DC Scientists from NASA's Mars explorer projects announced
Tuesday that they had found definitive evidence for "a lot of water"
at some point in the planet's history. Co-investigator Steve Squyres of Cornell
University said water could have "drenched" the surface, possibly
creating a habitable landscape for some time.
Tiny spherules and lens-shaped cavities, found in an outcrop in the crater where the rover Opportunity landed on Mars, are two pieces of evidence supporting the presence of large amounts of liquid water sometime in the planet's history. Image courtesy of NASA.
"Ever since Opportunity touched down," Squyres said in a press conference at NASA headquarters, "we have been puzzling over it. The last puzzle piece fell into place a few days ago." The layered rocks in the crater where the rover Opportunity landed in January hold "tantalizing clues," he said, to answer the question: "Were these altered by liquid water? The answer to that is definitely yes."
Squyres emphasized that they do not know how long or when the water may have been present, or if the water was at the surface or existed as groundwater. Nevertheless, he said, the team has found four criteria at the outcrop that pin down evidence for liquid water present in large quantities.
First, tiny spherules, which the team at the Jet Propulsion Lab nicknamed "blueberries,"
have been identified as concretions. Initially thought to be lapilli (droplets
or shards of volcanic material), the concretions are instead spheres made of
concentric layers that formed around a tiny nucleation seed. Opportunity sliced
through a "blueberry" to find the layers, Squyres said, which might
have solidified in chemical leaching and precipitation processes that took place
in water-filled rock layers.
John Grotzinger of MIT, geologist for the Mars explorer team, pointed out that the nodules still resting within the beds do not disrupt the layers, which is an indication that they formed in place. Had they been deposited fully formed, the impact of a falling lapilli fragment or concretion would have disturbed the sediment layers, he said. The team plans to further examine an eroded "blueberry bowl" that has caught the nodules as they eroded out of the layered outcrop, Grotzinger said.
The second line of evidence comes from tiny vugs or cavities worn away by chemical erosion, which Squyres said looked "as if objects the size and shape of pennies had been embedded inside and had gone away." Some of these cavities were fat in the middle with tapered ends, Grotzinger noted, which is a shape similar to gypsum crystals on Earth. The team has hypothesized that the vugs came from crystals that grew in a liquid water-filled environment by chemical precipitation, pushing aside the rock layers, and then eroded away.
The other two pieces of evidence came from Opportunity's onboard technology. Alpha particle analysis and the MiniTES (or miniature thermal emissions spectrometer) indicated high levels of sulfur in the rocks ground away by the rover's rock abrasion tool. The rock holds the "all-time Mars record" for sulfur at five times the amount of sulfur present in Mars soil, said Mars explorer team member Benton Clark of Lockheed Martin Space Systems. Opportunity's Mössbauer spectrometer also found evidence for the mineral jarosite, or iron sulfate hydrate. "For sulfide hydrates," Squyres said, "you have got to have water."
Clark also said that the high amount of salt up to 40 percent in the rock means it can no longer be considered volcanic. "The only way on Earth to get so much salt is evaporation," he said, noting that the minerals that Opportunity detected often represent an evaporite sequence.
The team emphasized that their results come from a very small section of Mars. "We're talking about 20 centimeters" of outcrop thickness, Squyres said. Grotzinger said that seeing other sites will be important, including a future trip to a part of the outcrop that seems to have crossbedding, a kind of sedimentary layering that may be from particle deposition by flowing wind, air or water currents. However, the evidence from Opportunity's first outcrop confirms what previous observations of hematite deposits indicated Mars was once a warm planet with a lot of liquid water on or near its surface.
"We can't say much about age or duration [of the liquid water-affected deposits] until we get to other outcrops," Squyres said, adding that the only definitive way to date the rocks would be to bring a sample back to Earth.
Joy Crisp, the manager of the project, laid out a short-term goal to get to a nearby crater the team has named Endurance, which is about 100 meters in diameter and 30 meters deep, in hopes of finding the same rock outcropping there as well or something different. "The rovers may not last that long," she said, "but we'd like to try." Opportunity and its twin Spirit, halfway around the planet, may overextend their 90-day life expectancies; Spirit landed Jan. 3, and Opportunity about three weeks later.
NASA News coverage
"Mars update: A pixel at a time," Geotimes Web Extra, Feb. 11
"Mars geologist in action," Geotimes Web Extra, Jan. 4
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