 Giant crater explains strange shape of Mars
(China Daily)
Updated: 2008-06-27 07:41 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 yesterday'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 8,500 km across and 10,600 km long - the size of Asia, Europe and Australia combined. It is believed to be four times bigger than the current titleholder, the South Pole-Aitken basin on Earth's moon. Astronomers have long puzzled over Mars' landscape ever since images beamed back in the 1970s showed different-looking halves. An orbiting spacecraft later observed the northern lowlands were on average two miles (3 km) lower than the southern highlands and had a thinner crust. Scientists who had no role in the studies said the latest research strengthens the case for a colossal Martian impact, but it does not rule out the other theory that hot rock from inside the planet could have welled up and formed the different crusts. The idea of an ancient impact was first advanced by Steve Squyres of Cornell University and Don Wilhelms of the US Geological Survey in the 1980s. Squyres, currently the lead scientist for the twin Mars rovers, had always hoped other scientists would "pick that ball up and run with it." For one study, a team of scientists from the Massachusetts Institute of Technology and NASA's Jet Propulsion Laboratory recreated what the Martian surface would have looked like before the volcanoes formed using gravity and surface measurements from spacecraft. They determined the impact basin is oval-shaped, similar to what would be expected if a space object had hit at an angle. "The shape is really one of the key pieces of evidence that it was probably formed in a giant impact," said MIT postdoctoral researcher Jeffrey Andrews-Hanna, whose original "gut feeling" favored the other theory. A separate group led by the California Institute of Technology developed 3-D simulations to determine the "sweet spot" of conditions that would form the basin. In the third study, a team of researchers led by the University of California, Santa Cruz, found that shock waves from such an impact would disrupt the southern crust. All three teams believe there was a single giant blow and not several small hits because there's no evidence of other basins. Agencies (China Daily 06/27/2008 page10) |
