Thanks to the Curiosity rover, NASA has new evidence of an ancient flowing stream on Mars. Until now, scientists have had to make do hypothesizing about the channels left on the martian surface. The finding site lies between the north rim of Gale Crater and the base of Mount Sharp, a mountain inside the crater. The new images sent back show the discovery of gravel that was once part of what NASA says was an ancient stream.
The black oval indicates the targeted landing area for the rover, known as the "landing ellipse." The cross shows where the rover actually landed. Red indicates higher elevations; the cooler colors point to transitions to lower elevations.
Link outcrop of rocks on Mars (left) alongside an image of similar rocks on Earth (right), which offers a typical example of sedimentary conglomerate formed of gravel fragments in a stream. The Link region on the left features rounded gravel fragments, or clasts, up to a couple of inches within the rock outcrop.
An annotated version of the image highlights a piece of gravel that is about 0.4 inches (1 centimeter) across. It was selected as an example of coarse size and rounded shape. Rounded grains (of any size) occur by abrasion in sediment transport, by wind or water, when the grains bounce against each other. Gravel fragments are too large to be transported by wind. At this size, scientists know the rounding occurred in water transport in a stream.
Path on Mars of NASA's Curiosity rover toward Glenelg and the discovery of what appears to be an ancient martian streambed.
Goulburn offered the first hint that water might have transported the sandstone material making up the outcrop. The exposed rock outcrop named Link piqued interest because the rounded shapes offered evidence of water transport. The other exposed rock outcrop, named Hottah, contains many rounded pebbles. That offered more evidence of water as the rounded pebbles, which are up to 1.6 inches large, are thought to be too big to have been transported by wind, according to NASA.
For comparison's sake, check out this image of a dry streambed on an alluvial fan in the Atacama Desert, Chile. The Rover Curiosity has seen a couple of rock outcrops on Mars close to its Bradbury Landing site that also record a mixture of sand and pebbles. Scientists believe the pebbles got transported by water and were most likely deposited along an ancient streambed.