The Curiosity rover's science team has come to a consensus: There was once a large amount of flowing water in Gale Crater. They can't say when, or for how long, but based on the types, shapes and sizes of rocks they've been seeing, they can say, finally, there was flowing water on Mars.
We aren't talking about a small amount of water either: The stream running from a canyon in Mount Sharpe called Peace Vallis could have been up to hip deep. The water eroded jagged rocks into smooth stones and deposited them in a fan shape across the floor of Gale Crater, Curiosity's landing site.
The team has examined rock outcroppings from three areas along the rover's path, all of which indicate that water had eroded rocks into gravel, then deposited them in layers. These layers cemented together forming what's called a conglomerate rock, similar to a rough cement.
NASA/JPL-Caltech/Univ. of Arizona The water that deposited these stones likely came from Mount Sharp. The "alluvial fan" of water marks coming from Peace Vallis canyon has been imaged by the Mars HiRise orbiter and the Mars Reconnaissance Orbiter, and was one of the main reasons the Curiosity team landed in Gale Crater.
NASA/JPL-Caltech/UofA "Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them. This is the first time we're actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of stream bed material to direct observation of it," Curiosity science co-investigator William Dietrich of the University of California, Berkeley, said a NASA teleconference.
These type of alluvial fans are also seen in California's Death Valley and several other places on Earth where water flows from mountains into canyons.You can see the similarity between the Mars and Earth sites below:
"A long-flowing stream can be a habitable environment," Mars Science Laboratory Project Scientist John Grotzinger, of the California Institute of Technology in Pasadena, said in the teleconference. "It is not our top choice as an environment for preservation of organics, though. We're still going to Mount Sharp, but this is insurance that we have already found our first potentially habitable environment."