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USGS Sedimentologist David Rubin Serves as External Expert During NASA Announcement of Evidence for Flowing Water on Mars
U.S. Geological Survey (USGS) scientist David Rubin participated as an external expert in a National Aeronautics and Space Administration (NASA) press conference on March 23 in which the agency announced the first firm, direct evidence for flowing water on the surface of Mars. That evidence consists of bedding structures—particularly, fine layers called trough cross-laminae—in the rock outcrop rimming Eagle Crater, where NASA's Mars exploration rover Opportunity landed at about 9 p.m. PST on January 24. The structures have been documented in images taken by Opportunity and studied by the rover science team over the past weeks. John Grotzinger, rover science-team member from the Massachusetts Institute of Technology (MIT), presented the sedimentologic evidence in a series of images, starting with a time-lapse movie shot by Dave Rubin and Jon Nelson (USGS) of sand in a flume organizing itself into ripples with sinuous crests as water flows over it. The movie was followed by a video simulation, also produced by Rubin, of migrating, sinuous-crested ripples seen from overhead and in cutaway sections that show bedding patterns which match those in the Martian rocks. (The movie and simulation can be downloaded from the Movies of Bedforms and Crossbedding Web page.) Next, Grotzinger showed images of the Martian rocks themselves, calling viewers' attention to the trough cross-laminae, which he described as "opening-upward smiles," that particularly moved the scientists to interpret the layers as having been deposited by flowing water. Grotzinger said, "We feel very confident that this adds up to a story about ripples moving in water rather than in wind." He said that the ripples had been formed in water at least 5 cm (2 in) deep, possibly much deeper, by currents flowing at speeds of 10 to 50 cm/s, or about 1 mph. The sedimentologic findings build on chemical findings announced three weeks earlier, in a press conference on March 2, when NASA announced the presence in the outcrop of abundant sulfate salts and widely varying concentrations of bromine—characteristics of rocks on Earth that formed by the evaporation of seawater. The chemical evidence showed that liquid water once soaked the rocks, but the scientists could not say at that time whether it was surface water or ground water. Sedimentologic evidence gathered shortly after the March 2 press conference caused the scientists to conclude that the outcrop had formed in surface water, an environment even more likely than ground water to have been capable of supporting life. NASA's associate administrator for the Office of Space Science Ed Weiler said, "We thought, since these conclusions were very profound, that we ought to go through a peer-review process" before announcing them. NASA's lead scientist for Mars and the Moon, Jim Garvin, lined up six reviewers, including two from the University of Texas, Austin and Dallas, and one each from the Open University in London, Los Alamos National Laboratory, the Geological Survey of Canada, and the USGS. The USGS reviewer, Dave Rubin, is an expert on sedimentology and bedforms on the Western Coastal and Marine Geology Team at the USGS Pacific Science Center in Santa Cruz, CA. Rubin expressed his support for the interpretations presented by Grotzinger at the press conference, saying: "When John and Jim first sent me [the images], I was astonished. There on Mars were sedimentary structures just like we see on Earth. You can go out to your nearest beach or creek and take a shovel and dig in and see some of these same kinds of structures." As an example, Rubin showed a photograph of trough cross-laminae in sand deposited by the Colorado River that look much like those seen in the images of Martian rocks.
Being a conscientious reviewer, Rubin also played devil's advocate, offering an alternative interpretation of the Martian rocks' trough cross-laminae: that they might have been deposited by wind. But the shape and scale of the Martian bedding structures—the sets of cross-laminae are trough shaped and only a few centimeters thick—led him to favor deposition by water. He said, "Probably the best [alternative explanation] I could come up with would involve very small windblown bedforms, and probably the best way to keep the windblown bedforms small would be to have water just beneath the surface. So, even in the best counterexample I could come up with, there probably would be water at the surface, if not above the surface." Rubin concluded that the rover science team's interpretation of the Martian rocks as water-laid deposits was the best explanation for the rocks' textures. According to Grotzinger, the environment at the time the rocks were forming could have been a salt flat, or playa, sometimes covered by shallow water and sometimes dry. Such environments on Earth, either at the edge of oceans or in desert basins, can have currents of water that produce the type of ripples seen in the Martian rocks. The scientists cannot tell yet how deep the water was, when or how long it flowed, or what the climate was like at the time. Although running water might seem to imply a warm climate, Steve Squyres of Cornell University, principal investigator for the science payload on Opportunity and its twin Mars exploration rover, Spirit, pointed out that the water could have been flowing under a cover of ice. [See "Microbial Life in Perennially Ice Covered Lakes in the McMurdo Dry Valleys, Antarctica," for an example of an ice-covered life-supporting environment on Earth.] Squyres also noted that the environment in which the Eagle Crater rocks appear to have formed would be not only a habitable environment, one capable of supporting life, but also a good environment for preserving the evidence of past life. As salt crystals precipitated in the rocks, for example, they would trap chemicals from the seawater that might hold clues to the presence of life. Addressing a question about the possible presence of fossils in the Martian rocks, Rubin said that, on Earth, there could be microscopic fossils, such as mats of algae, in rocks formed in similar environments. Then he got a twinkle in his eye and added, "Some of the rocks [formed on Earth in] these same kinds of environments also have dinosaur footprints." That drew a laugh from the other panelists, who were probably relieved when Rubin went on to say, "But there's no reason to think that there would be anything like that here." Squyres agreed that if there are any fossils in the Martian rocks, they would probably be microscopic and too small for Opportunity's instruments to see. Finding signs of past or present life will likely require sending human geologists or, in the near term, a robot to bring back samples for study on Earth.
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in this issue:
USGS Assists in JASON Expedition USGS Participates in Career Day
Comprehensive Study of Florida Keys Coral Reefs |
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U. S. Department of the Interior | U.S. Geological Survey
cover story:
