A New Finding on the Crustal Formation of the Lunar Highland 1
Updated : 2009.09.18
Observation date :

Results of the Multiband Imager onboard the lunar orbiting satellite "SELENE" (Kaguya) were published in the British science journal "Nature" on September 10 (British Standard Time).

The global distribution of pure anorthosite on the Moon
(Ohtake et al., Nature 461, 236-240, 10 September 2009)

As a result of examining the central peaks, walls, and other parts of 69 craters, the Multiband Imager observation instrument team clarified for the first time in the world that anorthosite (*1) consisting of nearly 100 percent anorthite is widely distributed in the lunar highland crust.
The lunar highland crust is believed to have been formed in the earliest stage of the lunar evolution by anorthite crystallizing from a magma ocean (*2) and floating due to the density difference with the magma ocean. It had been conventionally estimated that the lunar highland crust consists of 90 percent anorthite and 10 percent other minerals, wherein pyroxene and olivine also crystallized as well as anorthite. However, this finding has clarified that a new formation mechanism of the lunar highland crust must be constructed, which can efficiently and selectively collect only anorthite in order to form crustal material consisting of only anorthite.

From the image abobe, we can tell that highly pure anorthosite always exists at a depth of 4-30 km in the highland region (indicated as Highland in the figure) where the original crust formed from a magma ocean that was believed to be exposed.
Based on the sizes of the craters examined, the depths of crust excavated by cratering can be estimated to be about 4-30 km.
On the other hand, some of the small craters examined have lesser amounts of anorthite (small squares in yellow and orange) as well as highly pure anorthosite. The reason for this is believed to be that their chemical compositions reflect mixing of the highland crustal material and other various kinds of rocks by meteorite bombardments that occurred on the shallow surface portion of the Moon over a long period. Note that the purity of anorthosite is estimated based on the strength ratios of absorption bands characteristic of individual minerals.

*1 Anorthosite : A white igneous rock rich in minerals called anorthite made of calcium, aluminum, silicon, and oxygen is called anorthosite.

*2 Magma Ocean : It is believed that after the Moon was formed, there was a period when the Moon became hot and melted to form a magma ocean.