From Scientific American:
The Phoenix Lander may have dominated Mars news in recent weeks, but a new study performed here on Earth has turned up a whopper of a finding: The Red Planet seems to have been the victim of a massive hit and run more than four billion years ago. That is the conclusion of researchers who have finally mapped the edges of something known as the Martian hemispheric dichotomy. That feature—in which the crust thickness drops from 30 to about 10 miles (50 to 20 kilometers) over a large area that is the most visible feature on Mars—has been known to astronomers for more than 30 years and was long suspected to be due to an asteroid impact that flung most of the crust out the area.
Scientists could not say for sure, however, because the dichotomy’s exact shape was unclear: As much as a third of its edge was obscured beneath a 20-mile- (30-kilometer-) high pile of volcanic rock (the second largest feature on Mars) known as the Tharsis Rise. To uncover the dichotomy’s true edge, researchers from the Massachusetts Institute of Technology (M.I.T.) and NASA’s Jet Propulsion Laboratory in Pasadena, Calif., used geologic data to probe the structure of the crust underneath Tharsis. They combined data on the surface height, or topography, with variations in mass revealed by disparities in the surface’s gravitational force, looking for telltale changes in mass under Tharsis. The analysis revealed an elongated round shape measuring about 6,600 by 5,300 miles (10,600 by 8,500 kilometers) and covering 42 percent of the planet. The team calls it the Borealis basin.