The landing site of the Mars rover Curiosity was once covered with fast-moving and possibly waist-high water that could have possibly supported life, NASA scientists announced Thursday.
While planetary scientists have often speculated that the now-desiccated surface of Mars was once wet, Curiosity cameras provided the first proof that flowing water was present on a least one part of Mars for "thousands or millions of years."
The finding led Mars Science Laboratory mission top scientist John Grotzinger to conclude that Curiosity had already found a potentially "habitable" site -- a central goal -- well before heading to its primary destination. While the area may not have other attributes needed for life, he said, the team now has a "hall pass" on the question of flowing water, and the Gale Crater landing site seemed even more appealing.
"A long-flowing stream can be a habitable environment," he said. "We're still going to Mount Sharp [a 3-mile high mound at the center of the crater] but this is insurance that we have already found our first potentially habitable environment."
Curiosity team scientists determined that flowing water was once present near the Gale Crater site based on the telltale size, shape and scattering of pebbles and gravel, especially those found in conglomerate rocks at three different sites. The roundedness of the pebbles is especially significant, they said, and strongly suggest that the rocks were carried down a roughly 20- to 25-mile stream or river and were smoothed along the way.
William Dietrich, professor of geomorphology and member of the Curiosity imaging science team, presented some rounded earthly pebbles, which he said are similar to those found in the images.
"Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them," Dietrich said. "This is the first time we're actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it."
Curiosity made its dramatic landing in early August, and has spent much of its time since testing out systems and instruments and preparing for its two-year drive.
But the rover's suite of cameras began sending back images of the conglomerate rock with small pebbles soon after landing, and they provided sufficiently detailed pictures to convince scientists that the pebbles and gravel had a watery past.
Gale Crater was selected as a landing site in part because satellite imaging had earlier found what appeared to be a sizeable cut in the crater wall that looked like a dried river or stream bed. Similar features have been found in many other Martian locations.
The team thought the rover had not landed exactly on that dried delta -- or "alluvial fan," as geologists describe it -- but the finding of the water-borne rocks is forcing them to rethink the size of the fan.
Ironically, the confirmation of Mwater flows came in the early days of a mission that had discarded the long-standing NASA directive to "follow the water" in Mars exploration. While finding and studying the signatures of past water flows is important for Curiosity's goal of identifying habitats that could have supported life, the mission motto is now "follow the carbon." That element is present in all organic compounds and is expected to have been essential to any possible Martian life.
The rover's ultimate destination is the 3-mile high mound in the center of the crater, but it will first detour to an unusual geological meeting of three rock types -- including one at the very end of the fan. Scientists think that rock may be formed from fine clays, the lightest material carried by the water and so the last to drop out.
Announcement of the long-ago presence of surface water is an early coup for the mission, but is consistent with the rover's unusually good fortunes. "Our biggest anomaly has been that we have no real anomalies," said Michael Meyer, lead scientist for NASA's Mars Exploration Program. "Definitely surprising, given the complexity of what we're doing."