In a news conference Thursday afternoon, NASA scientists announced that the Curiosity rover has found convincing evidence that an ancient stream once flowed by where the rover stands.
Mastcam images taken earlier this month of two bedrock outcropping, named 'Hottah' and 'Link', show that they are composed of many smaller rocks cemented together — what geologists call sedimentary conglomerates.
The rocks that are cemented together to form Hottah and Link are coarse-grained gravel, and are rounded and worn. We know, from studying such rocks here on Earth, that this is due to them rubbing and knocking together as they are moved by either wind or water.
"The shapes tell you they were transported and the sizes tell you they couldn't be transported by wind. They were transported by water flow," said mission investigator Rebecca Williams, of the Planetary Science Institute in Tucson, Arizona.
"From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep," said William Dietrich, who is also one of the Curiosity mission's science investigators, from the University of California, Berkeley. "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."
The stream bed rests at the foot of Mount Sharp, the mountain in the middle of the Gale Crater. Earlier pictures taken from orbit showed an alluvial fan of material flowing down from a channel, known as Peace Vallis, that leads up to the crater's northwest edge (seen at about 11 o'clock in this image).
The mission scientists may instruct Curiosity to examine the conglomerates to see what is cementing them together, which may give them a clearer understanding of the watery environment they formed in.
Regardless of this evidence, the scientists find it unlikely that they'll find evidence of life in this ancient stream-bed.
"A long-flowing stream can be a habitable environment," said John Grotzinger, project scientist for the Curiosity mission team. "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."