From finding a trail of evidence supporting the presence of water on Mars a few billion years ago, Curiosity’s discovery of subrounded or rounded pebbles provides definitive proof that the red planet once had a river. According to a May 31 paper in Science , multiple exposures of a sedimentary rock (conglomerate) containing densely-packed rounded pebbles, varying in size from 2 mm to 40 mm in diameter, are particularly significant as they provide indisputable evidence of a palaeo-river. While other discoveries such as water-bearing minerals in veins at the Yellowknife Bay area in Gale crater and smectite clay in John Klein rock samples are in themselves noteworthy, they do not reveal if the water body was moving. In fact, water-bearing minerals in veins do not tell us about surface water flow. On the other hand, the very presence of big, rounded pebbles that lie overlapping along with coarse sand in the rock tell a completely different and definitive tale of Mars’s palaeo-environment. For one, the pebbles not only prove the presence of water but also shed light on the nature and quantum of moving water. Since only water transportation can abrade pebbles as big as 40 mm, and based on estimates for the riverbed’s gradient at the site of discovery, scientists have been able to postulate several characteristics about the river. First, it should have flowed at a velocity of up to 0.75 metres per second, the minimum force required to move pebbles of that size. Second, in order to initiate motion, the river should have had a flow depth of less than 0.90 m. Hence, the amount of water flowing in the river was indeed considerable. Finally, the fact that the pebbles have been abraded to produce subrounded or rounded edges despite having varying characteristics in terms of composition and shape strongly suggest that the river flowed for several kilometres.
In all, the mere presence of rounded pebbles indicates that Mars’s atmospheric conditions at some point in the past were so very different from today that they permitted liquid water to flow on the surface. In geology, the possibility of discovering prized fossils and pieces of evidence such as this lies in careful selection of the study area. In the present case, the final landing site was decided based on the presence of Mount Sharp, a layered mound within the Gale crater and its proximity to the alluvial fan, the Peace Vallis fan. The pebble discovery confirms the March 12 find of a non-acidic, fresh-water environment based on the finding of smectite clay from a John Klein rock specimen drilled by the rover. Discovering the Holy Grail of space science — a planet capable of supporting life in the past — is no longer a distant dream.