Scientists think big impact caused two-faced Mars
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 creating a giant hole over 40 percent of the surface.
If there ever was a civilization on Mars, this would have made mincemeat of it.