NASA/JPL-Caltech/Cornell Univ./Arizona State Univ.

ScienceShot: A Momentous Crossing on Mars

Dick writes about Earth and planetary science for Science magazine.

Today, scientists announced in a NASA teleconference that the veteran Opportunity Mars rover can now make a unique claim: Its 36 kilometers of roving has carried it across the most important geologic transition in the history of the Red Planet. Team members reported that analyses of a third-of-a-meter-long rock that they call Esperance (rendered here in exaggerated color) is rich in a sort of clay that could only have formed when lots of neutral-pH water bathed martian rock. Up to now, Opportunity had been finding only ancient rock leached by acidic brines far more hostile to life and ill-suited to nurturing the origin of life. So the rover has now sampled both sides of the momentous planetary transition from a wet, benign environment more than 4 billion years ago to a colder, drier, harsher one since then. NASA's Curiosity rover arrived on Mars last August to examine the same transition preserved in Gale crater but won't reach its target rocks until this time next year.

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