03 September 2004

Why are impact craters almost always circular?

Editor,

I would appreciate learning why planet surfaces are pocked with craters but with few if any gouges. Why are there so few glancing impacts, which I want call "gouges," by meteors and asteroids visible on surfaces of planets?

Intuitively it seems that not all near passing bodies would be drawn down to direct impact creating the symmetrical craters that we do see. It seems logical to assume that there must be loose bodies that are passing at an angle and speed to hit with a glancing, gouging and non penetrating blow. Will appreciate dialogue.

John Hu

This is a good question for a web search, which quickly revealed the reason. For example, http://cse.ssl.berkeley.edu/AtHomeAstronomy/activity_05.html discusses this phenomenon: "One would normally expect the crater to have an oblong shape on extremely wide-angle impacts. In fact, all craters that we have seen on the Moon and Earth are pretty much circular. The reason is that an explosion occurs on impact and the forces associated with an explosion are always spherically symmetrical." There are exceptions to this rule. For example, oblong craters can occur when ejecta from an impact explosion strikes the surface at a glancing angle and with a velocity too low to cause the object to explode. The size of the impactor doesn't seem to matter. For example, studies of microscopic craters formed by micrometerorites in the surfaces of various satellites that have been retrieved from orbit reveal circular impacts. Editor.