The University of Arizona
The Evolution of LPL: 1973-2000



Ground-Based Research
The Department   Graduate Students   Spacecraft Missions  

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.