Ancient pool. The Gusev Crater site.

Two Sweet Spots on Mars

Dick writes about Earth and planetary science for Science magazine.

In the end, recommending places for NASA to land its two Mars Exploration Rovers next year turned out to be a no-brainer. In a meeting last week near NASA's Jet Propulsion Laboratory in Pasadena, California, planetary scientists agreed that two--and only two--sites stood head and shoulders above the rest. Although one of the selections has some lingering safety issues, the alternatives were too often “big, flat, ugly, and boring.”

Planetary scientists have pored over 185 potential landing sites for more than 2 years looking for technically practicable, reasonably safe, and scientifically interesting choices. In accordance with NASA's “follow the water” strategy for ferreting out sites where life may have existed, the workshop attendees came down solidly in favor of the Terra Meridiani (now called Meridiani Planum) and Gusev Crater sites (Science, 10 May 2002, p. 1006), according to John Grant of the National Air and Space Museum in Washington, D.C., co-chair of the workshop.

New data helped clinch the case. Evidence gleaned from 30-year-old spacecraft data suggests that the hematite spotted from orbit at Meridiani Planum has an aqueous origin--perhaps an ancient hot spring. And new imaging from the Odyssey spacecraft alleviated concerns that all the deposits on the floor of Gusev, where water pooled billions of years ago, might now be covered by deep dust or volcanic ash. Small impacts have obviously blasted out debris that a rover could inspect. While safety concerns have not gone away--the crater floor looks rougher and windier than ideal for landing--the certainty of ancient waters sealed the deal for the scientists.

Safety and science worked against the two remaining alternatives. Doubts arose at the workshop about whether the windswept Isidis impact basin really would have water-washed rocks from the adjacent highlands, as hoped. The fourth potential target--sited in smooth, flat, and boring Elysium for its low winds--looks too inscrutable to merit the trip.

In early April, NASA's space science chief Ed Weiler will decide whether to take a chance on Gusev or play it safe and head to Elysium after all.

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