Astronomers have long known that the northern hemisphere of Mars was low and fairly flat, while the southern was higher, rougher, and heavily cratered. Explaining how that came to be, however, has been a problem.
New studies suggest an extremely violent cause. A rock one half to two thirds the size of Earth's Moon slammed into Mars about 3.9 billion years ago, creating what the proponents of this theory say is the largest impact basin in the Solar System, covering about 40 percent of the surface. It is so large, they say, that no one recognized it for what it is until recently. Computer models of both the theoretical impact and the internal structure of Mars seem to support the impact hypothesis.
If this new theory is correct, Mars was slammed at roughly the same time a Mars-sized object probably collided with Earth. The ejecta from that collision eventually formed the Moon.
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