Now, laboratory modeling -- simulating high-speed impacts that might have occurred on Mathilde -- shows that the craters could have been punched in like dimples into a Styrofoam ball rather than excavated out.
Researchers at the Boeing Company's Shock Physics Group in Seattle, Washington have found that under certain conditions, craters form in porous material by compressing it, rather than throwing it out. The research, lead by Kevin Housen, a research fellow at Boeing, is published in this week's issue of the journal Nature.
One of the unique characteristics of porous materials was noted in the 1950s when the United States and Britain were testing atomic weapons on coral atolls in the Pacific Ocean. Craters in these very porous corals formed essentially by being crushed up as shock waves passed through. In these craters, virtually all the material was compacted, not ejected.
This inspired the idea, Housen said, that perhaps the same kind of collapse might occur with impact craters on Mathilde if it really is very porous.
"Instead of forming a crater by digging a hole out and throwing the material around the crater you basically just squish it up, compact it and compress it with very little ejecta being formed," he said.
To test this idea, Housen and his colleagues used a high-speed centrifuge with a hyper-velocity gun mounted in the center. Spinning the centrifuge to produce an artificial gravity 500 times that of Earth, the team shot small nylon projectiles at a target material that had the same density as Mathilde.
If you look at the physics of how craters form," Housen explained, "and want to conduct a small-scale experiment that correctly models a very large crater on Mathilde, you have to form the crater at elevated gravity. You want to reproduce the weight of material that is being excavated on Mathilde."
Boosting the gravity makes a small amount of material behave like it has much larger mass.
"You can think of it as a wind tunnel for impact cratering," Housen said. "People test the aerodynamics of how airplanes work by making very small scale models. But they adjust properties of the fluid -- the air that's flowing over them and the velocity so that they get things simulated in the right way."
"By doing this experiment at 500 times normal gravity we can simulate directly one of these large craters that form on Mathilde," he said. The craters do indeed form without ejecta blankets.
The holes on Mathilde appear to have been formed right next to one another, without affecting each other at all. Joe Veverka, who leads the imaging team for the NEAR spacecraft, said this kind of impact modeling is needed to explain Mathilde's curious craters.
"We would expect a huge, catastrophic collision of this kind to cause major modification to the craters nearby, but we don't see this happening," said Veverka, who is chair of the astronomy department at Cornell University.
These very localized effects of large impacts suggest that Mathilde is made of something that dampens the shock of impact, "like a body made of shock absorbers," Veverka said.
People have suggested that a very porous body might be able to dampen the shock of impact so that one crater might form next to another without causing major landslides, Veverka said. This explanation has never been tested with experimentation, however.
While he has not seen the results of Housen's work, Veverka said that this type of study is key to determine how Mathilde's craters formed.
The craters formed in Boeing's centrifuge also match Mathilde's craters in this sense. In the porous material, the effects of craters remained very localized. Craters could form right next to each other without disturbing one another, Housen said.
The impact craters Housen's team produced do remarkably resemble those found on Mathilde, but some scientists who have studied the asteroid worry that their explanation for the craters may raise more questions than it answers.
William Merline, an astronomer who studies asteroids at Southwest Research Institute in Boulder, Colorado, has his doubts.
"It's a little hard to believe that you keep compressing the thing and it still has a density of 1.3 (grams per cubic centimeter)," Merline said. "It would seem like you would have to start with an even lower density body."
Mathilde's low density is hard enough to explain, Merline said. It is even harder to imagine a body so porous that its density is the same as water. (Water has a density of 1 gram per cubic centimeter.)
Housen, though, thinks that might be possible. "If you calculate how much Mathilde should have compacted just by the formation of those large craters that you see, it turns out that the density might have been about 20 percent lower than what it is now."
An earlier Mathilde, uncompressed by cratering, would have had a density very close to that of water.
"People tend to think that when the solar system formed, when you have gas and dust settling down in a very quiescent kind of environment, you may tend to build up these fluffy kind of objects," he said. "Unless you get something that's fairly large -- much bigger than Mathilde -- it won't have enough self-gravity to really squish itself up much."
Mathilde might be an early step in the evolution of heavier asteroids, Housen suggests. Perhaps after a few more billion years of swinging around the solar system and being pounded and compressed, it may come to resemble the more dense, less porous rocks that have landed on Earth as meteorites.
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