Scientists think big impact caused two-faced Mars AP

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 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.
New calculations reveal the crater known as the Borealis basin measures 5,300 miles across and 6,600 miles long — the size of Asia, Europe and Australia combined. It’s believed to be four times bigger than the current titleholder, the South Pole-Aitken basin on Earth’s moon.