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 here, named “Hottah”. Rounded pebbles within this sedimentary conglomerate indicate sustained abrasion of rock fragments within water flows that crossed Gale Crater.
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).