Astronomy 171
Solar System Astronomy
Prof. Paul Martini

Lecture 29: The Moon

Key Ideas:

Surface of the Moon:: Old, heavily cratered highlands; Younger, darker Maria; Thick regolith of pulverized rock
Interior of the Moon: Crust and Mantle, but no Core(?); No magnetic field
Origin of the Moon

Moon Rocks

Our most detailed knowledge of the Moon comes from ~382 kg of samples returned by 9 space missions:
United States:: 6 Apollo Landings (Apollo 11-17) 1969-1972; 12 astronauts visited maria and highlands
Soviet Union:: 3 Luna robotic missions (1970, 1972, 1976); returned samples by robotic capsule

Regolith

Layer of dust and fragmented rock: Product of repeated meteor impacts; Covers the surfaces of most moons and astroids in the Solar System
Lunar Regolith: Single-mineral grains and rock fragments; Impact Breccias: heat-fused grains and rocks; 2-8 km thick in Maria; More than 15 km thick in the Highlands

Lunar History

The Moon's surface was shaped by impacts
Maria:: Younger (3.1-3.8 Gyr) and lightly cratered; Formed after the epoch of heavy bombardment
Highlands: Older (3.8-4.0 Gyr) and heavily cratered; Solidified ~4.3 Gyr ago, before which time the Moon was molten

Lunar Maria

Youngest Moon Terrains: Dark basalts rich in iron and magnesium; No water or hydrated minerals; 10x more Titanium than Earth rocks
Magma flows from deep crust fractures: Caused by asteroid impacts; Ages of 3.1 to 3.8 Gyr

Cratered Highlands

Oldest Moon Terrains: Rocks pulverized by impacts; Older than the Maria (3.8 to 4.0 Gyr); Marks the end of an intense period of bombardment that started 4.6 Gyr ago
Unusual mineral content: The Moon was molten 4.35 Gyr ago!

Is there a Lunar Core?

The Moon has no global magnetic field now: Doesn't have a molten core like the Earth
Did it have a core in the past?: "Fossil" magnetism in very old Moon rocks; Means they cooled in a strong magnetic field
The Moon had a molten core and magnetic field 3.6 to 3.8 Gyr ago, but it does not have one now.

Origin of the Moon

Moon origin theries must explain these facts:: Moon has much less iron than the Earth; Moon lacks water and other "volatiles"; Moon rocks most resemble the Earth's mantle; Identical proportions of Oxygen isotopes in Earth and Moon rocks (different from meteorites)
Four formation models have been proposed: All are testable with observations

Theories of Moon Formation

Co-Formation: Earth and Moon formed together in place; Cannot explain the lack of iron and volatiles
Capture: Earth gravitationally captured the Moon; Cannot explain lack of iron and volatiles, or the identical oxygen isotope ratios; Also hard to do (but not a theory-killer)
Fission:: Moon split off from a fast-spinning proto-Earth; Composition issues OK except the volatiles; Also hard to do
Giant Impact:: Proto-Earth hit by a Mars-sized body; Moon formed from the debris; Currently the favored theory (!)

Giant Impact Theory

The impact theory has many strong points: The impactor's iron would have been in its core, and so sunk into the Earth's mantle; A molten post-impact Moon would have boiled off all of its volatiles; Moon formed of mostly Earth's mantle debris, explaining the compositional similarities
Still questions, but it does the best so far.

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.