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Signs of “ancient stream” on Mars

Sept. 27, 2012
Courtesy of NASA
and World Science staff

NASA’s Cu­ri­os­ity rov­er mis­sion has found ev­i­dence that a stream once ran briskly across the ar­ea on Mars where the rov­er is driv­ing, ac­cord­ing to sci­en­tists.

There is oth­er ev­i­dence for the early pres­ence of wa­ter on Mars, but this ev­i­dence—im­ages of rocks con­tain­ing an­cient streambed grav­els—is the first to point to a flow strong enough to move peb­bles, re­search­ers said. They’re stu­dy­ing the im­ages of stones ce­mented in­to a lay­er of what they call con­glom­er­ate rock. The stones’ sizes and shapes are be­lieved to of­fer clues to the speed and dis­tance of a long-ago stream’s flow.

NASA'sCu­ri­os­i­ty rov­er found what sci­en­tists called ev­i­dence for an an­cient, flow­ing stream on Mars at a few sites, in­clud­ing the rock out­crop pic­tured he­re, which the sci­ence team has named "Hot­tah" af­ter Hot­tah Lake in Canada’s North­west Ter­ri­to­ries. It may look like a bro­ken side­walk, but this ge­o­log­i­cal fea­ture on Mars is ac­tu­al­ly ex­posed bed­rock made up of smaller frag­ments ce­mented to­geth­er, or what ge­ol­o­gists call a sed­i­men­ta­ry con­glom­er­ate. Sci­en­tists the­o­rize that the bed­rock was dis­rupted in the past, giv­ing it the ti­tled an­gle, most like­ly via im­pacts from me­te­orites. The key ev­i­dence for the an­cient stream is thought to come from the size and round­ed shape of the grav­el in and around the bed­rock. Hot­tah has pieces of grav­el em­bed­ded in it, called clasts, up to a cou­ple inches (few cen­time­ters) in size and lo­cat­ed with­in a ma­trix of sand-sized ma­te­ri­al. Some of the clasts are round in shape, lead­ing the sci­ence team to con­clude they were trans­port­ed by a vig­or­ous flow of wa­ter. The grains are too large to have been moved by wind. (Im­age cred­it: NA­SA/JPL-Caltech/MSSS )

NASA’s Curiosity rover mission has found evidence that a stream once ran briskly across the area on Mars where the rover is driving, according to scientists. There is other evidence for the early presence of water on Mars, but this evidence—images of rocks containing ancient streambed gravels—is the first of its kind, researchers said. They’re studying the images of stones cemented into a layer of what they call conglomerate rock. The stones’ sizes and shapes are believed to offer clues to the speed and distance of a long-ago stream’s flow. “From the size of gravels it carried, we can interpret the water was moving about three feet per second, with a depth somewhere between ankle and hip deep,” said Curiosity science co-investigator William Dietrich of the University of California, Berkeley. “Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them. This is the first time we’re actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it.” The finding site lies between the north rim of Gale Crater and the base of Mount Sharp, a mountain inside the crater. The rounded shape of some stones in the area indicates long-distance transport from above the rim, the investigators said. There, a channel named Peace Vallis feeds into an alluvial fan, or fan-shaped deposit of material left behind when flowing water is slowed down by a flattened slope. The abundance of channels in the fan between the rim and conglomerate suggests flows continued or repeated over a long time, the researchers added. The finding comes from examining two outcrops, called “Hottah” and “Link,” with the telephoto capability of Curiosity’s mast camera during the first 40 days after landing. Those observations followed up on earlier hints from another outcrop, which was exposed by thruster exhaust as Curiosity, the Mars Science Laboratory Project’s rover, touched down. “Hottah looks like someone jack-hammered up a slab of city sidewalk, but it’s really a tilted block of an ancient streambed,” said Mars Science Laboratory Project Scientist John Grotzinger of the California Institute of Technology in Pasadena. The gravels in conglomerates at both outcrops range in size from a grain of sand to a golf ball, the researchers said; some are angular, but many are rounded. “The shapes tell you they were transported and the sizes tell you they couldn’t be transported by wind. They were transported by water flow,” said Curiosity science co-investigator Rebecca Williams of the Planetary Science Institute in Tucson, Ariz. Members of the scientific team said they may use Curiosity to learn the composition of the material, which holds the conglomerate together, revealing more characteristics of the wet environment that formed these deposits. The stones are thought to provide a sampling from above the crater rim, so the group may also examine several of them to learn about broader regional geology. The slope of Mount Sharp in Gale Crater remains the rover’s main destination. Clay and sulfate minerals detected there from orbit can be good preservers of chemicals that are potential ingredients for life. “A long-flowing stream can be a habitable environment,” said Grotzinger. “It is not our top choice as an environment for preservation of organics, though. We’re still going to Mount Sharp, but this is insurance that we have already found our first potentially habitable environment.” During the two-year prime mission of the Mars Science Laboratory, researchers plan to willuse Curiosity’s 10 instruments to investigate whether areas in Gale Crater have ever offered conditions favorable for microbial