Mars Got Slammed by Object: Discovery News : Discovery Channel

Mars Got Slammed by Object: Discovery News : Discovery Channel: "About four billion years ago, scientists believe Earth got smacked so hard by an asteroid or comet that a piece broke off to form the moon. Apparently, things weren't much better on Mars.

Three related studies appearing in this week's edition of the journal Nature provide evidence that Mars bears the biggest scar in the solar system -- an oval-shaped, 6,200-mile long impact crater that shattered half the planet's crust, left its southern hemisphere 2.5 miles taller than the north and possibly created the largest known string of volcanoes.

'Mars wouldn't be the planet it is today had this not happened,' lead author Jeffrey Andrews-Hanna, with the Massachusetts Institute of Technology, told Discovery News."