2CUL2NAME

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 Thursday's journal Nature provide the most convincing evidence yet that an outside force was responsible. According to the researchers, an asteroid or comet whacked a young Mars some 4 billion years ago, blasting away much of its northern crust and crea...