On Tue, 20 Dec 2005 07:21:54 GMT, "Dave"
wrote:

When I look at the surface of a planet or moon I see only round craters.

Not all are circular. Messier A & B on the Moon are elliptical.
Large imapcts come with a lot of energy that has to go somewhere and
it goes into heating the material at the site rapidly to high
temperatures, making an explosion that leaves a circular crater.