Every now and then you here stories like this one telling of rocks from Mars landing on Earth. The rocks supposedly get here after being ejected from the surface of Mars by a violent impact, floating in space, and, finally, smacking into the Earth.
What about the Moon? The Moon is a lot closer than Mars, it has obviously been hit by tons of impacts, and it has less gravity than Mars making it easier for rocks to escape. Why don’t we hear about any Moon rocks landing on Earth?
Lunar Meteorites (courtesy of the Department of Earth and Planetary Sciences at Washington University in St. Louis).
I think we just don’t hear as much about Lunar meteorites because they aren’t as “sexy” as Martian meteorites. No one’s likely to claim they’ve found fossil microbes in Lunar meteorites. Besides, NASA’s got lots of Moon rocks they got the hard way, whereas we don’t (yet) have access to Martian rocks that way. (Although that web page does address some reasons why Lunar meteorites are still scientifically important, despite the 382 kilos of samples brought back by the Apollo astronauts.)
Three things to remember,
The Moon is a relatively large body, with an escape velocity of 2.4 km/s. Most lunar impacts aren’t violent enough for material to escape. Impacts that contain enough energy to blast material into space don’t often yield pieces large enough to survive entry into Earth’s atmosphere.
Until the Apollo missions, scientists didn’t really know how to positively identify lunar rocks.
Finding meteroties anywhere is a tricky business, even if you know the object’s trajectory. Most of the time, we don’t. Mostly, we rely on glacial deposition, and snowfields, when looking for extraterrestrial samples.
Between 1982 and 1991, only 12 lunar rocks were found on Earth. (* Moons and Planets, Hartmann, 1993)
In case you’re wondering, we know a Mars rock when we see it because the Viking Lander sent us information about ratios of noble gas isotopes.
Wow, great site, MEBuckner! Thanks for the responses, guys.
Whoops, wrong zombie, past voting age now.
Nm.
We are up to 398 recorded lunar meteorites, though many of those on the list are obviously different pieces from the same fall. Only 247 known Martians (with the same caveat.)