The Moon What are the characteristics of the

The Moon • What are the characteristics of the Moon, and how do they compare to Earth's? • What can the surface tell us about its history and interior? • How was the Moon formed? 1

Basic data Average distance from Earth = 384, 400 km Mass = 7. 3 x 1022 kg = 1. 2% Mearth Diameter = 3476 km = ¼ Earth’s Density = 3344 kg/m 3 Vesc = 2. 4 km/s No atmosphere. => Cannot be liquids on surface. "Camembert? " 2

Libration - 59% of surface visible • Elliptical orbit means the moon appears to wobble: spin rate constant but not orbital speed. Viewing angle doesn’t change in a constant way (angular size varies too). • Appears to nod up and down because the spin axis is not perpendicular to the orbital plane 3

How to Measure the Moon’s Mass From Newton’s form of Kepler’s 3 rd Law: a is mean Earth-Moon separation over orbit. P is sidereal month, m 1 is Earth mass. Solve for m 2. Or use same equation with artificial satellite orbiting the Moon with known period. Or, use fact that Moon and Earth orbit around center of mass (CM) of Moon/Earth system. (recall: m 1 r 1 = m 2 r 2, where r 1 and r 2 are distances to center of mass at any point in orbit) 4

Earth wobbles around Earth-Moon center of mass – Earth does not exactly follow elliptical orbit around Sun. But center of mass does. This wobble has period of 1 sidereal month (27. 3 days) and can be measured with precise positions of stars and planets. Can determine 5 center of wobbling motion, i. e. center of mass.

Result: average distance from Earth’s center to center of mass is r. Earth = 4730 km in the direction of the Moon, i. e. still within the Earth. So average distance from Moon to center of mass is r. Moon = 384, 400 km – 4730 km = 379, 670 km. Hence: 6

The surface of the Moon • No atmosphere => surface reflects geological history well • Near side has – lunar highlands (bright regions, mountains but no chains) – maria (dark regions) 2 -5 km lower than highlands – Craters (10 times more per unit area than highlands ) 7

Cratering rates Small meteroids common, large ones rare. So same true for craters: Crater size Occurrence 10 km every 10 million years 1 m every month Crater diameter about 10 x impactor diameter. Recall, if no water or wind erosion, the number of craters in an area tells you the age of the surface. Also no atmosphere for small meteoroids to burn up in. 8

highlands Are highlands or maria older? • More heavily cratered surface is older – so highlands older than maria Comparison of Earth and Moon surfaces • Much higher crater density in general means surface of the Moon is older than that of the Earth, no geological activity for a long time (plate tectonics, wind, water erosion) maria 9

Also on the surface: regolith • Regolith is a powdery soil with scattered rocks – 2 -20 m thick layer • Made from debris from impacts that create lunar craters • Impact speeds several km/s • Each crater is surrounded by an ejecta blanket 10

Evidence for past volcanism Mare Imbrium – the largest of the maria facing Earth, about 1100 km across. Maria are roughly circular – probably large early impacts. Also smooth and dark, like volcanic flows. 11

Lava “river” in a mare indicating old volcanic flows. 12 - Linear chains of craters (but a few are due to impacts), probably marks ancient fault, collapsed lava domes And more direct evidence from…

Moon rocks • • 382 kg of rock samples from 9 missions: 6 Apollo landings (Apollo 11 -17, 1969 -72) 12 astronauts visited maria and highlands 3 Soviet Luna robotic missions (1970, 72 and 76), returned rock samples by capsule 13

All returned samples are igneous surface was molten. Maria – basalts Similar to rocks formed by lava on Earth. Highlands – anorthosites Less dense than basalt. – breccias Different rocks fused together in meteoritic impacts (also in marias) Rocks also have no trapped CO 2, H 2 O vapor, unlike Earth rocks. Go see a lunar rock at the Natural History Museum! 14

Age of Lunar material • Radioactive dating can give more accurate ages than crater rates. • Samples from Mare 3. 1 – 3. 8 billion years old • Highland rocks typically 4 - 4. 3 billion years, oldest rock dated to 4. 46 billion years Oldest material on the Moon is almost as old as we believe the Solar System to be. 15

Moon's interior • Astronauts left seismometers – moonquakes (due to varying tidal force from Earth) weak but useful • Much thicker lithosphere than Earth, 800 km • Relatively small iron-rich core, 700 km diameter. Only 2 -3% of mass (32% on Earth). Moon differentiated, like Earth. Must have been molten. • Thicker crust than Earth, 60 -100 km – thicker on far side • Largely dead geologically. Why? 16

No convection moving plates around Moon is solid, because it has cooled off Cooling time heat content /rate of cooling volume /area R 3/R 2 R The smaller the world, the faster it loses internal heat, and the less geologic activity it will show on its surface. 17

Geological history 1. Formed 4. 5 Gyrs ago. How? We’ll come back to that. 2. Until ~4. 4 Gyrs ago: top few 100 km molten, from bombardment heat and short-lived radioactive elements. Solidified, and crust formed. 3. Heaviest bombardment ended 3. 8 Gyrs ago: highland craters and large basins formed. 4. 3. 8 -3. 1 Gyrs ago: Radioactive heating from longer lived elements led to intense volcanic activity, filling large basins to create maria. 18

5. 19 3. 1 Gyrs ago: volcanism stopped, Moon mostly geologically dead since then. No plate tectonics. Surface is one solid plate. Light bombardment

Origin of the Moon Any theory must explain: • Moon has lower density than Earth, must have much less iron • Moon lacks water and other “volatiles” (substances with low boiling temperatures like sodium, potassium, water, CO 2), meaning moon rock experienced higher temperatures than Earth generally did. • Moon rocks also lack iron and high density materials, different from Earth on average, but similar to mantle 20

Theories 1. The Fission Theory: The Moon was once part of the Earth, then was separated Problem: Earth is not spinning fast enough, lack of volatiles 2. The Capture Theory: The Moon was formed elsewhere, and then captured by Earth's gravitation Problem: Extremely unlikely, given the Moon's size is a considerable fraction of Earth's. 3. The Co-creation Theory: Moon and Earth condensed together from the Solar Nebula Problem: Moon has different density and composition; where's the iron? 21

4. The Collisional Ejection Theory: Mars-sized planetesimal struck Earth, Moon formed in disk of ejected material. 22

Moon formation movie 23

Some of the strong points • Spectacular collisions must have happened at end of planet building. Few dozen large bodies that had been built collided to make the few planets we have (or were ejected). • If Earth differentiated at time of collision, little iron near surface. Agrees with low density and iron content of Moon • Moon formed of Earth's mantle debris => compositional similarities (oxygen isotope ratios) • Lower abundances of volatile elements on Moon - rock vaporized by collision would have lost these elements 24

Crust on far side is much thicker. Volcanic material could not reach surface. Reason seems to be that entire dense interior sags a bit towards Earth in a kind of differentiation due to Earth’s gravity. 25

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Water Ice on the Moon LCROSS impact mission Data from LCROSS - 2009 Absorption of infrared beam by water ice in plume from impact. About 32 ounces of water per ton. Probably from comets. 27

The Roche limit • The limiting radius inside which the tidal forces are stronger than gravity holding object together • Solid objects torn apart inside this radius (and cannot grow by accretion!) • Rings of Saturn: composed by many small particles 28

Tidal forces revisited • Tidal forces are differences in the gravitational pull at different points in an object 29

The effects of tidal forces Earth is 80 times as massive as Moon => Earth's tidal effects on Moon are large. The Moon is elongated toward the Earth. Rotation rate tidally locked to its rate of revolution Moon is moving away from the Earth is slowing down in its rotation 30

Why doesn’t Earth’s strong tidal force tear Moon apart? Consider two rocks of mass m on either side of Moon: Tidal force causing acceleration away from each other is Gravitational force on each rock is Ratio is or 3 x 10 -5 31

The Roche Limit (Sec. 12. 9 in text) Can also ask: how close would they have to be to Earth to be drawn apart? Solve for distance from Earth for which this ratio is > 1: this is the Roche Limit, for the Moon due to Earth’s tides Note: Roche Limit compares tidal force with gravitational attraction. For small objects, chemical bonds are much stronger than gravity – Roche Limit doesn’t apply. 32

Crust on far side is much thicker. Volcanic material could not reach surface. Reason seems to be that entire dense interior sags a bit towards Earth in a kind of differentiation due to Earth’s gravity. 33