Detailed analysis and review have borne out researchers’ initial interpretation of pebble-containing slabs that NASA’s Mars rover Curiosity investigated last year: they are part of an ancient streambed. The rocks are the first ever found on Mars that contain streambed gravels. The sizes and shapes of the gravels embedded in these conglomerate rocks — from the size of sand particles to the size of golf balls — enabled researchers to calculate the depth and speed of the water that once flowed at this location.
“Our analysis of the amount of rounding of the pebbles provided further information,” said Sanjeev Gupta of Imperial College, London, a co-author of the new report. “The rounding indicates sustained flow. It occurs as pebbles hit each other multiple times. This wasn’t a one-off flow. It was sustained, certainly more than weeks or months, though we can’t say exactly how long.”
The stream carried the gravels at least a few miles, or kilometers, the researchers estimated.
The atmosphere of modern Mars is too thin to make a sustained stream flow of water possible, though the planet holds large quantities of water ice. Several types of evidence have indicated that ancient Mars had diverse environments with liquid water. However, none but these rocks found by Curiosity could provide the type of stream flow information published this week. Curiosity’s images of conglomerate rocks indicate that atmospheric conditions at Gale Crater once enabled the flow of liquid water on the Martian surface.
NASA's Mars Science Laboratory (MSL) Curiosity rover found evidence for ancient, water-transported sediment on Mars at a few sites, including the rock outcrop pictured above, named "Hottah". Rounded pebbles within this sedimentary conglomerate indicate sustained abrasion of rock fragments within water flows that crossed Gale Crater pictured at the top of the page.
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 in the left foreground. The scale bar at lower right is 5 centimeters (2 inches).
This view of Hottah is a mosaic of images taken by the right (telephoto-lens) camera of the Mast Camera instrument (Mastcam) on Curiosity during the 39th Martian day, or sol, of the rover's work on Mars (Sept. 14, 2012 PDT/Sept. 15 GMT). It has been enhanced for presentation in white-balanced color, which yields a view as if the rock were seen under outdoor lighting conditions on Earth, which is useful for scientists to distinguish rocks by color in familiar lighting.
During a two-year prime mission, researchers are using Curiosity’s 10 science instruments to assess the environmental history in Gale Crater on Mars, where the rover has found evidence of ancient environmental conditions favorable for microbial life.
The Daily Galaxy via http://www.nasa.gov/mission_pages/msl/multimedia/pia17062.html