Moon

I. Origin

A. Theories

1. Capture - Moon formed elsewhere and was captured into Earth's orbit

2. Coaccretion - Moon and Earth formed together from a swarm of planetesimals.

3. Collisional ejection - Mars sized planetesimal hits the Earth, and the moon accretes from the resulting orbital debris disk.

4. Disintegrative capture - a large planetesimal passes through Earth's Roche limit, is disrupted, and the moon accretes from the orbiting debris.

5. Fission - a rapidly rotating Earth becomes unstable and loses the material that forms the moon.

B. Constraints on Formation

Geological:

1. Oxygen isotopes - the ratio of ¹⁸O to ¹⁷O within the planets varies as a function of distance from the Sun. Earth and moon's ratios are identical.

2. Moon is very depleted in metal; metallic Fe is only a few percent by weight.

3. The moon is highly depleted in volatile elements, even compared to Earth. This suggests that lunar formation involved heating the moon to high temperatures.

4. Both Earth's mantle and moon's mantle are very depleted (relative to chondrites) in siderophile elements. Siderophile, or iron-loving, elements are elements that react easily with iron. Depletion of these elements is indicative of a highly differentiated body.

5. Earth and moon have similar amounts of refractory elements, although the moon may be slightly enhanced. Refractory elements have high vaporization temperatures, occur in similar proportion in the Sun and meteorites, and should be in constant relative proportions in planetary bodies.

Other Constraints:

6. The moon is unusually large compared to the Earth.

7. The current angular momentum of the Earth-moon system is high.

C. Table of how well various theories satisfy the constraints:

	Capture	Coaccretion	Fission	Collisional ejection	Disintegrative capture
Lunar size	B	B	D	I	B
Earth-moon angular momentum	C	F	F	B	C
Volatile depletion	C	C	B	B	C
Fe depletion	F	D	A	I	B
O isotopes	B	A	A	B	B
Siderophile and refractory elements	C	D	A	C	C
Physical plausibility	D	C	F	I	F

A = excellent, B = good, C = satisfactory, D = poor, F = failing, I = incomplete information.

II. The big events in lunar history:

A. Formation of a magma ocean - ~4.6 Ga. The lunar highlands are ~75% plagioclase feldspar [(Na,Ca)Al_xSi₂O₈] and in places are true anorthosite (>95% plagioclase). Simple partial melting of the mantle could not produce this, the entire upper several hundred km of the moon must have been molten, forming a magma ocean. The source of this heating was presumably accretion.

B. Late, heavy bombardment - 4.1 to 3.8 Ga. Terra (highlands) formation was completed by ~4.2 Ga. By sampling of ejecta and stratigraphy, 15 large basins (>300 km diameter) formed between 4.1 and 3.8 Ga. It is debated whether this represent a late peak period of cratering or just the normal tail-off of accretion.

Estimated relative ages for some of the larger basins:

Orientale

Imbrium 3.85 Ga

Serenitatis 3.87

Crisum

Humorum

Nectaris 3.92

C. Mare volcanism - 3.9 to 3.0 Ga, with most occurring ~3.8 Ga. Radioactive heating caused partial melting of the mantle; basalts caused partial melting of the mantle; basalts flooded many low-lying areas, forming the maria. Mare volcanism probably began before all the large basins finished forming, and steadily declined from 3.8 to 3.0 Ga.

III. Other interesting facts about the moon:

A. The lunar crust is thicker on the farside (~110km) than the nearside (~90 km). Nobody knows why.

B. Maria covers ~15% of the lunar surface, almost all on the nearside. At its thickest it is ~4-5 km thick, and generally (but not always) occurs where the highland crust is thinnest.

C. The surface of the moon is continually pounded by small meteorites (<1 cm) and quite often by somewhat larger features. Thus, the upper km is highly fractured and termed "megaregolith." The upper few m has been tilled into soils and is termed "regolith".

D. There are very few tectonic or volcanic features on the moon. Tectonic features are mostly ridges and graben related to settling of the mare. Volcanic tubes and rilles associated with mare emplacement are common. Small volcanic domes can be found in places.

Mercury

Very limited data; images exist for 1/2 the planet from flyby's by Mariner 10.

I. Interior:

A. Mercury has a large iron core, ~ 42% of its volume (Earth's is ~16% of its volume).

B. Mercury has a permanent magnetic field, weaker than Earth's but still capable of deflecting the solar wind about the planet.

II. Rotation: Tidal interaction with the Sun and perhaps Venus resulted in a 3:2 relationship between rotational (58.6 days) and orbital (88 days) periods. The end result is that the time between sunrises on Mercury is 176 days.

III. Appearance:

A. Optical, thermal, and reflecting properties very similar to the moon.

B. It is a matter of debate whether or not any true volcanic plains exists.

Several scarps indicative of shrinking; over time the radius of the planet may have decreased by as much as 2 km.

Ice and shadows on Moon and Mercury

Radar returns from the poles of both the Moon and Mercury show highly reflective material that depolarizes a circularly polarized radar signal. This is strongly characteristic of water ice, and the ice apparently occurs in permanently shadowed regions.

The neutron spectrometer on the Lunar Prospector mission detected high amounts of hydrogen at the lunar poles, which provides strong confirmation of ice at the poles, apparently in the form of small crystals in the regolith (i.e., there's icy dirt, not a skating rink).

For manned exploration of the moon, the poles turn out to be a good place to put a lunar base. There are regions of near-permanent sunlight for power, and regions of permanent shadow nearby that could have ice (useful for humans and for fuel).