How the Red Planet Got 2 Faces

http://space.newscientist.com/article/mg19325965.300-how-the-red-planet-got-two-faces.html

Scientists now think that mars' lopsidedness could be explained by the fact that the planet was struck by an asteroid. Computer models show that an asteroid could have caused the depression in the northern hemisphere if it struck the planet in a "glancing blow". It was determined that it could not have been a direct hit because that would have caused massive lava flows that would have erased any sign of an impact.