List of craters on the Moon

Terrae

The lighter-colored regions of the Moon are called terrae, or more commonly just highlands, since they are higher than most maria. Several prominent mountain ranges on the near side are found along the periphery of the giant impact basins, many of which have been filled by mare basalt. These are believed to be the surviving remnants of the impact basin's outer rims.^[16] In contrast to the Earth, no major lunar mountains are believed to have formed as a result of tectonic events.^[17]

From images taken by the Clementine mission in 1994, it appears that four mountainous regions on the rim of the 73 km-wide Peary crater at the Moon's north pole remain illuminated for the entire lunar day. These peaks of eternal light are possible because of the Moon's extremely small axial tilt to the ecliptic plane. No similar regions of eternal light were found at the south pole, although the rim of Shackleton crater is illuminated for about 80% of the lunar day. Other consequences of the Moon's small axial tilt are regions that remain in permanent shadow at the bottoms of many polar craters.^[18]

Impact craters

Lunar crater Daedalus on the Moon's far side

Main article: List of craters on the Moon

Earth's Moon's surface is marked by impact craters^[19] which form when asteroids and comets collide with the lunar surface. There are about half a million craters with diameters greater than 1 km on the moon. Since impact craters accumulate at a nearly constant rate, the number of craters per unit area superposed on a geologic unit can be used to estimate the age of the surface (see crater counting). The lack of an atmosphere, weather and recent geological processes ensures that many of these craters have remained relatively well preserved in comparison to those on Earth.

The largest crater on the Moon, which also has the distinction of being one of the largest known craters in the Solar System,^[20] is the South Pole-Aitken basin. It is on the far side, between the South Pole and equator, and is some 2,240 km in diameter and 13 km in depth.^[21] Prominent impact basins on the near side include Imbrium, Serenitatis, Crisium, and Nectaris.

Regolith

Blanketed atop the Moon's crust is a highly comminuted (broken into ever smaller particles) and "impact gardened" surface layer called regolith. Since the regolith forms by impact processes, the regolith of older surfaces is generally thicker than for younger surfaces. In particular, it has been estimated that the regolith varies in thickness from about 3–5 m in the maria, and by about 10–20 m in the highlands.^[22] Beneath the finely comminuted regolith layer is what is generally referred to as the megaregolith. This layer is much thicker (on the order of tens of kilometres) and comprises highly fractured bedrock.^[23]

Astronauts have reported that the dust from the surface felt like snow and smelled like spent gunpowder.^[24] The dust is mostly made of silicon dioxide glass (SiO₂), most likely created from the meteors that have crashed into the Moon's surface. It also contains calcium and magnesium.

Presence of water

Main article: Lunar ice

The continuous bombardment of the Moon by comets and meteoroids has most likely added small amounts of water to the lunar surface. If so, sunlight would split much of this water into its constituent elements of hydrogen and oxygen, both of which would ordinarily escape into space over time, because of the Moon's weak gravity. However, because of the slightness of the axial tilt of the Moon's spin axis to the ecliptic plane—only 1.5°—some deep craters near the poles never receive direct light from the Sun and are thus in permanent shadow (see Shackleton crater). Water molecules that ended up in these craters could be stable for long periods of time.

Clementine has mapped craters at the lunar south pole^[25] that are shadowed in this way, and computer simulations suggest that up to 14,000 km² might be in permanent shadow.^[18] Results from the Clementine mission bistatic radar experiment are consistent with small, frozen pockets of water close to the surface, and data from the Lunar Prospector neutron spectrometer indicate that anomalously high concentrations of hydrogen are present in the upper metre of the regolith near the polar regions.^[26] Estimates for the total quantity of water ice are close to one cubic kilometre.

Water ice can be mined and then split into its constituent hydrogen and oxygen atoms by means of nuclear generators or electric power stations equipped with solar panels. The presence of usable quantities of water on the Moon is an important factor in rendering lunar habitation cost-effective, since transporting water from Earth would be prohibitively expensive. However, recent observations made with the Arecibo planetary radar suggest that some of the near-polar Clementine radar data that were previously interpreted as being indicative of water ice might instead be a result of rocks ejected from young impact craters.^[27] The question of how much water there is on the Moon has not been resolved.

In July 2008, small amounts of water were found in the interior of volcanic pearls from the Moon (brought to Earth by Apollo 15).^[28]