[meteorite-list] Witnessed fall lunars?

From: Randy Korotev <korotev_at_meteoritecentral.com>
Date: Tue, 07 Sep 2010 16:06:15 -0500
Message-ID: <201009072105.o87L5vX11021_at_levee.wustl.edu>

>MikeG asks:

>"Is there a theory for why there have been no witnessed falls of lunar
>meteorites? It seems odd to me that we have 4 Martian witnessed falls
>(Shergotty, Chassigny, Zagami, Nakhla, and almost Lafayette) and no
>lunars."

One issue is that these 5 meteorites are 5 kg, 4 kg, 18 kg, 10 kg,
and 0.8 kg in mass. Only 3 lunars are >4 kg in mass.

Another issue (probably more important) is that lunar escape velocity
is only 2.4 km/s and very little material ejected from the Moon is
going much faster than that. This velocity compares with 20-40 km/s
for asteroidal meteorites. Is a rock entering the atmosphere at 2.4
km/s going to noticeably incandesce? I don't know. I believe that
the space shuttle hits the atmosphere at ~7.7 km/s.

Melanie asks:

"I asked this a while ago on Greg Catterton's forum, and I was told that rocks
from the moon aren't as solid (tough) as Mars rocks, and therefore are less
likely to survive entry... yet what about all these Howardites?"

Although breccias, most of the lunar meteorites are very tough
rocks. Any rock that survives being blasted off the Moon isn't going
to disintegrate in Earth's atmosphere any more than an asteroidal or
martian meteorite.

Steve says:
"The moon is close to the earth and material knocked off the moon has
a relatively short time to reach the earth."

Compared to what? Some lunar meteorites took a million years or more
to reach Earth.

"Mars is farther away and not protected by a companion and its closer
to the asteroid belt so it receives many more impacts than the moon."

Not "many more." Only a factor of two greater for Mars, but the
average velocity of the impactors is only 60% as great.



Randy Korotev
Washington University in St. Louis
Received on Tue 07 Sep 2010 05:06:15 PM PDT