WHAMM!

Four billion years ago Mars was hit by an asteroid that thinned the crust over 41% of its surface from 30 miles to 10 miles.

http://www.sciam.com/article.cfm?id=giant-asteroid-flattened&sc=rss




Video of the impact via Cheryl Morgan:

http://www.nature.com/nature/newsvideo/mars-impact.mov