Ground-Based Research, Page 4

Dolores Hill, on the K/T boundary

I helped out with K/T boundary samples. That was very, very exciting work, mostly by Alan Hildebrand. He was a very serious fellow who was quiet and kind. He would travel all around the world to all of the known K/T boundary sites, and bring back either chunks of rock out of a cliff, or material that he had removed from a cliff, in these little baggies. He would go to places that are very humid, so he’d have to leave the baggies open to let the moisture out. He would separate those out into different strata, and actually analyzed those samples. He developed a radiochemistry technique to analyze the siderophile elements, which are the metal-loving elements.

Even though in the popular press [Luis] Alvarez gets an awful lot of credit for identifying that there was high iridium in that layer, Alan—and also later David Kring joined him with Bill Boynton as his advisor—was actually the one to pull together information from a lot of different perspectives, and a lot of different data, to identify the actual site of the crater. I don’t think people really understand the whole story there, because, like I said, he was kind of a quiet person. He doesn’t look for credit, but he deserves it.

He analyzed all these materials, and the thing that confused people for so long was that the chemical signature indicated both an ocean impact and a terrestrial impact. It didn’t make any sense. One camp would say, “Well, obviously the big impact hit in the ocean, and we have no hope of finding a crater,” and the other said, “No, there’s terrestrial signature, it had to have been on land, it’s here somewhere.” It kind of bounced back and forth, and each ignored one side of the data.

He carefully looked at it all while this confusion was going on. He would attend conferences and a couple times people would come up and say, “I have these core samples that are kind of funny. We think we saw some interesting material there.” He’d take these core samples and put them out on the table, and he looked at those and realized that what was previously identified as volcanic soils was in fact impact.

Another person from an oil company contacted him one time and said, “There’s this funny circular depression. You might be interested in that.” That turned out to be the crater [the Chicxulub crater on the Yucatan Peninsula, Mexico]. It had hit half on the continental shelf and half in the ocean. Because of that, the material that was spread around the world had different signatures. So all these little bits of information he studied, magnetic anomalies, gravitational anomalies, just converged on this site. It was really fun, to see the whole process come together. His dissertation was exciting. It was just little bits of random information that came together.

Jay Melosh, on the SNC meteorites

When I came to the University of Arizona, a big problem had come up that I had been working on or considering. Round about that time, 1980, it was recognized that some very strange meteorites—the so-called SNC meteorites, shergottite, nakhlite, and chassigny—had come from a larger body. The proposal had been made that perhaps they came from Mars.

At that time, the best understanding of how impact craters worked said that there was no way to eject a rock at anything like Martian escape velocity and have it survive its flight. Gene Shoemaker said in no uncertain terms—I can still remember his voice echoing through the room—that it was absolutely impossible to get a rock off of Mars without it either melting or vaporizing. Yet these rocks had neither melted not vaporized, and it became more and more clear as 1980 turned to ’81, and ’81 to ’82, that they in fact really had come from Mars. ’83 was the clincher—dissolved atmospheric gases were found in one of the Martian meteorites that were an absolute dead-ringer for the Martian atmosphere. Basically all scientific resistance collapsed.

I was attracted to this problem where the best theory and the observations disagree, and started working on mechanisms by which the rocks could be ejected. In the end I recognized that it was the interaction of the shockwave in the rock with the surface of the planet that actually was responsible for ejecting the material into the upper atmosphere, a process I call spallation.

I worked on that pretty exclusively for the first couple of years I was here, 1982 to ’86 or so. I was working pretty hard on that mechanism by which rocks get ejected from their parent bodies. That’s now pretty much the accepted mechanism; we have a lot of direct observational evidence that that process is, in fact, what happens.