June 5, 2013
Pebbles, Sand In Martian River Network Show Signs Of Ancient Water
April Flowers for redOrbit.com - Your Universe Online
The most convincing evidence yet that the frigid deserts of Mars were once a habitable environment crossed by flowing water are presented by pebbles and sand scattered near an ancient Martian river network. The findings of this study, led by scientists at NASA´s Mars Science Laboratory mission, were published in the journal Science.
The research team says that the discovery of sand grains and small stones that bear the telltale roundness of river stones are too heavy to have been moved by the wind. They estimated the water that produced the sediment moved at a speed between that of a small stream and a large river, at a depth of roughly and inch to nearly three feet.
The rocks and sand are among the best evidence so far that water once flowed on Mars, said Kevin Lewis, a Princeton associate research scholar in geosciences and a participating scientist on the Mars mission. This evidence suggests 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 sediment analyzed was collected from a Martian plain abutting a sedimentary deposit known as an alluvial fan, which is comprised of sediment leftover when a river spreads out over a plain then dries up. Alluvial plains are common on Earth in arid regions such as Death Valley.
Compared to modern Mars, however, Death Valley is a refreshing spring. Ancient river channels carved into the land on and around Mount Sharp — a 3.5-mile high mound similar in size to Mt. McKinley in Alaska — were revealed by satellite images taken in preparation for the 2012 landing of the Curiosity rover. Mount Sharp became the landing site for the NASA rover, whose major objective is to explore Mars´ past habitability.
Ancient river channels notwithstanding, liquid water is most likely rare on Mars´ currently cold and dusty landscape where wind is the dominant force. The winds are so fierce that a 2013 paper in the journal Geology, co-authored by Lewis, suggested that Mount Sharp is most likely a giant dust pile produced by the violent, swirling winds, rather than a remnant of a massive lake as has been previously thought.
Winds, however strong, cannot move sediment grains with a diameter larger than a few millimeters. According to Lewis, the sand and stones the team analyzed had diameters ranging from one to 40 millimeters (up to 1.57 inches), or roughly the size of a mustard seed to being only slightly smaller than a golf ball. The roundness of the sand and pebbles also suggested a prolonged eroding force.
“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.”
The researchers must determine when and where the river came from, and how it dried up. Such a project will be a “major scientific project over the coming year,” Lewis said. The future research will also center 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.”