The Mars Dichotomy
Astronomers have known for decades that the northern hemisphere of Mars is very different than the southern hemisphere. The north is flat with few ancient volcanoes, while the south is characterized by highlands with many volcanoes. There are many theories to explain the stark difference, but the question remains unresolved.
Scientists are now proposing a new theory based on computer models. According to their model, all the features of Mars can be explained if early Mars were struck in the southern hemisphere by a massive body.
Giovanni Leone and his team, publishing in Geophysical Research Letters, show that if a body comprised of mostly iron, about one tenth the size of Mars, and traveling at five kilometers per second struck the Martian south pole, it would have released enough energy to cause a magma ocean to flow over the southern half of the planet. This magma ocean could have created the highlands and the intense volcanism that would have lasted for three billion years.
Leone also claims his model predicts when Mars lost its magnetic field, about 4.1 billion years ago.
The impact would have occurred over 4 billion years ago, adding to the overall mass of the planet and resulting in its current geological features.
This model competes with the older theory, first put forward in 1984, that Mars was struck near the north pole by a large object.
At present there is no definitive evidence to resolve the various theories to explain the current geophysical makeup of Mars. The new model is just that, a model. It can explain features we already know about, but the real test will come if the model can make predictions that have yet to be verified.
Interestingly, the Earth too was struck by a very large body (Mars-sized) in the early years of the solar system, and likely resulted in the Moon. The early solar system likely contained many potential planets, some crashing into each other, while others may have been flung from the solar system.
Only 8 planets survived in stable orbits, and many of them show signs of large impacts early on. It seems likely that Mars did not survive this period unscathed. The details of where it was struck and by what remain open questions, however.
Source: The 2 faces of Mars