Pebbles and sand on Mars best evidence that a river ran through it

Peb­bles and sand scat­tered near an ancient Mar­t­ian river net­work may present the most con­vinc­ing evi­dence yet that the frigid deserts of the Red Planet were once a hab­it­able envi­ron­ment tra­versed by flow­ing water.

Sci­en­tists with NASA's Mars Sci­ence Lab­o­ra­tory mis­sion reported May 30 in the jour­nal Sci­ence the dis­cov­ery of sand grains and small stones that bear the tell­tale round­ness of river stones and are too heavy to have been moved by wind. The researchers esti­mated that the sed­i­ment was pro­duced by water that moved at a speed between that of a small stream and a large river, and had a depth of roughly an inch to nearly 3 feet.

Co-author Kevin Lewis, a Prince­ton asso­ciate research scholar in geo­sciences and a par­tic­i­pat­ing sci­en­tist on the Mars mis­sion, said that the rocks and sand are among the best evi­dence so far that water once flowed on Mars, and sug­gest that the planet's past cli­mate was wildly dif­fer­ent from what it is today.

"This is one of the best pieces of evi­dence we've seen on the ground for flow­ing water," Lewis said. "The shape of these rocks and sand is exactly the same kind of thing you'd see if you went out to any streambed. It sug­gests a very sim­i­lar envi­ron­ment to the Earth's."

The researchers ana­lyzed sed­i­ment taken from a Mar­t­ian plain that abuts a sed­i­men­tary deposit known as an allu­vial fan. Allu­vial fans are com­prised of sed­i­ment left­over when a river spreads out over a plain then dries up, and are com­mon on Earth in arid regions such as Death Valley.

Yet Death Val­ley is a refresh­ing spring com­pared to Mars today, Lewis said. Satel­lite images taken in prepa­ra­tion for the 2012 land­ing of NASA's Curios­ity Mars rover had revealed ancient river chan­nels carved into the land on and around Mount Sharp, a 3.5-mile high mound sim­i­lar in size to Alaska's Mt. McKin­ley that would become the rover's land­ing site. A major objec­tive of the Curios­ity mis­sion is to explore Mars' past habitability.

Nonethe­less, liq­uid water itself is most likely rare on Mars' cur­rently cold and dusty land­scape where wind is the dom­i­nant force. Lewis was co-author on a paper in the May 2013 edi­tion of the jour­nal Geol­ogy that sug­gested that Mount Sharp, thought to be the rem­nant of a mas­sive lake, is most likely a giant dust pile pro­duced by Mars' vio­lent, swirling winds.

Strong as it might be, how­ever, wind can­not move sed­i­ment grains with a diam­e­ter larger than a few mil­lime­ters, Lewis said. The sand and stones he and his col­leagues ana­lyzed had diam­e­ters rang­ing from one to 40 mil­lime­ters, or roughly the size of a mus­tard seed to being only slightly smaller than a golf ball. The round­ness of the sed­i­ment also sug­gested a pro­longed erod­ing force, Lewis said.

"Once you get above a cou­ple of mil­lime­ters the wind will not be able to mobi­lize sed­i­ment. A num­ber of the grains we see in this out­crop are sub­stan­tially big­ger than that," Lewis said. "That really leaves us with flu­vial trans­port as the most likely process. We knew Curios­ity was land­ing near the fan, but to land right on top of these rocks that sug­gest the pres­ence of water was really fortuitous."

If the sed­i­ment does mean a river ran through Mars, the researchers must next deter­mine when, where it came from and how it dried up, a project that will be a "major sci­en­tific project over the com­ing year," Lewis said. The mys­tery also cen­ters on the poten­tial rela­tion­ship of the river to the scars on Mount Sharp: Did the river flow down it? Was the mound a source of water after all?

"This evi­dence tells us that there were a diverse set of geo­log­i­cal processes hap­pen­ing at roughly the same time within the prox­im­ity of [the land­ing site], and it gives us a pic­ture of a much more dynamic Mars than we see today," Lewis said. "Find­ing out how exactly they relate will be an excit­ing story."