National Geographic : 2013 Jul
40 national geographic • JULY 2013 while I miss and whack the hand that holds the chisel. Band-Aids and time usually solve the problem. On Mars we really don’t want the drill or the percussion hammer to hit the rover, ever. The arm was built with as little slop as possible in the joints, and those thousands of lines of software were checked and checked again—but we still didn’t know exactly how it would all work on Mars until we tried it. For one thing, gravity there is about one-third as strong as it is on Earth. And so the dozens of activities we had practiced already in California, we practiced again on Mars, in very small steps. If working on Mars weren’t so amazing, it would be enough to make you scream sometimes. But after six months we were ready to drill a rock. So what is this precious powder we come in search of, like early explorers to the Spice Is- lands? Curiosity is looking for evidence that life could once have existed on Mars—for environ- ments that could have supported microbes and for organic molecules the microbes might have made. We’re not searching for life itself; that would take instruments even more advanced than Curiosity’s. Its job is to help us figure out where a future mission should look for life. A habitable environment includes three im- portant ingredients: water, a source of energy, and the chemical building blocks of life, such as carbon. Earlier missions proved that Mars was once wet. Orbiters photographed ancient river valleys; rovers found minerals that contained wa- ter in their crystal structure. Curiosity is testing for the other two ingredients of habitability. Since the surface of Mars today is not hospitable, we’re hunting for ancient rocks that preserve records of a wetter, more Earth-like environment. We’re expecting to find such rocks in the stacked sedi- ment layers of Mount Sharp, at the center of Gale crater. But we stumbled on some not far from our landing spot, and so we’re drilling there first. We have to drill to find the good stuff. Drill- ing gets at material inside the rock that is less degraded and more likely to contain a faithful record of an ancient environment. From the study of Earth’s ancient environments, the major focus of my research for more than two decades, I’ve learned how difficult it is to discover such a record—and especially to find organic molecules that may have been made by ancient organisms. Even on Earth, which we know was teeming with microbial life billions of years ago, we find the traces in only a few locations. The paradox is that water, an essential ingredient for life, can also destroy organic carbon molecules. In just the places where we might look for life, places where water has flowed through sand or silt, pre- cipitating minerals that bind the particles into rock, the water has often erased the organic trac- es of life—with rare exceptions. On Earth we’ve learned how to hunt for those exceptions. It’s a long shot, but we’re hoping Curiosity will find organic molecules on Mars. They can be made by nonliving processes too, so finding them wouldn’t prove there was once life on Mars. But it would tell us where to look. We’ve already proved, with the first rock we drilled, that Mars was once habitable. A flat mudstone, shot through with veins of a mineral that formed in water, the rock looks like some- thing from a mining district. Curiosity’s analysis showed that the water was not too acidic for life—it would have been drinkable. It contained sulfur compounds that on Earth are an energy source for some microbes. It contained a carbon Caltech geologist John Grotzinger is chief scientist for the Curiosity mission. Fieldwork on Earth has taken him to every continent but Antarctica. From the day we landed, this place looked different from all the others we’d visited on previous missions to Mars. The images reminded us of home.