Mars Shown To Be Pear-Shaped
By PAUL RECER
AP Science Writer

WASHINGTON (AP) _ A global study of Mars shows that the red planet is shaped like a pear, with towering volcanic mountains in the south, and a smooth lowland in the north that may once have been an ocean. A three-dimensional map of Mars, drawn from measurements taken by the Mars Global Surveyor program, shows that the planet is a land of extremes, with the highest, lowest and smoothest land forms found in the solar system. There's about 19 to 20 miles difference between the highest and the lowest points on Mars, about 1{ times the range of elevation seen on Earth, said David Smith, a NASA scientist and the lead author of a study appearing Friday in the journal Science. Smith said that an asteroid or other space rock smashed into Mars billions of years ago, blasted out a crater deeper than Mount Everest is high. The crater, called Hellas, is deeper than any other in the solar system, said Smith. If it had been gouged out of the United States, he said, it could stretch from the East Coast to the Rocky Mountains _ about 1,500 miles. The rock and dust blasted from the crater would be enough to cover the United States with a blanket up to two miles thick, Smith said. On the other extreme is a volcanic mountain, called Olympus Mons, that soars almost 17 miles above the average elevation of Mars, making it the highest volcano in the solar system. In the northern hemisphere, there is a huge basin that dips down to about six miles below the average elevation of Mars. Maria Zuber, a professor at Massachusetts Institute of Technology and a co-author of the mapping project, said the basin ``is consistent with the formation of a past ocean.'' Whether or not Mars ever had enough water to fill such a basin is still unconfirmed, she said, although the area is smooth and featureless, as if sea lapped for an eternity. ``The north basin is the smoothest surface known in the solar system,'' said Zuber. ``Any water on Mars would have flowed into the area that is low and smooth.'' Despite this, she said, ``we have not detected any beach feature,'' or area that appears to have once been the edge of an ocean. Zuber said Mars probably has quite a bit of water, trapped beneath the north and south poles as ice. She estimated that the combined deposits of water at the two poles would equal about 1.5 times the water frozen in the glaciers and lakes of Greenland. That quantity, said Zuber, would equal about 40 percent of the amount of liquid water estimated to have once flowed on Mars. She said much of this water was probably ``lost to space'' over the 4 billion-year history of the planet. Data for the Mars map was collected by an instrument called the Mars Orbiter Laser Altimeter. The device aboard the orbiting Global Surveyor spacecraft zapped the Martian surface with a laser beam and then calculated the time for the light to bounce back to a receiver. The device uses this time interval to determine how far the light traveled and, thus, the elevation of the surface below. The accuracy was estimated at six feet over flat, smooth surfaces, and about 42 feet elsewhere. More than 27 million measurements were taken for the current map, but Smith said that the map will be refined further because the instruments are still taking readings. Smith said that data from the evolving map will be used to find future Mars landing sites for robot spacecraft. The first use will probably come in 2003 when a lander is set to touch down near the south pole.