NASA's Curiosity rover has now confirmed what scientists have long suspected -- that water anywhere from ankle to waist deep once flowed on Mars' surface.
The conclusion, scientists said at a briefing Thursday, is based on images showing what looks like an ancient gravel stream bed. One of those stream bed slabs, named Hottah, appears to be made of gravel cemented together by water that once ran freely on Mars and settled on the floor of Gale Crater, likely several billion years ago.
Mission scientist WiIliam Dietrich of the University of California-Berkeley said it looks like the water was moving about 3 feet per second, "with a depth somewhere between ankle and hip deep."
Dietrich said there have been a lot of hypotheses about the water flows on Mars. "This is the first time we're actually seeing water-transported gravel on Mars," he said in a statement on the discovery. "This is a transition from speculation about the size of the streambed material to direct observation of it."
Curiosity arrived on Mars in August and is now on its 51st Martian day, known as a "sol." The evidence that a warmer and wetter Mars once enjoyed floods of water inside Gale Crater adds to the $2.5 billion rover's efforts to find evidence of chemistry hospitable to life now or in the past on Mars.
"The rock formed in the presence of a vigorous flow of water on the surface of Mars," said mission scientist John Grotzinger of Caltech. The find confirms past observations of orbiting spacecraft of Gale Crater that helped lead to its selection as a landing site for the rover, now on a two-year mission to search for evidence of past habitable conditions on Mars.
Essentially, the rover is travelling over a gully wash's fan of stones, ones that traveled down from a canyon on the crater wall "several billion years ago," says Michael Malin of the rover imaging team. Most likely the canyon waters flowed sporadically over thousands to millions of years, depositing gravel in a broad fan of stones covering the floor of the crater. "We had anticipated this was where some water-lain sediments would be," Malin says.
The rover is equipped with a laser, drill and on-board lab to investigate chemistry of Martian rocks such as Hottah, named after a Canadian geological formation. In coming weeks the rover will steer toward more rock deposits suspected to represent these gravel outcrops, looking to test chemical conditions for past habitability on Mars, Grotzinger says. "This is really just the start of the science mission for the rover."
The final goal of the rover is to examine layers of clay that underlay the foothills of Mount Sharp, the 3.4-mile-high mountain in the center of Gale Crater. It is expected to arrive in those foothills in about a year.