The hypothesis, I believe, is that the stuff in Allan Hills 84001 was fossilized back when Mars was wet but it was only ejected a few million years ago. The im

The hypothesis, I believe, is that the stuff in Allan Hills 84001 was fossilized back when Mars was wet but it was only ejected a few million years ago. The important part is that the inside of the rock never got too hot. As long as there are bacteria which can take being frozen for long periods of time they ought to be able to survive. As to larger impacts I really can't speak with any authority and I'm not aware of any studies which would say. But my intuition would be that the greater volumes in large the ejecta from large impacts would heat up less due to more insulation. I should probably put this on /r/askScience and see if anybody knows more.