Recent evidence suggests the vast disparity seen between the northern and southern halves of the planet is caused by the long-ago impact of a gigantic space rock into Mars. … The northern hemisphere of the planet is smooth and low, and some experts think it may have contained a vast ocean long ago.
Meanwhile, the southern half of the Martian surface is rough and heavily-cratered, and about 2.5 miles to 5 miles (4 km to 8 km) higher in elevation than the northern basin. … Nimmo’s model showed how shock waves from the impact on the northern hemisphere would travel through the planet and disrupt the crust on the other side, causing changes in the magnetic field.
“The impact would have to be big enough to blast the crust off half of the planet, but not so big that it melts everything,” Nimmo said. “We showed that you really can form the dichotomy that way.” – FOX