Did a giant impact create the two faces of Mars?

The impact of a giant asteroid could explain why Mars has two very different faces. The northern hemisphere is much flatter and lies lower than the southern hemisphere, with a difference in elevation between the two of about 5 kilometres. In the 1980s, scientists suggested a giant impact by an asteroid about 300 kilometres across in Mars's early history could have led to a permanent depression in the planet's northern hemisphere. Reasearchers using computer simulations found that such an impactor would produce huge amounts of lava – enough to cover the planet in an ocean of molten rock somewhere between 14 and 48 kilometres thick. That would have ended up erasing any record that an impact happened in the first place. But simulations carried out by other teams suggest the giant impact hypothesis could still do the job if it struck only a glancing blow. One that would produce much less heat and therefor much less lava flow. n this scenario, the total amount of lava produced is equivalent to a 5-kilometre-deep layer distributed over the whole planet. This would be small enough to avoid erasing the depression.

submitted by Shaun A. Saunders