Lunar Meteorites
Every year since the late 1970s, a team of US based scientists is organized by the Smithsonian and NASA to take a trip down to Antarctica over that continent’s summer. They move along the ice sheets looking for rocks, and go to the places where the glaciers ablate also looking for rocks. Why?
Because if you’re in the middle of a glacier with no hills anywhere nearby, and you find a rock, where did it probably come from? The sky. Good chance that rock is a meteorite. Go to places where glaciers are ablating, and the rocks that feel onto the ice sheets over thousands of years are concentrated, leaving a deposit containing bunches of meteorites.
This rock was found in the Allan Hills region of Antarctica in early 1982, and it was given the designation ALHA 81005 (the location, year of the expedition, and sample number). It was quickly identified as being something unique compared to the other rocks of that area, as no one had ever found a sample like it in Antarctica, or in any other meteorite collection. However, once it was analyzed, scientists realized they had found a whole bunch of rocks similar to it on six other occasions; this sample contains clasts that are exactly like the Apollo moon rocks. This sample was the first recognized Lunar Meteorite.
Today there are more than 300 lunar meteorites that have been recognized. The main locations they have been identified from include Antarctica and the deserts of Asia and Africa, for the same reasons as Antarctica – if you’re walking along a sand dune or a dry lakebed, and there’s a rock, there’s a decent chance it fell from above. No lunar meteorites have ever been identified from North America, South America, or Europe; that doesn’t mean they don’t fall there, but instead it means it’s nearly impossible to distinguish them from ordinary rocks unless there’s something to tell you it’s a meteorite. That means if you ever think you’ve found a lunar meteorite, you haven’t – because there’s literally no way to tell by looking at it, you need detailed chemical analysis to figure that out.
Lunar meteorites can be recognized as distinct from terrestrial rocks by their mineralogy and chemistry. This rock, ALH 81005, is a breccia, first indicating that it was fractured by a violent event after it was already solid. Those type of events do occur on Earth in faults, volcanoes, and debris flows, but this breccia was produced when the rocks were shattered by asteroid impact.
ALH 81005 also contains pieces that represent many different rock types found on the Moon, most of which are not present on Earth. Unlike rocks from Earth, most lunar rocks are dominated by only 4 minerals; Olivine, Pyroxene, Plagioclase, and Ilmenite. Common minerals from earth like quartz and calcite are missing from lunar rocks.
There are chunks of the lunar highlands – the bright areas are plagioclase rich, and these represent pieces of the lunar crust that formed when plagioclase floated to the surface as the moon was solidifying. There are chunks of lava flows with large amounts of ilmenite – these high titanium lavas are a record of processes in the lunar mantle just after it formed, where dense rocks with lots of titanium sank to the bottom and melted once they heated up. Finally, there are even agglutinates – bits of lunar soil that have been melted and stuck together as they’re exposed to impacts by space dust.
Lunar meteorites include all of the rock types sampled by the Apollo missions, and also a few new ones; the Apollo missions all landed on the lunar nearside, close to the giant Imbrium crater, and as a consequence they never sampled the full surface. The presence of new rock types in these samples does tell us we have more work to do to fully understand the geology of our nearest neighbor.
Because some of the lunar meteorites have fallen in deserts, they can be picked up and eventually sold to private collectors. All of the Apollo samples are considered property of NASA even if they are loaned to museums or other facilities around the world, so selling them is not legal and if you somehow bought one, NASA might well come and claim it back. If you wanted to buy a piece of the moon, it won’t be cheap, but Lunar Meteorite slices can be purchased by private collectors and researchers.
-JBB
Image credit:
https://www-curator.jsc.nasa.gov/antmet/lmc/f2%20alha81005.pdf
(The image includes a 1 centimeter sized cube for scale)
Reference:
https://sites.wustl.edu/meteoritesite/items/how-do-we-know-that-its-a-rock-from-the-moon/
