PULLMAN, Wash. — We may already have ‘met’ Martian organisms, according to a paper presented Sunday (Jan. 7) at the meeting of the American Astronomical Society in Seattle.

Dirk Schulze-Makuch of Washington State University and Joop Houtkooper of Justus-Liebig-University, Giessen, Germany, argue that even as new missions to Mars seek evidence that the planet might once have supported life, we already have data showing that life exists there now—data from experiments done by the Viking Mars landers in the late 1970s.

“I think the Viking results have been a little bit neglected in the last 10 years or more,” said Schulze-Makuch. “But actually, we got a lot of data there.” He said recent findings about Earth organisms that live in extreme environments and improvements in our understanding of conditions on Mars give astrobiologists new ways of looking at the 30-year-old data.

The researchers hypothesize that Mars is home to microbe-like organisms that use a mixture of water and hydrogen peroxide as their internal fluid. Such a mixture would provide at least three clear benefits to organisms in the cold, dry Martian environment, said Schulze-Makuch. Its freezing point is as low as -56.5o C (depending on the concentration of H2O2); below that temperature it becomes firm but does not form cell-destroying crystals, as water ice does; and H2O2 is hygroscopic, which means it attracts water vapor from the atmosphere—a valuable trait on a planet where liquid water is rare.

Schulze-Makuch said that despite hydrogen peroxide’s reputation as a powerful disinfectant, the fluid is also compatible with biological processes if it is accompanied by stabilizing compounds that protect cells from its harmful effects. It performs useful functions inside cells of many terrestrial organisms, including mammals. Some soil microbes tolerate high levels of H2O2 in their surroundings, and the species Acetobacter peroxidans uses hydrogen peroxide in its metabolism.

Possibly the most vivid use of hydrogen peroxide by an Earth organism is performed by the bombardier beetle (Brachinus), which produces a solution of 25 percent hydrogen peroxide in water as a defensive spray. The noxious liquid shoots from a special chamber at the beetle’s rear end when the beetle is threatened.

He said scientists working on the Viking projects weren’t looking for organisms that rely on hydrogen peroxide, because at the time nobody was aware that such organisms could exist. The study of extremophiles, organisms that thrive in conditions of extreme temperatures or chemical environments, has just taken off since the 90s, well after the Viking experiments were conducted.

The researchers argue that hydrogen peroxide-containing organisms could have produced almost all of the results observed in the Viking experiments.


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  <li>Hydrogen peroxide is a powerful oxidant. When released from  dying cells, it would sharply lower the amount of organic material in their  surroundings. This would help explain why Viking&rsquo;s gas chromatograph-mass  spectrometer detected no organic compounds on the surface of Mars. This result has  also been questioned recently by Rafael Navarro-Gonzalez of the University of Mexico, who reported that similar  instruments and methodology are unable to detect organic compounds in places on  Earth, such as Antarctic dry valleys, where we know soil microorganisms exist.<br />
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  <li> The Labeled Release experiment, in which samples of Martian soil (and putative  soil organisms) were exposed to water and a nutrient source including  radiolabeled carbon, showed rapid production of radiolabeled CO2 which  then leveled off. Schulze-Makuch said the initial increase could have been due  to metabolism by hydrogen peroxide-containing organisms, and the leveling off  could have been due to the organisms dying from exposure to the experimental  conditions. He said that point has been argued for years by Gilbert Levin, who  was a primary investigator on the original Viking team. The new hypothesis  explains why the experimental conditions would have been fatal: microbes using  a water-hydrogen peroxide mixture would either &ldquo;drown&rdquo; or burst due to water  absorption, if suddenly exposed to liquid water.<br />
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  <li> The possibility that the tests killed the organisms they were looking for is  also consistent with the results of the Pyrolytic Release experiment, in which  radiolabeled CO2 was converted to organic compounds by samples of  Martian soil. Of the seven tests done, three showed significant production of  organic substances and one showed much higher production. The variation could  simply be due to patchy distribution of microbes, said Schulze-Makuch. Perhaps  most interesting was that the sample with the lowest production&mdash;lower even than  the control&mdash;had been treated with liquid water.</li>
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