NASA’s Mars rover Curiosity has found the first definitive signs that, billions of years ago, water once flowed at the bottom of Gale crater. Fast.
At press briefing on 27 September, the science team said that a layer of rock, a conglomerate containing large pieces of gravel within a matrix of sandy material, suggests that a fast moving stream once spilled over the Gale crater walls.
Project scientist John Grotzinger says the science team reached consensus after inspecting the size and texture of gravel pieces in one layer, named Hottah, that had been tilted up and exposed (pictured). “To us, it looks like someone came along to the surface of Mars with a jackhammer,” he says.
The terrain was expected, since imagery from orbit had spotted fan-shaped deposits spilling through a crack in the crater rim. Another orbiter instrument had found evidence for rock units with high-thermal inertia — a potential sign of consolidation like that found in the conglomerate. There was even evidence suggesting something like the conglomerate in a spot scoured away by retrorockets during the landing.
And while evidence for past and present water on Mars has abounded, this is an example of a new watery environment. The rounded gravel pieces offer evidence that the stream was coursing at speeds around a metre per second, with the water ankle to hip deep, says William Dietrich, a science team co-investigator from the University of California, Berkeley.
Ultimately, the Curiosity team wants to move up the flank of Aeolis Mons, where there are clays that could represent better geochemical conditions for the preservation of organics. Those layers could also represent millions of years of history, rather than the thousands of years probably represented by the streambed.
Curiosity, in its 51st day on the surface, is fast approaching Glenelg, a triple-junction of different rock units. Grotzinger says the next major task will be finding a loose, sandy sample and feeding it to the rover’s geochemical instruments, a process that will probably take two or three weeks.