Ground-Based Research, Page 5
Dolores Hill, on meteorite research
The very first project was one where they had had some difficulties. It was to study compositions of individual chondrules and chondrule rims. We developed a technique that worked out very well I think. We would disaggregate or take apart meteorites and gently break the sample so that the chondrules would fall out, and then we tried to identify which ones might have a dark rim around it.
At that time it wasn’t known whether these were rims that had condensed onto the chondrule after it formed, or whether these just were particles that had been accreted onto the surface, or what. We wanted to find out. We would separate these chondrules and irradiate them. We’d send them to a nuclear reactor on campus at the University of Missouri, and when they came back, we would take the little chondrule and glue it to a push-pin, and grind it on a little piece of grinding paper. That would be our sample.
It was an ingenious technique—it wasn’t my idea—but the paper was not radioactive so it didn’t matter to us. We saw the sample that was deposited onto the paper. I would grind these little chondrules—they’re only about a millimeter, two millimeters across—on to a series of maybe four or five of these grinding papers, and then I would count those papers under gamma ray spectrometers. By looking at those we could tell what the outer layers of the chondrule were made out of. Then we popped the chondrules off of the push-pin and made little thin sections of them, and looked at them under an electron microscope.
At the same time, we were studying calcium-aluminumrich inclusions, otherwise known as CAIs. We made slices of this little thing on a special saw that we have, and then we broke off this rim area. We analyzed that through a radiochemistry technique that Bill Boynton developed to look for rare earth elements, a very critical group of elements. He pioneered that procedure.
We had a very famous person work with us as a post-doc, named David Wark. He’s since passed away. He actually studied the first rims on CAIs, and the rims are named after him: Wark-Lovering rims. He did several experiments that no one will ever repeat, because they were so difficult and tedious.
We were very privileged to work on the very first lunar meteorite that was discovered in Antarctica. It was called Allan Hills 81005. That was the first time we ever identified a lunar meteorite that had been ejected off of the Moon onto the Earth’s surface. We were able to do that because we had Apollo samples to compare it with. It was pretty exciting. We knew we should have these samples but we had never identified them. So that was a wonderful project, very exciting.
That also then, I think, laid the groundwork for, again, being privileged to study another meteorite called Calcalong Creek, which is a lunar meteorite in Australia. That one was really famous for a long time because it was, at the time it was discovered, the only meteorite not found in Antarctica from the Moon. There’s no good reason why they’re only found in Antarctica, so it was exciting to finally spread out. Since then many more have been discovered in the Sahara Desert. There should be more in other places, we just haven’t found them. We’ve worked on Martian meteorites as well.
Jay Melosh, on the origin of the Moon
While I was here, the idea that the Moon had been born of a giant impact came up. It was suggested in a meeting around ’84, and soon there was serious talk about how the Moon was made. I got involved with that, actually in collaboration with Chuck Sonett. I didn’t go to the actual meeting. I had some responsibility to give a lecture at the Flandreau Planetarium on Martian meteorites, so I couldn’t go to the meeting on the origin of the Moon which took place in Hawaii. But a number of people came through here on the way back from that meeting, and I heard about the speculations.
I decided right then and there that all the speculations were flat wrong. The proposals that had been made were transparently incorrect. I figured I could spend a weekend and write up a paper to get rid of all this nonsense.
In the process of that weekend working—it actually took place mostly on an airplane, I was going to another meeting—I realized that there was a way out. What people were talking about was patently not going to work, because the people who were talking about this didn’t understand impacts very well, but there was a way in which impacts could launch enough material to form the Moon. Furthermore, in the process of launching it you would have to vaporize most of the material, and if you condense that material in a vacuum it would have the chemical signature that accounts for the difference between the Moon and the Earth.
I ended up writing a paper. Chuck Sonett was the co-author. We wrote up this paper in which we actually agreed with the impact origin, and described in detail how it could happen, and the chemistry. That was published in the book that contained the proceedings for the conference, even though I didn’t go to the conference.