• Dick writes about Earth and planetary science for Science magazine.

Geologic Enigma Is Target of Next Mars Rover

22 July 2011 2:17 pm

Mar's Gale crater.

With the Smithsonian Institution's National Air and Space Museum providing a suitably themed backdrop, NASA officials announced today that its next Mars rover, Curiosity (a.k.a. Mars Science Laboratory), will be sent to 155-kilometer-wide Gale crater next November to try to find out if life has existed or ever could have existed on the red planet. The $2.5 billion mission will be gambling on both new technology and a landing site still cloaked in considerable mystery.

As many had expected, NASA announced that Edward Weiler, its associate administrator for space science, had decided to send the 900-kilogram Curiosity to land beside a 5-kilometer-high pile of layered sediments in Gale crater. That was the site preferred by the mission's top scientists, engineers, and managers after a 5-year site-selection process that also involved the planetary science community. But unlike the landing site-selection process for the two previous Mars rovers, Opportunity and Spirit, this time participants in the publicly open part of the process were not asked by NASA to pick a favorite among the four sites that remained after they had whittled the list down from 60 proposed sites.

Although any one of the four finalists would have worked out, Gale has a combination of attributes that gave it an edge, says Curiosity's project scientist, John Grotzinger of the California Institute of Technology in Pasadena. Participants in the open part of the selection process had tended to split on the question of what makes for an ideal site. Some favored sites more likely to show signs of abundant water-altered minerals, which point to past environments that could have hosted life. But sites with a simple, well-understood story of how they had formed geologically offered scientists reassurance that they can make sense of the landscape when the rover gets there. That would help to look for the mission's other prime target, organic matter left by ancient life.

Gale was the only site with some of both desirable attributes, Grotzinger says. Being one of the lowest spots on the planet, Gale almost certainly held a lake early in Mars history, he says. And the huge mound of sediments is nicely layered, so Curiosity can rove up it, sampling several different ancient environments marked by a variety of water-altered minerals. "We can sample three different habitable environments," Grotzinger says. "None of the other sites has that."

The catch is that Gale's geologic story is far less clear than its closest competitor, Eberswalde crater. Unlike Eberswalde, there are no definitive geologic signs that Gale ever held a lake. And no one can say how the layered mound formed—in a lake or not; by streams bringing in sediments; or by winds bringing in dust, volcanic ash, or impact debris. "I'm scratching my head as to what the heck it is," says planetary scientist John Mustard of Brown University, the immediate past chair of NASA's Mars Exploration Program Analysis Group. "There are a bunch of enigmas about how it formed."

Enigmatic or not, the site selection announced today will be pivotal for NASA's Mars exploration program. If Curiosity finds organic matter left by ancient life, missions will no doubt revisit Gale in the planned 3-decade-long effort to return samples from Mars at a cost of tens of billions of dollars. If organic matter isn't found this time, the next rover will have to start from scratch with another type of site.

While NASA is taking a perhaps inevitable gamble on the single site it must choose, the agency is also laying bets on new technology. Curiosity has a new system for entering the martian atmosphere under navigational control, descending on a parachute, and then landing on the rover's wheels while dangling at the end of cables. That makes for quite the gee-whiz video, but the new technology is essential for precision landing of Curiosity and all future landers. And several instruments on the rover, like the chemical lab that laser zaps rocks from a distance, are first-time experiments whose success could be crucial to future sample return missions.

Many of the technology questions will be answered in August of next year when Curiosity is scheduled to land on Mars. Then at least 2 years of scientific exploration would begin.

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