atmosphere Questions
Q1. What are the primary chemical constituents of Earth’s atmosphere? What is
unusual about its composition, compared to other planets? Why is this unusual material
present?
A1. Earth’s atmosphere is made mostly of nitrogen and oxygen gases. The unusual part of
this is the presence of free oxygen in the atmosphere. Free oxygen is very reactive, and
should have disappeared long ago. Its presence suggests that some mechanism is
continuously creating more oxygen for the atmosphere. Of course, that process is life.
Photosynthesis by plants releases oxygen into the atmosphere.
Q2. Describe the thermal structure (changes in temperature with altitude) of Earth’s
atmosphere. What unusual feature does our atmosphere have in this regard, compared
to other planetary atmospheres?
A2. As you move upward from the surface of Earth, the temperature cools for several
kilometers before it reverse and begins to increase (in the stratosphere). This increase
ultimately reverses into another decline in temperature, which in turn changes into an
increase in temperature at high altitude. The net effect is that the temperature oscillates back
and forth over a fairly narrow range of values.
Q3. What are the astronomical causes of climate changes, such as the periodic
occurrence of ice ages?
A3. Periodic climate changes are caused by an effect known as the Milankovich cycle. The
combined effect of small changes in the eccentricity of Earth’s orbit around the sun, the
precession or wobbling of Earth’s axis of rotation, and small changes in the tilt of Earth’s
axis of rotation can cause the subtle changes in climate that lead to an ice age.
Q4. Earth’s atmosphere is composed mostly of nitrogen and oxygen. Why is there so
little carbon dioxide in our atmosphere, compared to other planets? Why is there so
much oxygen?
A4. Even though carbon dioxide is the most abundant gas released in volcanic processes, it is
not the most abundant gas in our atmosphere because it slowly dissolves in the liquid water
on Earth and ultimately forms carbonate rocks. The existence of free oxygen in our
atmosphere is purely a result of biological activity, specifically photosynthesis.
Q5. What happened to Earth’s original primary atmosphere? Where did the present
secondary atmosphere come from? What happened to all the carbon dioxide that
should be in our atmosphere?
A5. The original atmosphere of Earth was lost when the solar wind of the early sun stripped
away all the gas from around Earth. A secondary atmosphere was slowly released by
volcanoes as gases were released from the molten rock. Most of this gas is carbon dioxide,
but it is gradually dissolved in the oceans and precipitates out as carbonate rock instead of
building up in the atmosphere.
Q6. What three factors may cause Earth’s climate to go into an ice age?
A6. The 3 factors that must combine to send Earth’s climate into an ice age are: (1)
precession, or a change in the direction Earth’s axis points in space; (2) nutation, or a slight
wobble in the inclination of Earth’s axis of rotation with respect to the plane of its orbit; and
(3) a slight change in the eccentricity or shape of its orbit around the sun.
Q7. How did the earth's atmosphere become rich in molecular oxygen (O2)?
A7. Oxygen in Earth’s atmosphere is the result of biological activity on Earth. Plants release
oxygen as a by-product of photosynthesis. Since oxygen so readily reacts with many different
materials, it would quickly disappear if it were not constantly being replenished.
Q8. Describe the changes in the temperature in Earth’s atmosphere with increasing
altitude.
A8. As you rise from the surface of Earth the temperature decreases until it reaches a local
minimum at about 6 miles (10 km). The temperature then begins to increase up to about 35
miles (60 km), where it begins to decrease again. Finally at about 60 miles (100 km) it begins
to increase again for the rest of the way up until it finally merges with the interplanetary
medium.
Q9. Earth’s atmosphere is described as consisting of 4 layers. What are these 4 layers?
Describe a distinguishing characteristic for each layer.
A9. From the surface upward, the layers in Earth’s atmosphere are: troposphere
(distinguished by the water cycle, which controls our weather); stratosphere (distinguished by
the ozone layer); mesosphere (the top of the greenhouse where CO2 radiates energy to space);
and thermosphere (so thin it is easily heated by sunlight).
Q10. How does the thermal structure of Earth’s atmosphere compare to that of Venus’
atmosphere?
A10. Venus’ surface is much hotter than Earth’s. The atmospheres of both planets cool with
increasing altitude above the surface. Between sea level and about 75 km altitude, Earth’s
atmosphere cools and heats in several layers. In the same span, Venus’ atmosphere becomes
progressively cooler. At about 75 km they have both reached the same temperature.
Q11. What event do astronomers believe caused the extinction of the dinosaurs? What
evidence supports this idea?
A11. Astronomers believe that dust and steam raised into the atmosphere by the impact of a
large asteroid cooled the climate enough to cause the extinction of the dinosaurs and most
other species 65 million years ago. Several lines of evidence point in this direction. The most
persuasive is that the layer of rock deposited during that period, wherever it is found on Earth,
is rich in the element iridium. Iridium is extremely rare on Earth, but is much more abundant
in iron-rich meteorites.
Q12. What controls the temperature of Earth’s atmosphere? Describe both the physical
principle and the sources of energy.
A12. The temperature of any object is determined by the balance of input energy and emitted
energy. The temperature of the object rises or falls until the object is able to radiate as much
energy as it receives. The sources of energy for our atmosphere are sunlight and heat leaving
the surface of Earth from the interior. Each layer of the atmosphere heats up until it can
radiate as much energy as it receives form the sun and the layers below.
Q13. What are the major constituents of Earth’s atmosphere? In what way are they
unusual for an atmosphere?
A13. Our atmosphere is composed primarily of nitrogen (78%) and oxygen (22%). While
nitrogen is a common gas for an atmosphere, free oxygen molecules are quite unusual.
Oxygen is a very reactive substance and normally is quickly consumed in reactions with other
substances in the environment. Its presence in our atmosphere is the result of continual
production of more oxygen by plant life on Earth. Without that production mechanism, our
atmosphere would be quickly cleaned of all its free oxygen molecules.
Moon Questions
Q1. In considering the origin of the Moon, why is unlikely that Earth captured the Moon as it
passed us?
A1. The Moon is so large relative to the Earth that it would be very difficult for Earth's gravity to
capture it as it moves past us. This is analogous to a human being trying to catch a bowling ball.
Q2. How does the impact theory for the origin of the Moon explain general properties of the
Moon?
A2. If a large object impacted on Earth and splattered debris into Earth orbit to form the Moon,
most of the impactor's core would fall to the center of Earth, leaving the Moon with a very small
core. Most of the volatile metals (like gold and silver) would be vaporized, leaving the Moon
deficient in those elements.
Q3. How does the co-accretion (or double planet) theory suggest the Moon formed? What are the
major deficiencies of this theory for the origin of the Moon?
A3. The co-accretion theory suggests the Moon formed as a separate object in close association
with the forming Earth. That is, they formed at the same time and at the same distance from the sun.
This theory fails to explain why the Moon’s core is so much smaller than Earth’s. It also fails to
explain the more subtle differences in the chemical composition at the surface, for example, the
deficiency on the Moon of volatile metals like gold and lead.
Q4. Describe how the Moon formed according to the currently accepted theory. How does it
explain the properties of the Moon?
A4. The currently accepted theory for the formation of the Moon suggests that a Mars-sized body
collided with Earth. Some of the material of the colliding body and the surface of Earth were blasted
into orbit around Earth, and later collected together to form the Moon. Most of the core material of
the impactor would fall to the center of Earth, producing a very small core for the Moon. Most of the
volatile metals would be evaporated by the impact and lost.
Q5. Explain the concept of resonance
A5. Resonance occurs when a small force is applied at the same point in a cycle of motion. For
example, a child kicks in the air at the back of each swing on a playground swing. This small force
gradually increases the amplitude of the swing to a large effect.
Q6. Why do we believe that the core of the Moon is very small?
A6. The average density of the Moon is only barely larger than the average density of a typical rock.
Since material in a core should be significantly denser than rock, the core of the Moon cannot be
very large or the overall density would be larger.
Moon Surface Questions
Q1. How do we know that most of the Moon’s craters formed very early in its history?
A1. By comparing the density of craters (for example, number of craters per million square miles) on
different parts of the Moon’s surface with the ages of the rocks found in those regions, we can
determine that most of the craters formed very early in the history of the Moon.
Q2. What formed the dark regions on the face of the Moon? Other than being darker, how are
they different in appearance from the lighter regions?
A2. The dark regions on the Moon are lava flows from the early history of the Moon. They have far
fewer craters than other regions on the Moon, because the lava covered the earlier craters.
Q3. Describe the events which occur during the formation of a medium sized crater.
A3. The energy of motion of the impactor is released on contact in an explosion which destroys the
impactor and excavates a crater about 10 times bigger than the original object. The material
expelled from the crater is hurled upward, and then falls back to the surface to form a raised ejecta
blanket surrounding the new crater. Bedrock under and around the crater is shattered, and the rock
closest to the crater may even be melted by the energy released in the explosion.
Q4. How can we determine the relative age of different parts of the Moon from Earth-based
observations alone?
A4. The older a region is, the greater the number of craters it will have. Relative ages can be
determined by just counting the number of craters in a given area.
Q5. Why are all the maria on the side of the Moon that faces Earth?
A5. Maria formed as lava oozed to the surface through cracks formed in the crust of the Moon by
giant impacts. Because of Earth’s gravitational attraction, the crust is thinner on the side of the
Moon which faces Earth. With a thinner crust it was more likely that cracks produced during an
impact would reach down to the mantle where molten material was present.
Q6. Describe the processes which occur during the formation of a crater. Why do some craters
have flat-bottomed floors?
A6. When an object strikes the surface of a planet, it is moving so rapidly that the force of the
collision causes it to explode. The energy of this explosion blows a great deal of material up into the
sky — both the impactor and some of the surface material. Shock waves from the explosion also
shatter the rock under and around the new crater, and push the surrounding material into a raised
rim around the crater. The material blasted out of the crater falls back to the surface to form an
ejecta blanket around the crater. In some cases enough of this material falls back into the crater to
give it a flat bottom as it partially fills in the new crater.
Q7. Describe the properties of the material found on the surface of the Moon. What role have
meteorites played in determining these properties?
A7. The surface material on the Moon is composed of ordinary rock broken into a fine powder or
dust. The rocks on the surface of the Moon were ground down by the countless impacts of tiny
meteorites.
Q8. How is the age of a rock determined? Explain the reasoning behind your procedure.
A8. The age of a rock can be determined by measuring the amount of a radioactive substance in the
rock compared to the amount of its decay product in the rock. This ratio tells us how many half lives
(the length of time required for half of the radioactive substance to decay) have occurred since the
rock formed. Laboratory measurements of the half life then allow us to find the age of the rock in
years.
Q9. How is a central mountain peak formed in a crater?
A9. When a large meteorite impacts another body, the energy of motion of the impactor causes an
explosion which excavates a crater far larger than the original body. When the extra weight of the
material from the crater is removed, the remaining crust rebounds by springing upward. If this
rebound is frozen in place as the crust re-cools, the central peak will remain.
Q10. How do we know the maria are much younger than the highlands on the Moon?
A10. The relative age of any solid planetary surface can be determined from the relative density of
craters on the surface. The highlands have many more craters per unit area than do the maria. That
means that the surface is much older, which has allowed the number of craters to build up over a
longer time interval.
Q11. How can a resonance affect the motion of an object? Describe a resonance in the solar
system, and describe how the motion has been altered.
A11. A resonance occurs when a small force acted repeated at the same point in the cycle of motion
of an object. Even though the force is very small, its effect accumulates over time to produce a
noticeable change in the motion of the object. The force of Earth acting on the "heavy" side of the
Moon has slowly pulled that side to always face Earth as the Moon orbits Earth. The action of the
sun on Mercury has produced a similar result there, except that Mercury rotates three times for
every two orbits around the sun.
Q12. Why is the front side of the Moon different in appearance from the back side?
A12. When the Moon was forming, its interior was pulled slightly off center because of the strong
pull of gravity of Earth. As a result, the crust on the side which faces Earth is thinner than the crust
on the back side. That made it easier for giant impacts on our side of the Moon to crack the early
crust so that maria could be formed. There also appear to be more giant impacts on our side of the
Moon, perhaps as a result of the gravitational focusing of incoming material by Earth.
Q13. Describe the sequence of events when a large meteor collides with the Moon to create a
crater.
A13. Because of the violence of the impact, the meteor explodes on impact. This explosion blasts
out a crater about 10 times larger than the original meteor. This explosion also pushes surrounding
surface layers upward, producing a raised rim around the new crater. Powerful shockwaves travel
downward into the crust of the Moon, shattering the rocks under the impact. The heat generated by
the impact may melt the rocks left in the crater. Material blasted out of the crater falls back to the
surface to form an ejecta blanket around the new crater.
Q14. Why is the surface of the Moon covered by a thick layer of powdery dust?
A14. ince the Moon has no atmosphere, even the tiniest meteor strikes the surface at high speed.
These tiny specks of rock create tiny craters in the surface material of the Moon and chip off tiny
flakes of material. The accumulation of these flakes over billions of years is the regolith or dust
found on the surface of the Moon.
Mercury Questions
Q1. What spacecraft observations do we have of Mercury? What limitations were present for
those observations?
A1. The only spacecraft that has visited Mercury is Mariner 10, which made three fly-by passes of
the planet in the mid-1970's. Unfortunately, each fly-by observed the same side of the planet. In
addition, these observations are limited by the relatively crude technology available at that time.
Q2. Compare and contrast the interior structure of the Moon and Mercury.
A2. While both Mercury and the Moon are made of rocky surfaces and iron cores, the proportions
of the two parts are entirely different. Mercury has a very large iron core, while the Moon has a very
small one.
Q3. Why does Mercury not have an atmosphere when Titan, a similar sized body which orbits
Saturn, does have one?
A3. The presence of an atmosphere depends on the balance between gravity (which holds the
atmosphere) and temperature (rapid motion of atoms allows them to escape). Mercury is close to
the sun, so the temperature is quite high. Titan is far from the sun and has a very cold atmosphere.
This Titan is able to hold onto its atmosphere while Mercury cannot.
Q4. Why is Mercury both one of the hottest and one of the coldest planets in the solar system?
A4. Because Mercury does not have an atmosphere, the heat gained from the Sun during the
daytime is quickly lost at night. Since Mercury is very close to the Sun, the day side is heated to a
high temperature. At night the surface cools to very low temperatures.
Q5. How is the age of a rock determined? Explain the reasoning behind your procedure.
A5. The age of a rock can be determined by measuring the amount of a radioactive substance in the
rock compared to the amount of its decay product in the rock. This ratio tells us how many half lives
(the length of time required for half of the radioactive substance to decay) have occurred since the
rock formed. Laboratory measurements of the half life then allow us to find the age of the rock in
years.
Q6. Why do we think Mercury may have a molten core? Why does that surprise us?
A6. Mercury's weak magnetic field indicates that some part of its core remains molten, since
magnetism is generated by currents in a fluid iron core. That is surprising since we would have
expected a small planet like Mercury to have lost its internal heat and solidified throughout by now.
Q7. Describe how scarps were formed on Mercury. Are there any similar features elsewhere in
the solar system?
A7. Scarps formed on Mercury's surface when it cooled very quickly soon after formation. As it
cooled, the planet shrank slightly and the stress was relieved when the surface cracked in various
places. While the same process has not occurred elsewhere in the solar system, similar looking
cracks are often associated with earthquake faults on Earth. Valles Marineris on Mars is also a (much
larger) crack caused by expansion or up-thrusting of the surface.
Q8. How can a resonance affect the motion of an object? Describe a resonance in the solar system,
and describe how the motion has been altered.
A8. A resonance occurs when a small force acted repeated at the same point in the cycle of motion
of an object. Even though the force is very small, its effect accumulates over time to produce a
noticeable change in the motion of the object. The force of Earth acting on the "heavy" side of the
Moon has slowly pulled that side to always face Earth as the Moon orbits Earth. The action of the
sun on Mercury has produced a similar result there, except that Mercury rotates three times for
every two orbits around the sun.
Q9. Compare the interior structure of Mercury to that of Earth. What observation gives us this
information about Mercury? How is the difference in structure between Mercury and Earth
explained?
A9. Mercury has a much larger core, in proportion to its size, than does Earth. This conclusion is
reached by comparing the average density of Mercury to that of Earth, after compensating for the
compression caused by Earth’s greater gravity. The large core of Mercury occurred because it
formed closer to the sun, where the temperature was higher. Fewer rocky materials could condense
to help form a planet there, although iron could still easily condense under those conditions.
Q10. Why doesn't Mercury have an atmosphere?
A10. Mercury is so close to the sun that any atmosphere it had would be very hot, which means that
atoms in its atmosphere would be moving very rapidly. However, Mercury is also a rather small
planet, which means that its gravity is not very strong. It does not have an atmosphere because its
gravity is too weak to hold onto a high temperature atmosphere.
Q11. What is the origin of the jumbled hills on Mercury?
A11. The jumbled hills originated as a result of the giant impact which created the Caloris Basin on
Mercury. The seismic waves created by this impact were so powerful they traveled all the way
around Mercury. When they reconverged on the side of Mercury exactly opposite the impact site,
they we sufficiently strong to break the surface into large blocks. This area of disruption became the
jumbled hills.
Mars Question
Interior
Q1. Why do we suspect that Mars has a small core? Explain your reasoning.
A1. Since the average density of Mars is only slightly greater than the average density of a typical
rock (3.8 for Mars versus about 3 for a typical rock), it could not have a significant core of dense
material without raising its overall density to a higher value.
Atmosphere Questions
Q1. Mars is frequently engulfed in global dust storms. Why do these storms occur?
A1. In the winter on Mars, a significant fraction of the atmosphere condenses onto the polar ice cap.
The resulting low pressure over the pole causes a strong wind as air moves from the opposite pole
(where summer heat is causing that ice cap the evaporate). These winds moving from one polar cap
to the other cause global dust storms.
Q2. Why is Mars’ atmosphere so thin today, compared to Earth’s?
A2. A terrestrial planet’s atmosphere is released by volcanoes. In the case of Mars, its gravity is not
strong enough to permanently retain an atmosphere. When the volcanoes became extinct, the
atmosphere continues to leak away but no new gases were released to replace those that were lost.
Q3. Why are there large seasonal changes in the atmospheric pressure on Mars?
A3. Mars is far enough from the Sun and cold enough for CO2 to condense into ice during the
Martian winter. So much condenses that the atmospheric pressure changes appreciably.
Correspondingly, when the ice cap sublimes in the summer the atmospheric pressure increases
dramatically.
Q4. Describe the evolution of Mars atmosphere over time.
A4. Mars once had a dense, warm, humid atmosphere. While volcanoes were active, new gasses
were released to replace those lost from the atmosphere due to the weak gravity or which
condensed on the surface. When the volcanoes died, that replacement mechanism was lost. Water
was gradually lost by the process outlined in the previous question and as permafrost in the soil. The
weak gravity of the planet was unable to hold onto the atmosphere, which slowly evaporated from
the planet.
Q5. Why is Mars’ atmosphere so very thin today? What evidence from surface features is there
that Mars once had a dense atmosphere?
A5. Mars is a small planet with weak gravity. Its atmosphere slowly leaks away to space. Now that
all of its volcanoes are extinct, there is no source of new gas to resupply what has left. Dry river
channels visible on the surface indicate that Mars’ climate was once warmer than today. If its
atmosphere had always been as thin as we see today, liquid water could not have flowed on the
surface.
Mars - Surface Questions
Q1. Part of Mars’ surface is relatively young, while the other part is much older. How do we know?
Why or how did this occur?
A1. We can tell that the northern half of Mars is relatively young because there are very few impact
craters there, compared to the southern half of the planet. The surface of the northern hemisphere
of Mars has been covered relatively recently by volcanic outflows from the many volcanoes found
there.
Q2. Mars has many volcanoes? How do we know they are all extinct? Why are they so much
larger than those found on Earth?
A2. We can tell that all the volcanoes of Mars are extinct because all of them have impact craters on
their flanks. If there had been recently active, at least a few of them would not have impact craters
on them. They become large, compared to Earth’s volcanoes, because there is no plate tectonics on
Mars. A volcano on Earth is constantly "cut off" as the crust moves past a single hot spot in the
mantle. Instead of single large volcano, plate tectonics on Earth produces a chain of smaller
volcanoes.
Q3. What evidence is there that liquid water once existed on the surface of Mars? What had to be
different about Mars in the past for liquid water to exist on the surface?
A3. Dry river channels on Mars’ surface suggest that liquid water once existed there. However, the
present atmosphere of Mars would not allow liquid water to exist on the surface. The atmosphere is
so thin that water would evaporate very quickly from the surface. The existence of dry river channels
on Mars suggests that the atmosphere must have been much denser in the past.
Q4. What evidence is there at erosion has occurred on Mars? Why would we not expect it to
continue today?
A4. The most obvious evidence of erosion on Mars are the dry river beds at various places. It is also
possible that the dust storms may produce a very mild erosion. The liquid erosion is not possible
today on Mars because the atmosphere is too thin to allow liquid water to exist on the surface.
Q5. What evidence is there that there has been no plate tectonic activity on Mars? What caused
the huge canyon system on Mars?
A5. The volcanoes on Mars are very large, compared to a typical terrestrial volcano, which indicates
that the crust did not move during the formation of the large volcanoes. However, there is evidence
of vertical motion or swelling associated with volcanoes. As one region expanded, adjacent regions
cracked from the stress created by the uplift. These cracks are visible as a huge canyon system on
Mars that stretches for thousands of miles.
Q6. In what two ways are craters on Mars different from those on the Moon?
A6. The ejecta of some craters on Mars shows evidence of fluid flow instead of explosive ejection.
This suggests that permafrost in surface layers on Mars melted at impact and the water carried loose
particles away from the crater. Some craters on Mars also show the effects of erosion, as blowing
dust fills them in. Neither of these phenomena are seen on the Moon.
Q7. Why are volcanoes on Mars so much larger than any on Earth?
A7. Since there is no plate tectonics on Mars, a volcanic vent remains in one location and builds a
single volcanic mountain. On Earth, several mountains can be made by a single vent as crustal plates
move over it. This plate motion prevents any one volcanic mountain from getting very large on Earth.
Q8. Describe the composition of Mars’ ice caps. What effect does their formation each winter
have on the rest of Mars?
A8. Mars’ ice caps are made of a combination of water ice and carbon dioxide ice (dry ice). So much
dry ice condenses each winter to reduce the air pressure over the pole by as much as 20 %. The
pressure difference between the poles (one freezing while the other "melts") causes a condensation
flow of air from one pole to the other.
Q9. Describe the two types of dry river channels on Mars. How do we believe they occurred?
A9. Some river channels on Mars show many tributaries in a highly developed system of channels,
reminiscent of young river systems on Earth. Other channels are huge outflow channels which seem
to have been carved by single, massive floods. In both cases, we believe that the flow of water
occurred when permafrost was melted (perhaps by impacts or volcanism) and reached the surface in
springs.
Q10. Give a general description of the surface appearance of Mars.
A10. Mars is heavily cratered in the southern hemisphere and heavily volcanic in the northern
hemisphere. One major volcanic highland is known as the Thasis bulge, which cause a long fracture
canyon on its edge. Dry river channels are found, along with extensive evidence of dust erosion.
Polar caps form at each pole during the winter.
Jupiter Questions
Q1. Why did the Galileo spacecraft take so much longer to get to Jupiter than did the
Voyager spacecrafts?
A1. After the Challenger accident, it was decided to launch Galileo with a small conventional rocket
instead of from the shuttle. As a result, it had to circle the sun three times to make close passes by
Venus and Earth in order to gain enough speed to reach the outer solar system. Voyager, on the
other hand, was launched with the much larger Saturn V rocket on a direct path to Jupiter.
Q2. Describe the two components of the Galileo spacecraft. What were they designed to
observe?
A2. The Galileo spacecraft consists of both an atmospheric probe and an orbiter. The probe plunged
straight into the atmosphere of Jupiter to observe the properties of the atmosphere (temperature,
composition, cloud particles, etc.). The orbiter continues to orbit Jupiter, taking observations of the
planet and its moons.
Q3. Describe the spacecraft exploration of Jupiter. What missions have visited? What
did they do there, that is, what kind of mission were they?
A3. Jupiter has been visited by 5 spacecraft. Pioneer 10 and 11 flew past Jupiter in the early
1970's, Voyager I and II flew past in the late 1970's, and Galileo arrived in the 1990's. Galileo
consists of both an atmospheric probe and an orbiter that continues to obtain observations of
Jupiter today.
Jupiter - Interior Questions
Q1. The interior of Jupiter is divided into three layers. Describe the physical properties
of the material in each layer. In this context, what is meant by the term metallic?
A1. From the outside, the layers are composed primarily of liquid molecular hydrogen,
liquid atomic metallic hydrogen, and liquid rock and ice. Metallic hydrogen occurs under
very high pressure, when the hydrogen atoms are able to conduct electricity by sharing their
electrons.
Q2. Why is the core of Jupiter a mixture of rock and ice while Earth is just rock? Why
is Jupiter's core so much bigger than Earth?
A2. The type of solid material available to form a planet depends upon the temperature at
that location in the cloud. The temperature of the cloud diminishes with increasing distance
from the sun. At Jupiter's distance from the sun, the temperature was low enough that the ices
could solidify. Earth is too close to the sun for that to have happened, so Earth is all rocky
material while Jupiter's core contains both rock and ice. When ice solidifies, there is a great
deal more solid material available to form a planet, since the ices are much more common
than rocky material. Hence, Jupiter's core is much larger than Earth.
Q3. How do the gas giant planets radiate more energy than they receive from the sun?
A3. Two mechanism of heat generation are present. The gas giants (except Uranus) are still
radiating some heat as they continue to slowly collapse. In addition, helium condenses into a
liquid and falls toward the interior somewhere in the molecular hydrogen layers. This falling
rain gains enough energy as it falls to significantly heat up the gas it falls through. This heat
eventually escapes from the surface of the planet, making it warmer than it would otherwise
be.
Q4. Describe the structure of the interior of Jupiter? What unusual properties does the
matter inside Jupiter have?
A4. Jupiter has a core of rock surrounded by ice. Outside the core is a large region of atomic
hydrogen which has the unusual property (for hydrogen) of being a good conductor of
electricity. For this region it is referred to as metallic hydrogen. Outside this region is a
region of more normal molecular hydrogen.
Q5. What two types of observations give us information about the interior structure of
Jupiter?
A5. We can learn about the interior of Jupiter both from the average density of the planet and
from measurements of the flattening of the planet, which measures the interior response to
Jupiter’s rotation.
Q6. Compare the core of Jupiter to Earth. Why is Jupiter’s core such a small
percentage of the overall planet?
A6. Jupiter’s core is about the same size as Earth’s core, but contains between 3 and 30 times
as much material. Obviously, the density of Jupiter’s core is quite a bit higher than ours,
because of the greater compression of a more massive planet. Even at the high end of the
range of possible masses, the core of Jupiter is still only about 10% of the mass of the whole
planet. The additional material was attracted to Jupiter during its formation, because its core
had grown large enough for its gravity to be strong enough to attract gas from the
surrounding cloud. Earth never made it that far.
Q7. What is meant by the term "metallic hydrogen?" What observable consequences
does the layer of metallic hydrogen have for Jupiter?
A7. A metal is defined as a material which conducts electricity effectively. Hydrogen
becomes electrically conductive under very high pressures. The layer of metallic hydrogen
inside Jupiter creates the very strong magnetic field observed around Jupiter
Q8. What causes convection? What effect does convection have on the surface
appearance of Jupiter?
A8. Convection occurs when heat is added too rapidly into a material. Convective motion
occurs to transport the excess heat away from the source. Convection in the outer layers of
Jupiter brings hot material toward the surface. Molecules in this hotter material have different
colors than the cooler layers that are descending. These color differences cause the banding
seen on the surface of Jupiter.
Jupiter - Io Questions
Q1. What is the source of heat that produces the volcanic activity on Io?
A1. Io’s orbit around Jupiter is locked in a 2:1 resonance with Europa’s orbit. Every other orbit they
line up at the same place in Io’s orbit. This causes Io’s orbit to become slightly elongated at that point.
The changing tides on Io caused by its changing distance from Jupiter cause the moon to slightly
expand and contract internally. The friction created by this internal motion heats the interior, which is
released by volcanic activity.
Q2. Why is Io the most volcanic object in the solar system? Explain the mechanism
which causes the volcanism.
A2. Io’s orbit is in a two-to-one resonance with Europa’s orbit around Jupiter. This resonance has
pulled Io’s orbit into a slightly elongated shape. When Io is closer to Jupiter it’s shape is distorted by
the tidal forces from Jupiter; when it is further away it’s shape relaxes somewhat. This internal flexing
of Io causes friction, which leads to the build up of heat, which in turn is released by numerous
volcanoes.
Q3. Why is Jupiter’s moon Io volcanically active?
A3. Its orbit has a 2 to 1 resonance with Europa’s orbit just outside its orbit. The repeated tugs of
Europa at a fixed point in Io’s orbit gradually have pulled its orbit into an elongated shape. As the
distance between Jupiter and Io changes during Io’s orbit, the pull of gravity it feels from Jupiter also
changes, causing the shape of the moon to be alternately squeezed and expanded. The internal friction
created by this changing shape generates internal heat, which powers the volcanic activity on the
moon.
Q4. What two lines of evidence tell us that Io is an extremely volcanically active moon?
A4. We know that Io is extremely volcanically active because we have seen numerous plumes from
currently erupting volcanoes and we have not seen any impact craters anywhere on the surface.
Jupiter - Europa Questions
Q1. What evidence is there that Europa has a liquid water ocean?
A1. The most compelling evidence for a liquid ocean on Europa is the appearance of the surface. It is
extremely smooth, as if it is unable to support the weight of tall features. It has many criss-crossing
ridges and grooves from the constant shifting of the surface. Pieces that look like icebergs are also
found at several locations on the surface.
Q2. How do we know that Europa is composed primarily of rock even though its
surface is entirely ice-covered?
A2. Evan though its surface is ice-covered, Europa’s average density is 3.0 — which is about the
density of a typical rock and too high for ice.
Jupiter - Ganymede Questions
Q1. Describe the surface appearance of Ganymede, both globally and locally (that is,
both on a large scale and on a small scale). Has the surface been recently active? How
do we know?
A1. On a large scale, Ganymede appears to be rather blotchy, with large vaguely round dark section
interspersed with lighter terrain. On a small scale, there are numerous sections of parallel grooves
perhaps caused by motion of the surface as the icy surface cooled. Any activity that produced these
features had to occur in the early history of the solar system because all parts of the surface of
Ganymede are extensively cratered.
Jupiter's Moons Questions
Q1. In what way do the four large moons of Jupiter represent a miniature solar system?
A1. The two outer moons are larger and composed mostly of ice, while the two inner ones are
somewhat smaller and composed mostly of rock. This pattern is similar to the pattern of terrestrial and
giant planets in the solar system. The large moons around Jupiter formed from a gas disk orbiting the
planet in much the same way that the planets formed from a gas disk orbiting the sun.
Q2. Describe (briefly) the unique or distinguishing characteristics of each of the seven