A Senate subcommittee asked for reasons to support sending humans to Mars, and, boy, did they get one from Ellen Stofan, NASA's former chief scientist.

Stofan, who now leads the Smithsonian's National Air and Space Museum, argued that if we truly want to find and understand any potential traces of ancient life on the Red Planet, robots can't do it alone — we'll need humans on the ground.

"While I'm optimistic that life did evolve on Mars, I'm not optimistic that it got very complex, so we're talking about finding fossil microbes," Stofan told a Senate subcommittee devoted to science issues on Aug. 1, adding that those fossils would be incredibly hard to find. [The Search for Life on Mars (A Photo Timeline)]

"That's why I do think it will take humans on the planet, breaking open a lot of rocks to try to actually find this evidence of past life," Stofan said. "And finding one sample is not good enough; you need multiple samples to understand the diversity."

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