The Alan Hills Meteorite- The first identified lunar meteorite (courtesy of NASA)
Lunar meteorites tend to come in two flavors: either the fine grained basalt of the relatively young lunar basins, or those that are rich in plagioclase that hail from the lunar highlands. To date only about 46 kg of lunar meteorite have been collected from the earth’s surface with the largest sample, Dal al Gaani 400, weighing in at about 1.425 kg (1).
The lunar meteorites that come from the highlands are composed mostly of a calcium rich plagioclase called anorthite. The rocks are then classified based on the amount of anorthite in the sample:
- >90% anorthite is called anorthosite
- <90% is classified as either norite if it’s poor in calcium rich pyroxene, or it’s called troctolite if it contains olivine.
The lunar basalts that come from the much younger basins are kind of the opposite of their highland cousins. Instead of being composed primarily of calcium-rich plagioclase, they contain calcium rich pyroxenes. Titanium is also found in these rocks.
Our knowledge of the moon came about primarily from the Apollo missions and the Luna missions. Both returned samples from various locations on the moon and have allowed us to compare those returned samples with the lunar meteorites found on the earth. It is believed that some of the highland meteorites come from the far-side of the moon, which gives us a look at the side we don’t see from our terrestrial vantage.
And now for that awesome picture I found off of NASA’s image of the day site:
Gorgeous picture of the moon taken by the Galileo spacecraft on it's way to Jupiter. (Image courtesy of NASA)
1. Hutchison, Robert. Meteorites: A petrologic, chemical and isotopic synthesis. 2004. Cambridge University Press. P. 296-297
Glacial Till 