[meteorite-list] Seeking Out Meteorites (Monica Grady Interview)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon Nov 6 12:49:46 2006
Message-ID: <200611061749.JAA17744_at_zagami.jpl.nasa.gov>

http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=2136&mode=thread&order=0&thold=0

Seeking Out Meteorites
Astrobiology Magazine
November 6, 2006

Summary (Nov 06, 2006): In this interview, Monica Grady talks about hunting
for meteorites on the moon in order to learn more about the ancient Earth.
She also explains why it took so long for us to find a meteorite
originating from Mars.

Seeking Out Meteorites

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.

MAC88105 -- A lunar meteorite found in 1989. Its origin has been
confirmed by comparing its composition with lunar rock brought back by
the Apollo astronauts. The collision that blew this rock off the moon
most likely occurred within the last 10 million years.Credit: NASA

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.
Received on Mon 06 Nov 2006 12:49:44 PM PST