Enlarge Jeff Andrews-Hanna This artistic representation of a giant impact on Mars was created from simulations by Marinova et al. (Nature, 2008). Mars is shown using a combination of Viking color images and shaded relief from the Mars Orbiter Laser Altimeter (MOLA). FOR MORE INFORMATION FOR MORE INFORMATION News from SPACE.com Science and space news on USATODAY.com An asteroid the size of Alaska slammed into Mars about 4.4 billion years ago, creating a 70,000-trillion-kiloton blast that forever deformed the Red Planet, suggest three studies out Wednesday. "It was a bad day for any Martians when this happened," says planetary scientist Francis Nimmo of the University of California-Santa Cruz, lead author of one of the studies in today's Nature journal. PHOTO GALLERY: This week in space Half the size of Earth, Mars has a curious split appearance, featuring a largely smooth northern plain and mottled southern highlands. Since close-up mapping of Mars in the 1970s, scientists have debated the origins of the split, with massive volcanism seen as the most likely culprit for the planet's northern lava plains. A few others scientists, including Mars rover expert Steve Squyres of Cornell University, suggested an asteroid impact instead. In one of the new studies, a team led by MIT's Jeffrey Andrews-Hanna sought to settle the debate by using gravity observation to electronically remove volcanic deposits — 18 miles of basalt — obscuring the original shape of the northern plains. The work revealed an impact basin 6,600 miles wide, "very smooth and regular and elliptical," says Andrews-Hanna. "It was not at all what we expected to find. The impact idea was pretty unpopular." Earlier gravity measures had shown the crust of Mars was thinner on its northern half, triggering the impact analysis led by Nimmo and another led by Caltech's Margarita Marinova. Both agreed that a roughly 1,500-mile-wide asteroid striking Mars' northern hemisphere at an angle would explain the crustal difference. "The tremendous melting generated by the impact would have very definitely generated the southern highlands," as the blast dumped rock from one side of the planet to another, Nimmo says. More proof of the blast could come from low-altitude gravity measurements of Mars' southern hemisphere, Nimmo adds, which would reveal eggshell-like cracks in the southern hemisphere's crust left over from the impact. "As much as I love this idea, I don't consider it proven by these reports," says Squyres, a planetary scientist at Cornell. "They strongly support the idea, but it's a hard thing to definitively prove if you are not there." If proven, northern Mars would contain the largest impact basin in the solar system, says Andrews-Hanna. "Essentially, it hit the reset button on life on Mars." The Mars impact came about the time a similar strike on Earth by an asteroid about half the size of the planet led to the creation of the moon. "The early solar system was a very dangerous place to be a planet," says Andrews-Hanna. Guidelines: You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. You share in the USA TODAY community, so please keep your comments smart and civil. Don't attack other readers personally, and keep your language decent. Use the "Report Abuse" button to make a difference. Read more