Elongated craters

[JohnD]

The latest Messenger pic shows several intersecting fields of secondary crater chains, many of which are elongated craters.
http://messenger.jhuapl.edu/gallery/sci ... 654912.png
I understood that the impact of a high speed object,typically one coming down from an infinate distance, always gave a circular crater even from very glancing impacts.
But secondary craters, now that we can see many more than we sued to be able to see, often show elengated craters.
Is there an explanation?

Could it be:
1/ resolution of imaging insufficient to show that the elongation due to two or more close ciruclar craters?
2/ Impacts from secondary ejection debris are somehow different?
2A/ because slow speed impacts are different?
2B/ because the object is molten, from the original impact?

Or something else!

John

[Chris Peterson]

I understood that the impact of a high speed object,typically one coming down from an infinate distance, always gave a circular crater even from very glancing impacts.
But secondary craters, now that we can see many more than we sued to be able to see, often show elengated craters.
Is there an explanation?

Hypervelocity impacts (that is, where the speed of the impactor is much greater than the speed of sound in the impacted surface) produce circular craters for all but very shallow collision angles. For the Moon, that angle is around 15° from horizontal. Naturally, crater chains tend to be the product of very shallow strikes, so it isn't surprising that they often show non-circular crater shapes.