Impact craters and Olivine: The geological side of the moon

The first lunar rocks were returned to Earth by the robotic Soviet Luna-missions between 1959 and 1976, then by the Apollo astronauts (the only scientist ever to visit Moon was the geologist Dr. Harrison Schmitt, during the Apollo 17 mission in 1974), and some meteorites found on Earth are today known to have come from the Moon.
But all these samples gave us information’s about the surface of the Moon, the interior, like the lunar mantle, has not yet been sampled and his composition and structure is only inferred by seismic wave-analysis and analogues to Earth, where the dominant mineralogical component found in the mantle is Olivine. The estimated up to 70 kilometres thick crust of the Moon consists of the iron rich dark Mare basalts (up to 3,7 billion years old) and the brighter feldspar dominated Anorthosite of the highlands, thought to be differentiated from the underlying mantle at least 4,4 billion years ago.

Now the Japanese space probe "Kaguya" has revealed possible direct information’s on the structure of the lunar mantle beneath the crust. "Kaguya" recorded the surface of the Moon over a period of one and a half years from an orbit 100 kilometres above the surface. The spectral analysis of the space probe showed that on the edges of impact craters there are often unusual accumulations of the mineral Olivine; in contrast in the centres of these craters no Olivine could be found. This new type of lunar rock also doesn’t coincide with the surrounding superficial geology.

Fig.1. Map of the ring-mountain Copernicus by Ph. Fauth (1909).

According to one proposed model, based also on the observed thin crust, 30-50 kilometres thick, in these areas, the now discovered Olivine rich rocks represent the upper part of the lunar Mantle, uncovered by impacts of meteorites and resulting disintegration of the “thin” lunar crust. The exposed mantle was subsequently covered only superficially by lava and impact-breccias, which filled predominately the interior of the crater.

References:

LUCEY, P. (2010): Planetary science: Mantle of the Moon exposed? Nature Geoscience. Published online: 4 July 2010 | doi:10.1038/ngeo904
FAUTH, P. (1909): The Moon in modern Astronomy - Summary of twenty years selenographic work, and a study of recent problems.D. van Nostrand Company - New York.