Where are the impact craters?
SEISMIC tremors caused by small impacts could disturb the surface material on a comet enough to “fluff it up”, burying or even destroying any craters or other features and creating the smooth plains, suggests Laurence Soderblom of the US Geological Survey, who was a member of the Deep Space One team. “The gravity is so low on a comet that it wouldn’t take much to move the surface material around,” he says. But if that’s the case, why do two craters survive on Tempel 1? “That’s part of the mystery that we have to solve. Perhaps they are not old but young craters,” says Soderblom. Comment: As explained earlier, the dust from Tempel 1 is best explained as electrically sputtered rock particles. The material in the comet jets is not unaltered surface material. Past observations of the presence of negative ions of oxygen and forbidden spectral lines are both evidence for electrical activity at a comet’s surface. The two craters strongly suggest that Tempel 1 is rocky. They also argue against
