Lunar Terrain
Section M 3.3.
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Notes on Lunar Terrain

Marvin Ostrega

The Moon has several distinctive landforms, which are distributed hetrogeneously across the Moon's surface. Maria, mountains, rilles and clefts are all major landforms on the Moon. Craters are not discussed.

The marias are smooth plains with very few mountains and solid lava flows. Unlike the more prevalent highland terrain, the maria is loaded with basaltic lavas which contain iron, titanium, and magnesium. These materials flowed from the Moon's interior and filled mare basins over 3 billion years ago, as a result of meteorite bombardment and readiosotope decay. The farside of the Moon is covered by only 2.6% of maria but on the nearside the maria covers 31.2% of the Moon's surface.

The terrae, or highlands, are covered with craters of all sizes, and contain large quantities of aluminum and calcium. They are the oldest parts of the Moon, not having been resurfaced by mare lava flows.

Mountains are very few on the Moon and are far outnumbered by large craters. Because of the Moon's lack of terrestrial land-forming devices, lunar mountains are quite dissimilar to terrestrial ones. It is possible that Lunar mountains are actually the remains of crater walls because they follow the remnants of gigantic craters.

Lunar montains have a gradual slope of 15-20 degrees compared to 30-35 degrees on Earth. This is in spite of the lower lunar gravity and thus higher stable angles of repose.

Faults and depressions on the Moon are called rilles and clefts. Previously thought to be dry river beds, sinuous rilles are lava channnels which were probably active during the maria formation. There are likely similar formations, lava tubes, which carried lava during the same period of mare basin flooding. These lava tubes may be hundreds of metres across.

Lunar Terrain

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