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