Lecture 4 The Lunar Environment. Moon Statistics Mass (Earth = 1)1.2298e-02 Equatorial radius (km)1,737.4 Mean density (gm/cm^3)3.34 Mean distance from.

Lecture 4Lecture 4The Lunar EnvironmentThe Lunar Environment

Moon StatisticsMoon StatisticsMass (Earth = 1) 1.2298e-02

Equatorial radius (km) 1,737.4

Mean density (gm/cm^3) 3.34

Mean distance from Earth (km) 384,400

Rotational period (days) = Orbital Period 27.32166

Average length of lunar day (days) 29.53059

Orbital eccentricity 0.0549

Tilt of axis (degrees) 1.5424

Orbital inclination (degrees) 5.1454

Equatorial surface gravity (m/sec^2) 1.62

Equatorial escape velocity (km/sec) 2.38

Visual geometric albedo 0.12

Mean surface temperature (day) 107°C

Mean surface temperature (night) -153°C

Maximum surface temperature 123°C

Minimum surface temperature -233°C

The Moon is the only natural satellite of Earth Orbit radius: 384,400 km, Diameter: 3476 km, Mass:

7.35e22 kg

Called Luna by the Romans, Selene and Artemis by the Greeks

Known since prehistoric times, it is the second brightest object

in the sky after the Sun.

As the Moon orbits around the Earth once per month, the angle between the Earth, the Moon and the Sun changes; we see this

as the cycle of the Moon's phases.

The time between successive new moons is 29.5 days (709 hours), slightly different from the Moon's orbital period (measured against the stars) since the Earth moves a significant distance in its orbit around the Sun in that time.

Lunar ExplorationLunar Exploration

  The Moon was first visited by the Soviet spacecraft Luna 2 in 1959.

It is the only extraterrestrial body to have been visited by humans.

The first landing was on July 20, 1969; the last was in December 1972.

The Moon is also the only body from which samples have been returned to Earth.

In the summer of 1994, the Moon was mapped by the spacecraft Clementine and again in 1999 by Lunar Prospector.

TidesTides

The Moon's gravitational attraction is The Moon's gravitational attraction is stronger on the side of the stronger on the side of the Earth nearest to the Moon and weaker Earth nearest to the Moon and weaker on the opposite side on the opposite side two bulges, two bulges, one in the direction of the Moon and one in the direction of the Moon and one directly opposite. one directly opposite. Earth's rotation plus friction carries Earth's rotation plus friction carries the Earth's bulges slightly ahead of the Earth's bulges slightly ahead of the point directly beneath the Moon.the point directly beneath the Moon. Therefore the force between the Earth Therefore the force between the Earth and the Moon is not exactly along the and the Moon is not exactly along the line between their centers producing a line between their centers producing a torquetorque on the Earth and an on the Earth and an accelerating force on the Moon. accelerating force on the Moon. This slows down the Earth's rotation This slows down the Earth's rotation by 1.5 milliseconds/century and raises by 1.5 milliseconds/century and raises the Moon into a higher orbit by about the Moon into a higher orbit by about 3.8 centimeters per year. 3.8 centimeters per year.

Ice, Crust and TerrainIce, Crust and TerrainEvidence from Clementine suggested that there may be water ice in some deep craters near the Moon's south pole which are permanently shaded. This has now been reinforced by data from Lunar Prospector. There is apparently ice at the north pole as well. The Moon's crust averages 68 km thick and varies from essentially 0 under Mare Crisium to 107 km north of the crater Korolev on the lunar far side. Below the crust is a mantle and probably a small core (roughly 340 km radius and 2% of the Moon's mass). Unlike the Earth, however, the Moon's interior is no longer active.          There are two primary types of terrain on the Moon: the heavily cratered and very old highlands and the relatively smooth and younger maria. The maria (which comprise about 16% of the Moon's surface) are huge impact craters that were later flooded by molten lava. Most of the surface is covered with regolith, a mixture of fine dust and rocky debris produced by meteor impacts. The maria are concentrated on the near side, where the crust is thinner.

Phenomenology of Asteroid Phenomenology of Asteroid Impacts.Impacts.

Asteroid fragment

rushes toward the

surface at 10 to 20 km per

second. Much faster

than the speed of sound in

Lunar rock

Material near the initial contact region is compressed at the rate of 10 -20 km/s. Pressure cannot be relieved because the asteroid speed is >> than the

speed of sound. By elementary thermodynamics, this compression, in addition to the rapid dissipation of the kinetic energy of the projectile,

results in rapid heating to stellar-level temperatures in a small zone around the impact point. A fireball contained in an intense shock wave forms.

Phenomenology of Asteroid Phenomenology of Asteroid Impacts.Impacts.

The fireball now explodes, with shock front becoming spherical as it expands. Asteroid body is shattered/vaporized. Sheets of rock are peeled back like the petals of a flower.

Masses of debris are ejected from the crater floor high above the surface and eventually fall back. Initial cavity is almost hemispherical. Rock layers bend upward and backward and when settled, form the raised rim around the crater

A Taxonomy of Impact CratersA Taxonomy of Impact Craters

3. >1,000km-scale multi-ring basin

Called an impact basin

1. Simple bowl craters, <10km

2. >20km, with central mountains

A Taxonomy of Impact CratersA Taxonomy of Impact Craters

3. >1,000km-scale multi-ring basin

1. Simple bowl craters, <10km

2. >20km, with central mountains

Examples of Crater TypesExamples of Crater Types

Lunar uplands.

Craters and RocksCraters and Rocks

Most of the craters on the near side are named for famous figures in the history of science such as Tycho, Copernicus, and Ptolemaeus Features on the far side have more modern references such as Apollo, Gagarin and Korolev In addition to the familiar features on the near side, the Moon also has the huge craters South Pole-Aitken on the far side which is 2250 km in diameter and 12 km deep making it the largest impact basin in the solar system and Orientale on the western limb (as seen from Earth) which is a splendid example of a multi-ring crater. A total of 382 kg of rock samples were returned to the Earth by the Apollo and Luna programs. These provide most of our detailed knowledge of the Moon. They are particularly valuable in that they can be dated. Even today, more than 30 years after the last Moon landing, scientists still study these precious samples. Most rocks on the surface of the Moon seem to be between 4.6 and 3 billion years old. This is a fortuitous match with the oldest terrestrial rocks which are rarely more than 3 billion years old. Thus the Moon provides evidence about the early history of the Solar System not available on the Earth.

Near SideNear Side

90 Degrees East90 Degrees East

Far SideFar Side

90 Degrees West90 Degrees West

Near SideNear Side

Origin of the MoonOrigin of the Moon

Prior to the study of the Apollo samples, there was no consensus about the origin of the Moon. There were three principal theories:

1. Co-accretion: The Moon and the Earth formed at the same time from the Solar Nebula

2. Fission: The Moon split off of the Earth3. Capture: The Moon formed elsewhere and was subsequently captured by

the Earth.

None of these work very well. But the new and detailed information from the Moon rocks led to the impact theory: that the Earth collided with a very large

object (as big as Mars or more) and that the Moon formed from the ejected material. The impact theory is now widely accepted.

Origin of the Moon – Giant Impact SimulationOrigin of the Moon – Giant Impact Simulation

Orpheus hurtles toward Terra to deal a glancing blow.Orpheus is half the mass of Terra.

First Contact: Orpheus is already greatly distorted by tidal forces. The off-center impact transfers a lot of angular momentum to Terra

TerraOrpheus

Origin of the Moon – Giant Impact SimulationOrigin of the Moon – Giant Impact SimulationOrpheus-Terra turns about; is stretched into an elongated streamer, and is mostly absorbed in the larger body to form Earth.

After half a rotation, only a thin “tail” is left. Most of this falls into a rapidly rotating Earth (< 18 hr day)

Origin of the Moon – Giant Impact SimulationOrigin of the Moon – Giant Impact Simulation

Many days later, we have three temporary moons and the permanent moon in an eccentric orbit.

Moon

Temporary moons

Later still, the Earth absorbs all the temporary moons and settles down. Note: the permanent Moon is 1% Earth’s mass bulks huge in the sky.

Origin of the Moon – Giant Impact SimulationOrigin of the Moon – Giant Impact Simulation

Over a longer time, strong forces due to Earth’s tides work to circularize the Moon’s orbit.

Beyond this point, the same tidal effects increase the orbit radius and lengthen the Earth’s period of rotation. This will continue until Earth’s day = the orbital period of the Moon.

At this stage, the Moon looms over 30 degrees of arc in the skies of Earth!!

Origin of the Moon – Giant Impact TheoryOrigin of the Moon – Giant Impact Theory

Studies of lunar rocks show that the moon originally had a molten surface. As the magma ocean cooled, intense volcanism continued for about 900 million years.

The Moon Today. This late afternoon scene on the moon typifies the moon as it has been for about 3 billion years. Volcanism has ended. Meteorite impacts are rare. The quiet landscape awaits the return of human explorers