Seeking Out Meteorites
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.”
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?
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?
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?
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.
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.
Read part 2 of this interview.