selenology

Man has always been fascinated by the Moon and have tried to exlain its origin and structure. Before the Apollo-landings the only way was to look at photos and the results of the few un-manned missions to the moon. The Apollo-austronauts brought back a lot of stones (400 kg) but they were only from a few places. Another problem is that you refer to what you know about the geology on earth but that can be treacherous. Things that look the same can have different expanations. Of course there is a great distinction between before and after the Apollo-project. For example in the theories about the origin of the moon.

There are principally three theories of the origin: Capture, Double-planet and Fission. Capture: According to this theory the moon was a planet orbiting the sun that was captured by the Earth's gravitation. But the gravitaional and tidal forces are dificult to match with this theory. And after the Apollo-projects evidence of the Moon's chemistry this theory was abandonned. Double-planet: This theory suggests that the earth and the moon originated from the same cloud when the solarsystem formed. But this theory was also hard to defend when the chemistry of the Moon was revieled. The Moon differ very much from the Earth as a whole.But it has a chemistry that correlates very well with the Earth's mantle. Seismic information has also influenced theories about the formation and evolution of the Moon.The third theory ,fission, is the one that is most valid. It postulates that the Moon originates from the Earth's mantle material. But there are more than one explanation of how that happened. Was the Moon-material "thrown away" by gravitaional forces or did it evaporate as the earth heated up when it was growing bigger and bigger or was there a very great impact that threw out the material.

Inner structure: The best way to find out about the inside of a planet is to register how the seismic waves behave. There is only residual tectonic activity due to cooling and tidal forcing, but some moonquakes is caused by meteor impacts. Artificial moonquakes have been made by deliberately crashing the Lunar Module into the moon. The results have shown that the Moon thas a crust 60 kilometers thick at the center of the near side. If this crust is uniform over the Moon, it would constitute about 10% of the Moon's volume as compared to the less than 1% on Earth. The seismic determinations of a crust and mantle on the Moon indicate a layered planet with differentiation by igneous processes. There is no evidence for an iron-rich core unless it were a small one.

The crust and the surface: The Moon was heavily bombarded early in its history, which caused many of the original rocks of the ancient crust to be thoroughly mixed, melted, buried, or obliterated. Meteoritic impacts brought a variety of "exotic" rocks to the Moon so that samples obtained from only 9 locations produced many different rock types for study. The impacts also exposed Moon rocks of great depth and distributed their fragments laterally away from their places of origin, making them more accessible. The underlying crust was also thinned and cracked, allowing molten basalt from the interior to reach the surface.

Because the Moon has neither an atmosphere nor any water, the components in the soils do not weather chemically as they would on Earth. Rocks more than 4 billion years old still exist there, yielding information about the early history of the solar system that is unavailable on Earth. With such an active early history of bombardment and a relatively abrupt end of heavy impact activity, the Moon is considered fossilized in time. Geological activity on the Moon consists of occasional large impacts that forms the regolith or lunar soil, which is unconsolidated mineral grains, rock fragments, and combinations of these which have been welded by impact-generated glass. It is found over the entire Moon. It is 2 to 8 meters thick on the maria and may exceed 15 meters on the terrae, depending on how long the bedrock underneath it has been exposed to meteoritic bombardment.

The dark, relatively lightly cratered maria cover about 16% of the lunar surface and is concentrated on the nearside of the Moon, mostly within impact basins. Mare rocks are basalt and most date from 3.8 to 3.1 billion years. Some mare flows actually embay young craters and may thus be as young as 1 billion years. The maria average only a few hundred meters in thickness but are so massive they frequently deformed the crust underneath them which created fault-like depressions and raised ridges.

The relatively bright, heavily cratered highlands are called terrae. Some fragments in highland breccias date to 4.3 billion years The craters and basins in the highlands are formed by meteorite impact and are thus older than the maria, having accumulated more craters. The dominant rock type in this region contain high contents of plagioclase feldspar and are a mixture of crustal fragments brecciated by meteorite impacts. Most terrae breccias are composed of still older breccia fragments. Other terrae samples are fine-grained cryst

alline rocks formed by shock melting due to the high pressures of an impact event. Nearly all of the highland breccias and impact melts formed about 4.0 to 3.8 billion years ago. The intense bombardment began 4.6 billion years ago, which is the estimated time of the Moon's origin.