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Lady Luck Watches Over Mars Rovers
by Phil BerardelliScience & Technology Editor The spectacular success of NASA's twin Mars rovers is due obviously to the skills and determination of the scientists and engineers involved in the mission, but luck also contributed to an amazing degree. For months, Spirit and Opportunity have been pouring forth unprecedented images and data that show, nearly beyond a doubt, the red planet once supported liquid water. Where only a year ago the possibility of a wet Mars remained just that, a possibility, now scientists are building a fairly strong - if unconfirmed - case that living Martian organisms just might be active. At first glance, it seems a natural progression: The twin rovers, which landed in January 2004, got rolling across the Martian soil and, using their formidable arrays of instruments, promptly began transmitting data that established the solid case for a once-wet Mars. Not quite. Stephen Squyres of Cornell University, principal scientist for the rover missions, disclosed Lady Luck's contribution to the process. "Our best science occurred after 90 sols," Squyres said, referring to the designed operational lifetime of the rovers and the term used for the Martian day -- 24 hours and 37 minutes. Speaking first at a news briefing and then to hundreds of attendees at the American Association for the Advancement of Science annual meeting, Squyres related how the latest mission could have turned up the same negative results as the three previous vehicles that managed to land safely on Mars - the twin Viking landers in 1976 and the Sojourner rover in 1997 - if not for some serendipitous occurrences. When Spirit landed in the Martian equatorial region called Gusev crater and began grinding away at nearby rocks, the readings it sent back for weeks all showed basaltic rock. "We landed on lava," Squyres said, adding the disappointment among the mission team at NASA's Jet Propulsion Laboratory in Pasadena, Calif., was palpable. It also was understandable. Squyres showed the audience an aerial image of Gusev taken by NASA's Mars Global Surveyor spacecraft. It resembles a sperm, with a 90-mile-wide circular head and a long, winding tail. The tail, however, also looks like a channel carved by water - a channel running into the crater. The rover team chose Gusev as Spirit's landing site because it looked like a dry lake bed. Dry continued to be the operative word for Spirit for a long time. At sol 100 - or 10 past the rover's expected lifetime - the mission team decided to change strategy, Squyres explained. They looked at an elevated formation named Columbia Hills, in honor of the lost crew of the space shuttle, that lay some 60 sols' driving distance away. "We decided we could trust the vehicle (to get there) and hit the gas," he said. The gamble paid off. After sol 156, at the base of Columbia Hills, Spirit "crossed a geological boundary," Squyres said. The basaltic content was replaced with increasing values of magnesium and sulfates, which is the unmistakable signature "of some kind of aqueous process," he said. The RAT, or rock abrasion tool, aboard the rover found a much easier time grinding the rocks because they were softer. Spirit's instruments showed increasing amounts of phosphorus, sulfur, chlorine and bromine, and minerals called goethite and jarosite - all related to water chemistry. Since that time - Spirit recently passed 400 sols on Mars -- the golf-cart-sized rover has transmitted data showing one water-associated rock after another, including the fine layering and scalloping that indicate wave action. The conclusion: Gusev crater, sometime during its history, was wet. Had Spirit lasted only 155 sols, it would not have transmitted any water evidence.
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