This stereo image from the Mast Camera (Mastcam) on NASA's Mars rover Curiosity shows a rock outcrop called "Hottah," cited as evidence for vigorous flow of water in a long-ago Martian stream. The scene covers an area roughly 1 yard or meter across at the near edge. It appears three-dimensional when viewed through red-blue glasses with the red lens on the left.
Curiosity found evidence for an ancient, flowing stream on Mars at a few sites, including this outcrop named after Hottah Lake in Canada. It may look like a broken sidewalk, but this geological feature on Mars is actually exposed bedrock made up of smaller fragments cemented together, or what geologists call a sedimentary conglomerate. Scientists theorize that the bedrock was disrupted in the past, giving it a tilted angle, most likely via impacts from meteorites.
The key evidence for the ancient stream comes from the size and rounded shape of the gravel in and around the bedrock. Hottah has pieces of gravel embedded in it, called clasts, up to a couple inches (few centimeters) in size and located within a matrix of sand-sized material. Some of the clasts are round in shape, leading the science team to conclude they were transported by a vigorous flow of water. The grains are too large to have been moved by wind. Erosion of the outcrop results in gravel clasts that protrude from the outcrop and ultimately fall onto the ground, creating the gravel pile at left.
Curiosity's Mastcam acquired component images of this scene on the 39th Martian day, or sol, of the mission (Sept. 14, 2012 PDT/Sept. 15 GMT). Mastcam has two cameras, a telephoto right eye (Mastcam 100) with a 100-millimeter-focal-length lens, and a moderately wide-angle left eye (Mastcam 34) with a 34-millimeter lens. This stereo image combines images from each eye. A right-eye-only version of the scene is at http://photojournal.jpl.nasa.gov/catalog/PIA16156 .