The Moon
Tidal CouplingSurface FeaturesImpact CrateringMoon RocksHistory and Origin of the Moon
Earth MoonSemi-major Axis 1 A.U. 384 x 103 kmInclination 0°Orbital period 1.000 tropical year 27.32 daysOrbital eccentricity 0.017Rotational period 23 h 56 min 4.1 s 27.32 daysTilt 23° 27’ 5.2°Radius 6378 km 1738 kmMass 5.97 x1024 kg 7.35 x 1022 kgBulk density 5.52 g/cm3 3.34 g/cm3Atmosphere N2, O2 trace Ne, HeAlbedo 0.40 0.066Surface temperature 250-300 K 120-390 KEscape speed 11.2 km/s 2.4 km/sMagnetic moment (equator) 8 x 1010 G.km3
Highlights of Lunar Exploration
Soviets had first contact with the Moon:
First spacecraft to fly past the Moon: January 1959
First spacecraft to (crash) land on the Moon: September 1959
First pictures of far side of the Moon: October 1959
The United States is (so far) the only country to send people to the Moon:
First person on the Moon: July 1969
Last person on the Moon: December 1972
Moon’s density is relatively low, and it has no magnetic field ⇒ cannot have sizable iron/nickel core
Crust is much thicker than Earth’s
Lunar Interior
Tidal Coupling
From the Earth, we always see the same side of the Moon.
The Moon rotates around its axis in the same time that it takes to orbit around the Earth.
Tidal coupling: Tidal forces have
slowed rotation down to same period as orbital period
This is an example of 1:1 spin-orbit resonance.
Tidal CouplingWhen the Moon formed, the Earth’s gravity, caused the Moon to form with its center of gravity closer to the Earth than its center of figure. This gravitational imbalance enabled the tidal force to slow the Moon into a 1:1 spin-orbit resonance.This diagram greatly exaggerates the difference. The center of mass and center of figure are separated by only ~7 km (0.2 %).
Acceleration of the Moon’s Orbital Motion
The Earth’s tidal bulges are slightly tilted in the direction of Earth’s rotation
Gravitational force pulls the Moon slightly forward along
its orbit
The energy loss from friction coupled with conservation of angular momentum, causes the Moon to drift slowly away from the Earth
Lunar Surface Features: Near SideTwo dramatically different kinds of terrain:• Highlands:
• Mountainous terrain
• Scarred by craters• Maria (pl. of mare):
• ~3 km lower than highlands
• Smooth surfaces• Basins flooded by
lava flows
The far side of the Moon has many craters but virtually no maria
Lunar Surface Features: Far Side
Impact CrateringImpact craters on the Moon can be seen easily even with small
telescopes.
Ejecta from the impacts can be seen as bright rays originating
from young craters
Craters• Craters are typically about 10 times as wide
as the meteoroid creating them, and twice as deep.
• Crater classes: (on the Moon)• Bowl craters: D < 15 km• Complex craters: 15 km < D < 300 km• Basins: D > 300 km
• Rock is pulverized to a much greater depth forming the lunar regolith or soil.
• Most lunar craters are ≥ 3.9 x 109 years old; much lower impact rates since then.
CratersBowl craters have a simple shape. Copernicus is an example of a complex crater. Notice the terraced walls caused by slumping and the central peak caused by a rebounding shock wave.
Central peak Terraced walls
Bowl craters
Overlapping bowl craters
Craters
Basins are characterized by 3 rings. One is the crater rim. The others are caused by rebounding shock waves like the central peaks of complex craters. Mare Orientale shows basin structure because it is only partially flooded with lava. Near-side basins are all flooded with lava out to the third ring.
Craters
The smallest craters on the Moon, such as this one on a small glass sphere, must be viewed using scanning electron microscopes
Small meteoroids are much more numerous than large ones and they are the ones that produce the regolith
History of Impact Cratering
Rate of impacts by interplanetary bombardment decreased rapidly after the formation of the solar system.
Most craters seen on the Moon’s surface were formed within the first ~½ billion years.
Cratering as a Geological ProcessIt is the dominant geological process on many bodies.Even though it is a minor process on Earth, there are ~180 visible craters on Earth. This is Barringer crater in Arizona which formed ~50,000 years ago. It is ~1 km in diameter.
Craters on EarthA comet nucleus impact produced the Chicxulub crater
~65 million years ago. It may have caused major climate change, leading to the extinction of many species,
including dinosaurs.
Gravity map shows the extent of the crater hidden below limestone
deposited since the impact.
300 km
CratersMeteoroids have produced most of the visible craters in the solar system.
The Earth has about 100 craters more than 0.1 km in diameter; erosion has made most of them hard to discern. One of the largest is Manicouagane in Quebec.
More than 3 billion years ago, the Moon was volcanically active; Hadley rille, as well as others, was formed then
Lunar Volcanism
Moon RocksAll lunar rocks brought back to Earth are igneous (= solidified lava). No sedimentary rocks ⇒ No sign that liquid water was ever present on the Moon.
Different types of lunar rocks:
Vesicular (= containing holes from
gas bubbles in the lava) basalts, typical
of dark rocks found in maria
Breccias (= fragments of different types of rock
cemented together), also containing anorthosites
(= bright, low-density rocks typical of the
highlands)
Older rocks become pitted
with small micrometeorite
craters
The History of the Moon
Alan Shepard (Apollo 14) analyzing a lunar rock, probably
ejected from a distant crater.
1. The Moon is small; low mass ⇒ rapid cooling; small escape velocity ⇒ no atmosphere ⇒unprotected against meteoroid impacts.
2. The Moon must have formed in a partially molten state (“sea of lava”);
3. Partial differentiation
4. No magnetic field ⇒ small core with little metallic iron.
5. Surface solidified ~4.6 – 4.1 billion years ago.
6. Heavy meteorite bombardment for the next ~1/2 billion years.
Formation of Maria
Impacts of heavy meteorites broke the crust and produced large basins that were flooded with lava. Flooding ended about 3.2 billion years ago.
Origin of Mare Imbrium
Terrain opposite to Mare Imbrium is jumbled by seismic
waves from the impact.
Its history is typical of the history of flooded basins on the near side
The Origin of the MoonEarly (unsuccessful) hypotheses:
Fission hypothesis:Break-up of Earth during early period of fast rotationProblems: No evidence for fast rotation; the Moon’s orbit is not in the equatorial plane
Condensation hypothesis:Condensation at time of formation of Earth
Problems: Different chemical compositions; the Moon is low in volatiles and iron
Capture hypothesis:Capture of the Moon that formed elsewhere in the solar systemProblem: Requires succession of very unlikely events
Current Theory of the Formation of the Moon
The Large-Impact HypothesisThe impact heated material enough to melt it
⇒ consistent with a “sea of magma”
The collision was not head-on⇒ large angular momentum of Earth-Moon system
The collision occurred after differentiation of Earth’s interior; it did not penetrate to the core
⇒ Moon will not have much iron