Lunar Meteorites – bits of the Moon found here on Earth

This post is a little off track from Moon Zoo images of the lunar surface, but I want to share some information about my particular favourite Moon related subject — lunar meteorites.

What are lunar meteorites?
Thousands of meteorites have been found all over the Earth — the vast majority are thought to have originated from the asteroid belt — but very rarely some are identified that come from bodies like Mars, the Moon, and large asteroids.

Lunar meteorites are chunks of the Moon that were blasted off its surface by meteorite impacts, that then flew through space from the Moon and were captured by Earth’s gravity. The rocks then fell through the Earth’s atmosphere — often losing some mass on the way — and then land here on Earth.

Allan Hills 81005 - the first lunar meteorite to be recognised. This rock was picked up in Antarctica in 1981. Credit: NASA

How many have been collected so far?
So far (as of 2010) Dr. Randy Korotev, a lunar meteorite researcher from Washington University in St. Louis, states that 136 individual lunar meteorite stones have been collected. As some fell through the atmosphere at the same time, these samples actually represent only 66 meteorites! It is likely that many many more lunar meteorites than this have fallen onto the Earth through time, but because of high weathering rates (wind, rain, organic disaggregation, etc.) they have most likely been weathered away and could no longer be identified as having a lunar origin. We are lucky to have the ones that we have!

How do we know they have come from the Moon (and are not just chunks of grey concrete!)?
Proof of lunar origin can be taken from several lines of evidence that often requires analysis in a laboratory.

1. The presence of a glassy crust suggests that the sample is of meteoritic origin and has passed through the Earth’s atmosphere causing frictional melting of the surface.
2. The rock does not have chondrules and therefore can be classified as an anchondrite.
3. The rock is poor in metal and therefore is classified as stony.
4. Typical bulk rock elemental ratios and mineral chemistries that are similar to those of the Apollo and Luna Moon samples.
5. An oxygen isotope ratio corresponding that of the Earth-Moon trend.
6. Typical mafic mineral Mn/Fe ratios are indicative of the volatile-poor nature of lunar samples that trend on a ‘lunar line’ distinct from terrestrial rocks and samples from Mars and asteroids.

North West Africa 4472 - a lunar meteorite collected in Africa in 2006. I have been studying this sample and you can find out more information about my study in this document. Image: K. Joy.

When did they land on Earth?
All lunar meteorites are thought to have been launched from the Moon in the last 20 million years. As there are believed to have been no large craters generated on the Moon during this period, it is assumed that all lunar meteorites are launched from craters only a few kilometres in diameter (all craters in the last 1 million years or so are thought to be <3.6 km in diameter). This implies that lunar meteorites have been ejected from relatively shallow depths, and so represent the upper layer of lunar crustal material.

What do they tell us about the Moon?
Manned (Apollo) and unmanned (Luna) missions to the Moon have returned about 382 kg of lunar rocks and soils. These were all collected from rather atypical regions on the lunar near-side, within and around the central lunar nearside, or from equatorial latitudes on the eastern limb. Therefore interpretations of the nature of the lunar crust and mantle have been made from a dataset from geographically restricted areas of the Moon. Lunar meteorites, in contrast, are derived from random sample sites on the surface, and thus provide a wealth of new information about the nature of the Moon, even though their precise provenance is as yet unknown.

Some cool lunar meteorite findings:

• Lunar meteorite Kalahari 009 provides a sample of the oldest basalt (volcanic lava) found on the Moon — it is 4.35 billion years old.
• Lunar meteorite NWA 032 is the youngest basalt (volcanic lava) found on the Moon — it is 2.9 billion years old.
• Many lunar meteorites that are rich in a mineral called plagioclase are believed to have originated from the farside of the Moon. These important stones provide information about how the lunar crust formed.

Some good lunar meteorite resources you might like to check out:

• Dr. Randy Korotev has an excellent website.
• Dr. Kevin Righter at NASA keeps track of all the samples found and prepares some nice summaries about each stone collected.
• There is a lunar meteorite 101 video you can watch to learn more about these exciting bits of Moon rock.

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