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Solving a mystery in the sands of Mars.

Solving an mystery in the sands of Mars

Results from the biology instruments that sampled and analyzed the Martian surface from the two Viking landing craft in 1976 failed to make a case for the presence of life on Mars. On the other hand, neither have scientists heretofore succeeded in completely explaining Viking's data by means of inanimate chemistry alone. Now, however, a group of researchers has proposed an answer that they say fits the data in detail, and without the need for Martian life.

In one Viking experiment, a nutrient solution labeled with carbon-14 moistened a bit of Martian "soil," causing the sample to emit gases containing the radioactive carbon in a manner suggesting metabolism. When the soil was again injected with the solution, the amount of gas in the test chamber dropped by 22 percent, and when the sample was heated to kill off any possible microorganisms, the gas production indeed stopped, suggesting life to some.

But when soil was injected with the carbon-14 nutrient solution in a second instrument, called the gas-exchange experiment, it startled Viking scientists by giving off an unexpected quantity of oxygen, which declined rapidly a few hours later. When those samples were heated, they still gave off oxygen, ut less of it, suggesting the presence of a chemical oxidant that was broken down by heat.

Viking scientists concluded the experiments failed to support the idea of life on Mars, but no one managed to explain fully what chemistry might account for the surprising data.

Physical chemist Robert C. Plumb of Worcester (Mass.) Polytechnic Institute, together with three of his present and former students, set out to match the Viking results as closely as possible. They used test chambers made of the same materials as Viking's, and report in the April 20 NATURE that their goal was to reproduce quantitatively the same chemical reaction rates, the same 22 percent reduction in the amount of gases produced during the "labeled release" test, the same responses to heating, and more.

A key step in the analysis, says Plumb, resulted from research reported in 1978 by Soviet researchers who proposed that oxygen could become trapped in microscopic pores in the rocky material of the Martian surface. The pores, according to the group, could readily open if exposed to water vapor, which was provided by the nutrient solution in the gas-exchange experiment. This would trigger the abrupt release of oxygen, one of the more dramatic events in the months-long operation of the biology package.

Viking scientists had suggested that superoxides trapped on the sample might have released the oxygen, but Plumb says "superoxides are not stable in the carbon dioxide atmosphere of Mars." Instead, his group cites a 1983 Soviet report, unrelated to Viking, that ultraviolet or X-ray radiation can break down metal nitrates into an oxidizing agent called peroxonitrite (the lander instruments could not detect nitrogen). This compound can oxidize the organic compounds in the nutrient and is destroyed by heat (which would shut off the carbon dioxide production) -- again reminiscent of Viking's data.

A third Viking biology instrument, designed to see whether labeled carbon dioxide might be metabolized by hypothetical Martian microorganisms, failed to show results that changed consistently with shifts in heat and humidity, Plumb says, noting another U.S. researcher's suggestion that the testwas probably disturbed by the heat-driven transport of dust through the device.

Even if there are no micro-Martians, adds Plumb, future Mars-landing missions should be instrumented to detect peroxonitrite and carbonate on the planet's surface, as well as to confirm a slight alkalinity that would account for the 22 percent carbon dioxide reabsorption reported by Viking.
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Author:Eberhart, J.
Publication:Science News
Date:Apr 29, 1989
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