[meteorite-list] Seeking Out Meteorites (on the moon)

From: Darren Garrison <cynapse_at_meteoritecentral.com>
Date: Tue Nov 7 01:00:25 2006
Message-ID: <6680l2pdqjg72p4fi3ecv757gfk0e7jm55_at_4ax.com>

http://www.spacedaily.com/reports/Seeking_Out_Meteorites_999.html

Seeking Out Meteorites

by Staff Writers
London UK (SPX) Nov 07, 2006
Monica Grady, a professor of planetary and space science at the Open University
in the UK, is one of the world's meteorite experts. In addition to studying the
finer details of these rocks from space that fall to Earth -- such as learning
the geochemistry of meteorites originating from Mars -- she is also interested
in the broader implications of her findings, and uses her research to learn more
about the possibility of life elsewhere in the universe.

In part one of this interview with Astrobiology Magazine, Grady talks about why
we need to search for meteorites on the moon. She also explains why it took so
many years to find a martian meteorite on Earth.

Astrobiology Magazine (AM): Rumor has it that the astrobiology community has
given you the title, "Reigning Queen of Meteorites."

Monica Grady (MG): Oh, that is completely unfair! (laughs) I don't know where
this title came from. It's not so much unfair as it's undeserved, because I
don't do anything that other people don't do. I research a subject that has an
astrobiological import and I enjoy talking to other people about it. But there
are loads of people in the astrobiology community who do work that is higher
profile than mine.

AM: Apparently you're doing something that's getting the attention of a wide
audience. For instance, making the case to go back to the moon to search for
meteorites.

MG: We know that meteorites have been falling on the Earth for 4.5 billion
years, but the oldest age of terrestrial meteorites is about 2 million years.
Those meteorites were found in Antarctica. For meteorites that fall in hot
deserts, their oldest age is about 100 to 200 thousand years. For meteorites
that have fallen in temperate zones, their oldest age is about a thousand years
-- a very short period of geological time.

Now if you go to the moon, you're going to be looking at rocks that fell
anywhere from 100 million years ago to 10 million years ago. The moon is an
airless body, so there's very little environmental change to objects there. So
we have a chance of looking at meteorites that fell a long time ago.
Theoretically, they shouldn't be much different from meteorites that fell
yesterday, because they all come from the same place in the solar system, but we
don't know. The orbits of asteroids evolve with time and populations of
asteroids evolve with time. So meteorites that fell 100 million years ago could
be slightly different in composition than those today, or there may be a greater
preponderance of one type over another that we don't have now.

Meteorites on the moon also could tell us about the flux of meteorites in
antiquity, so we might be able to fill in the gap between what we call the Heavy
Bombardment period and the average rate of bombardment. But that depends on
whether we are able to get an age for the lunar meteorites. We get a cosmic ray
exposure age when things are in space, and we get an exposure rate for
meteorites on Earth by looking at isotopes like chlorine-36. I've no idea if
we'll be able to get a lunar exposure age. I don't know how that would work.

AM: Can you explain why there are older meteorites on the moon? Does it have to
do with the moon being tectonically inactive, and so the surface is always the
same?

MG:Yes, the reason we don't have ancient meteorites on Earth is because the
surface of our planet is always changing. Meteorites that fell a long time ago
are now gone. But on the moon you've got a very stable, ancient surface. So the
idea of collecting ancient meteorites there is valid.

Just to illustrate, I was fortunate to be part of a team that looked for
meteorites in the Nullarbor region of Australia. One day we found four
meteorites in an area less than a square kilometer in size. I think the
Nullarbor Plain is 60 million years old. Those four meteorites were all
different -- they were from different parent bodies, different asteroids.

In Antarctica, the movement of ice brings meteorites together. But the Nullarbor
has no concentration mechanism for meteorites -- it's just a flat plain. The
meteorites might get blown a bit by the wind, but they essentially remain where
they land. So four meteorites from four separate parent bodies hit that square
kilometer over 60 million years. That's staggering. That square kilometer wasn't
calling out to be hit; it's completely representative of the Earth's surface.

That indicates the potential number of meteorites on the moon. It has a nice
flat surface like the Nullarbor, with no concentration processes other than
time, and no removal processes.

AM:I've heard that looking for meteorites on the moon is like looking for a
needle in a haystack. How hard would they be to find?

MG: The Mars rovers have found meteorites, so it's not unreasonable to search
for meteorites on the moon. It's going to be, not a difficult task, but a time
consuming task. But I think it's worthwhile.

The pictures we see of the moon are grey, and meteorites on Earth are black and
brown. But are they that way on the moon? On Earth they're black because the
friction from falling through the atmosphere causes them to melt and develop a
fusion crust. But the moon has no atmosphere, so would they develop a fusion
crust falling on the Moon? If not, they might be more difficult to spot. But I
think that most meteorites will stand out from the surrounding rock. I would
expect meteorites on the moon to be dominantly asteroidal, and we'll certainly
be able to spot asteroidal meteorites and primitive carbonaceous chondrites. The
difficulty will be in spotting any of the eucrites -- the basaltic rocks from
Vesta -- because they look so similar to lunar rocks.

I also would expect for there to be pieces that were broken off the Earth,
although it might be difficult to spot terrestrial basalts. But a nice
sedimentary limestone complete with fossils should stick out like a sore thumb.

There also would be pieces of Mars, just as there are pieces of Mars on the
Earth. I don't know whether there'd be meteorites from Venus or Mercury, but
it's an interesting possibility. There's a finite probability of Venusian and
Mercurian meteorites coming to Earth, so there's a finite probability of them
coming to the moon.

AM:What could such meteorites tell us of the geological or biological history of
early Earth or early Mars?

MG: We don't have really ancient rocks on the Earth, but there might be
something ejected from Earth 4.2 billion years ago that is now on the moon.
There might be some precursor rocks preserved on the moon, things that we now
see only the metamorphosed remnants of, perhaps the original igneous or
sedimentary rocks of the Earth. The same with Mars. There's only one really old
Martian rock that's fallen on the Earth, and that is ALH84001. There might be
rocks that are 2 or 3 billion years old from Mars that fell on the moon. So it's
a great opportunity to learn about early Earth and Mars.

AM:We didn't find a martian meteorite on Earth until 1979. Why did it take us so
long to find one? Did we not have the techniques to identify them, or are such
meteorites rare?

MG: People weren't open to the fact that meteorites from Mars could be here, so
we weren't looking for them. You've also got to remember that in 1979, the study
of meteorites was an immature science. It's only since around 1960 that the
study of meteorites and cosmo-chemistry started to develop. The last of the
Apollo materials arrived in 1972. Then after Apollo, in the early 70s, people
started studying extraterrestrial materials seriously. Collection of Antarctic
meteorites began on a regular basis in '76 or '77, although the first ones were
collected in 1969. So the subject of meteorites is still quite young.
Astrobiology is a baby in comparison with meteoritics, but meteoritics is still
not as mature as the wider science of astronomy.

People were still getting to grips with the idea of different asteroidal parent
objects, our theories of solar system formation, and formation of planets from
solar nebulae. We were arguing whether our solar nebula was hot and homogeneous
all the way through, or whether there were patches where you'd expect
compositional differences. There was not an understanding at all of the dynamics
of how the asteroid belt evolved. There was no recognition of the fact that we
got meteorites from specific asteroids like Vesta, no recognition of the
dynamics of interplanetary transport.

The idea of meteorites from Mars had been kicked around a little bit, but was
not thought to be possible. People said, "If we got them from Mars, we also
should get them from the moon," and we'd never seen any lunar meteorites. But
then the first lunar meteorite was collected in 1981, and was recognized in 1983
as being lunar, so that argument went away.

People started getting more precise dates for meteorites, and there was one that
consistently came out as a young crystallization age, and it had gases trapped
in it that were consistent with the martian atmosphere. So it had to come from
Mars. A paper on that was written in 1983, at the same time papers were coming
out about the lunar meteorite.

It's not surprising that it took time, but once the idea was floated, people
almost unanimously agreed that we must have a meteorite from Mars. People
unanimously agree that we have meteorites from the moon, but there are still
dissenters on the Mars rocks.

Related Links
Received on Tue 07 Nov 2006 01:00:32 AM PST