William P. Jeffs
Johnson Space Center, Houston
NASA Phoenix Water Ice, Soil Findings Point to Climate Cycles
HOUSTON — NASA’s Phoenix mission landed inside the Martian Arctic Circle on May 25, 2008. Phoenix’s instruments searched for water ice and for evidence of mineral nutrients essential to life in the Martian soil. Both were found.
Phoenix's goal was to determine the habitability of the northern polar region of Mars. The water ice found and the soil chemistry and minerals observed lead scientists to believe that the landing site had a wetter and warmer climate in the last few million years and could again in the future.
Interpretations of data that Phoenix returned during its five months of operation are reported in the July 3 issue of the journal Science. Scientists in the Astromaterials Research and Exploration Science (ARES) Directorate at NASA’s Johnson Space Center in Houston are among the co-authors of three of the papers: “H2O at the Phoenix Landing Site,” “Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site” and “Evidence for Calcium Carbonate at the Mars Phoenix Landing Site.” The ARES scientists are Doug Ming, Dick Morris, Paul Niles and Brad Sutter.
The announcement on July 30, 2008, that the science team had discovered water ice at the Phoenix landing site was a promising sign that the Mars environment could be habitable to life. In “H2O at the Phoenix Landing Site,” the authors cite evidence for subsurface water ice exposed by digging with the robotic arm scoop and several lines of evidence that support the interpretation that the Martian soil has thin films of water.
“It was fantastic to see that the water ice was actually present just below the surface and to watch it sublimate away, just like the ice in my freezer at home,” said Morris.
In “Detection of Perchlorate and the Soluble Chemistry of Martian Soil at the Phoenix Lander Site,” the authors state that chemical analysis of the soil revealed that most of the soluble chlorine is in the form of perchlorate. Perchlorate was detected in soil analyzed by the wet chemistry lab aboard the Phoenix Lander. Soil perchlorate is found on Earth but only at low concentration in the most hyper-arid regions like the Atacama Desert in northern Chile. Atmospheric processes responsible for perchlorate formation in the Atacama could also be operating on Mars.
Perchlorate, which strongly attracts water, makes up a few percent of the composition in all three soil samples analyzed by the wet chemistry laboratory. It could pull humidity from the Martian air. At higher concentrations, it might combine with water as a brine that stays liquid at Martian surface temperatures. Some microbes on Earth use perchlorate as food and it is also a strong oxidizer that releases oxygen when heated.
“The discovery of perchlorate salts was a big surprise,” said Ming. “We did not detect any organic molecules, but it is possible that if organic compounds were present in soil they were burned by the perchlorate oxidizer during heating and not detected by Phoenix instruments.” Organic compounds are the building blocks of life.
In “Evidence for Calcium Carbonate at the Mars Phoenix Landing Site,” the authors discuss the discovery of calcium carbonate in the soils around the Phoenix landing site. The authors suggest that the calcium carbonate detected “is most consistent with formation in the past by the interaction of atmospheric carbon dioxide with liquid films of water on particle surfaces." Carbonates are generally products of aqueous processes and may hold important clues about the history of liquid water on the surface of Mars.
“The detection of calcium carbonate was very exciting for us,” said Sutter. “The calcium carbonate concentration at the Phoenix landing site is about about four percent by weight, and it was especially rewarding to see that our laboratory work was crucial in making that determination.”
“The Phoenix discovery of carbonates in the soils of Mars has turned the previously popular notion of an acidic Mars on its head,” said Niles. “We are now faced with two contrasting chemical views – acidic and alkaline. The next decade of Martian science will be focused on unraveling these two intertwined but opposing views of the conditions of water on the Martian surface.”
For more about Phoenix, visit: http://www.nasa.gov/phoenix
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