Okubo and McEwen, Science

Sliced geology.
Joints or fractures once carried water through now-exposed rock to alter surrounding rock to form a lightened "halo."

Martian Cracks Show Signs of Once-Flowing Water

Dick writes about Earth and planetary science for Science magazine.

How many times can you "discover" water on Mars? Orbiting cameras found ancient, water-cut valley networks in the 1970s and young, presumably water-cut gullies in 2000. And in 2004, rovers discovered signs of acidic groundwater that occasionally oozed to the surface of ancient Mars (ScienceNOW, 23 March 2004) Now, the High Resolution Imaging Experiment (HiRISE) on board the Mars Reconnaissance Orbiter has found another setting for the elixir of any past martian life, thanks to the unprecedented level of detail being returned by that camera.

The first new result from HiRISE was presented in San Francisco, California, today at the annual meeting of the American Association for the Advancement of Science (which publishes ScienceNOW). Planetary geologist Chris Okubo of the University of Arizona in Tucson reported the discovery of cracks in layered sedimentary rocks exposed in Candor Chasma, a branch of the great Valles Marineris canyon system. HiRISE's 30-centimeter resolution--tens times better than the previous best--has provided stunning views of Mars since routine imaging began last November. In Candor Chasma, that enhanced detail revealed thin, straight, dark lines that cut across the light-dark layering of sedimentary deposits. Each cross-cutting dark line is surrounded by a light-toned "halo" a few meters across.

With dark, loose sand blowing around, the team could infer that the thin dark lines are low, sand-collecting fractures surrounded by higher, sand-shedding ridges. On Earth, such features form where water has flowed deeply through fractured rock. If the same thing happened on Mars, the mineral-laden water could chemically lighten the tone of surrounding rock and cement and strengthen it. Then weaker rock farther from the fracture would erode more easily, leaving a pair of ridges paralleling the fracture. The findings also appear online today in Science.

The martian details being returned by HiRISE are "making all of us geologists drool," says Marjorie Chan of the University of Utah in Salt Lake City, who works on ancient fractures on Earth like those HiRISE found. Geologists both terrestrial and planetary could soon be feasting on such once-watery fractures. HiRISE has spied a set of likely fractures on the eastern rim and floor of Victoria Crater, conveniently enough, the same crater now being explored by the Opportunity rover.

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