Did a giant impact create the two faces of Mars?

Although it is obvious that the northern and southern hemispheres of Mars are vastly different, it is still unclear as to WHY this is the case. The northern hemisphere is much flatter and lower in elevation than the rocky southern hemisphere. Although it was previously believed that a giant impact by a 300-km asteroid may have left a huge depression in the northern hemisphere, a computer simulation shows that such an impact would have caused molten material to cover the entirety of the planet, erasing any signs of an impact crater. However, if the asteroid were to have been an indirect hit, then it is possible that a small signature of the impact would not have been erased by the lesser amount of lava produced. This could be why the craters on Mars are still visible in the northern hemisphere, while much of the other topography surrounding it is quite flat.

http://space.newscientist.com/article/dn11387-did-a-giant-impact-create-the-two-faces-of-mars.html