The Earth’s Moon with a telescope

The Moon also has large, dark smooth areas covering about 17% of the Moon's surface that people originally thought were seas of liquid water so they are called mare (Latin for ``seas''---they are what make out the face on the Moon). Now it known that the mare are vast lava flows that spread out over many hundreds of square miles, covering up many craters that were originally there. The mare material is basaltic like the dark material on the Earth's ocean crust and that coming out many of our volcanoes (e.g., the Hawaiian islands).

From WikipediaBasalt (pronounced /bəˈsɔːlt, ˈbeisɔːlt, ˈbæsɔːlt/)[1][2] is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey.On Earth, most basalt magmas have formed by decompression melting of the mantle. Basalt has also formed on Earth's Moon, Mars, Venus, and even on the asteroid Vesta. Source rocks for the partial melts probably include both peridotite and pyroxenite (e.g., Sobolev et al., 2007). The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling mantle below ocean ridges.

Basalt from igneous rock

A comparison of the Earth’s moon with the four moons of Jupiter

Formation of a Crater on the Moon

The size of the craters having central peaks depends on the gravity of the planet or moon:

Moon Crater Copernicus

Notice the radial streaks issuing outward

The strong tides from the Earth pulled the early Moon's liquid interior toward the Earth, so the far side's crust is now about 130 kilometers thick while the near side's crust is about 65 kilometers thick. The thinness of the near side's crust is also why there are more mare on the near side than the far side. The near side was thin enough to be cracked apart when large asteroids hit the surface and formed the mare but the far side crust was too thick.

The Moon's density is fairly uniform throughout and is only about 3.3 times the density of water. If it has an iron core, it is less than 800 kilometers in diameter. This is a sharp contrast from planets like Mercury and the Earth that have large iron-nickel cores and overall densities more than 5 times the density of water. The Moon's mantle is made of silicate materials, like the Earth's mantle, and makes up about 90% of the Moon's volume. The temperatures do increase closer to the center and may be high enough to partially liquify the material close to the center. Its lack of a liquid iron-nickel core and slow rotation is why the Moon has no magnetic field.

Lunar samples brought back by the Apollo astronauts show that compared to the Earth, the Moon is deficient in iron and nickel and volatiles (elements and compounds that turn into gas at relatively low temperatures) such as water and lead. The Moon is richer in elements and compounds that vaporize at very high temperatures. The Moon's material is like the Earth's mantle material but was heated to very high temperatures so that the volatiles escaped to space.

FormationThere have been a variety of scenarios proposed to explain the differences

between the Moon and Earth. The one that has gained acceptance after much study is the giant impact theory. Earlier theories came in a variety of flavors.

The capture (pick up) theory proposed that the Moon formed elsewhere in the solar system and was later captured in a close encounter with the Earth. The theory cannot explain why the ratios of the oxygen isotopes is the same as that on the Earth but every other solar system object has different oxygen isotope ratios. The theory also requires the presence of a third large body in just the right place and time to carry away the extra orbital motion energy.

The double planet (sister) theory said that the Moon formed in the same place as the Earth but it could not explain the composition differences between the Earth and Moon.

The spin (daughter or fission) theory said that the Earth rotated so rapidly that some of its mantle flew off to the form the Moon. However, it could not explain the composition differences. Also, the spun-off mantle material would more likely make a ring, not a moon and it is very unlikely that the Earth spun that fast.

The giant impact theory proposes that a large Mars-sized object hit the Earth and blew mantle material outward which later re-coalesced to form the Moon. The Earth had already differentiated by the time of the giant impact so its mantle was already iron-poor. The impact and exposure to space got rid of the volatiles in the ejecta mantle material. Such an impact was rare so is was not likely to have also occurred on the other terrestrial planets. The one ``drawback'' of the theory is that it has a lot of parameters (impactor size, speed, angle, composition, etc.) that can be tweaked to get the right result. A complex model can usually be adjusted to fit the data even if it is not the correct one (recall Ptolemy's numerous epicycles). But the giant impact theory is the only one proposed that can explain the compositional and structural characteristics of the Moon.

From Wikipedia

More interesting moon data

Success - first manned landing on the Moon (manual landing required), exploration on

foot in direct vicinity of landing site; one EVA July 16, 1969