Why is Mars two-faced? Scientists say fresh evidence supports the theory that a monster impact punched the red planet, leaving behind perhaps the largest gash on any heavenly body in the solar system.

Today, the Martian surface has a split personality. The southern hemisphere of Mars is pockmarked and filled with ancient rugged highlands. By contrast, the northern hemisphere is smoother and covered by low-lying plains.

Three papers in yesterday's journal Nature provide the most convincing evidence yet that an outside force was responsible.