June 25, 2008 -- 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. The stark contrast between the hemispheres was first revealed by the Viking missions of the 1970s, which found low-lying plains in the north and older, heavily cratered highlands in the south. Later studies showed the southern hemisphere had thicker crust than the north as well as some odd magnetic readings. "Two main explanations have been proposed," said Francis Nimmo with the University of California, Santa Cruz. "Either some kind of internal process that changed one half of the planet, or a big impact hitting one side of it." The Skinny on Mars Phoenix |
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