Title: Why is the Southern Hemisphere of Mars is heavily struck by asteroids while the northern part is relatively smooth?
Tags: mars;planetary-science;asteroid;perseverance
Question: The northern hemisphere of Mars is smooth with flats lands but areas particularly below the Tharsis rise is heavily cratered, also Hellas basin, one of the largest impact craters is located in the Southern Hemisphere. My question is WHY only the Southern Hemisphere?

Comment: [This question on the astronomy SE might be of help](https://astronomy.stackexchange.com/q/26349)

Comment: Instinctively thought because of different ages, craters possibly hidden under lava just like on the moon. But when starting to search I realized it may be more complicated, possibly involving a huge impact. Sounds familiar :-) ? Am looking forward to the answer !

Comment: Don’t have time for a referenced answer, but I believe the popular theory is that a single huge impact basically left the Northern hemisphere as one huge crater wiping out all cratering From earlier smaller impacts.

Comment: It seems the northern basin (Borealis basin) hypothetically may be a combination of a huge impact and subsequent massive volcanism that may covered post-impact craters ...

Comment: This is just an idea. If Mars once had an ocean in its northern hemisphere, as is speculated,  any meteorites that would have hit the northern hemisphere during this period may not have survived hitting the surface of the ocean. In hitting the ocean much of the kinetic energy any meteorites had would have been lost during impact & thus would not have formed impact craters on the ocean floor.



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