New Thoughts On Mars

The three experiments have been extensively tested using terrestrial soil samples and, as a result, it is safe to say that it is extremely unlikely that a false positive result will be obtained.

The word "positive" translates as evidence of life processes in this summary by Harold Klein, biology team leader of the Viking mission, published about the time that the first U.S. lander went to work on the surface of Mars. Yet last November, after these same life-seeking experiments aboard both the Viking landers had shown apparently positive results in tests of Martian soil, Klein and other NASA scientists seemed unsure. In a Washington press conference summarizing the Viking findings, they announced that the results made it impossible to say that there was or was not life on Mars. That has remained NASA's official position. But unofficially, a handful of scientists support the view of Physicist Robert Jastrow, director of NASA's Goddard Institute for Space Studies. Says he in a forthcoming article in Natural History magazine: "Although the Viking experiments have contradictory elements, they seem to indicate that life, or some process closely imitating life, exists on Mars today."

Complex Reaction. Jastrow and those who hold similar views base their judgment not on new evidence but on an analysis of the biology experiments conducted by the Viking landers. The gas exchange test, based on the fact that terrestrial organisms give off gases as waste products, involved dropping a pinch of Martian soil into a warm, moist test chamber. The aim was to determine whether the sample would give off carbon dioxide, as animals would, or oxygen, as plants do. Scientists were surprised when the sample began releasing oxygen far more rapidly than plants would be expected to do. But they noted that the reaction might have a purely chemical, rather than a biological explanation; compounds called peroxides could have released the oxygen if they were present in the Martian soil.

More encouraging results came from a second test, in which a sample of Martian soil that had been moistened with a nutrient broth showed a rapid release of carbon dioxide. The result might mean that some kind of microbe was metabolizing the food provided by Viking. But cautious scientists noted that certain peroxides in the soil might also have caused the reaction to occur.

A third experiment was designed to measure the conversion of atmospheric carbon dioxide or carbon monoxide into organic matter. The results suggested that this process, which is carried out on earth in living cells, might also be taking place on Mars. In Jastrow's view, the experiment undermined the argument that peroxides might have been responsible for the results of the other tests. If peroxides were involved in this activity, certain catalysts had to be present and the reaction more complex.

Tests did not find organic, or carbon-based, molecules in the Martian soil. Terrestrial soil is laden with such molecules, which are the remains of living organisms. But scientists agree that this negative finding does not necessarily weaken the case for Martian biology. There may simply have been too few of these molecules in the soil to be detected by the gas chromatograph-mass spectrometer designed to look for them. The picture is complicated by the biology tests run at the second Mars landing site. In the gas exchange test, the soil released substantially less oxygen than the sample at the first site, but showed a higher level of possible microbial activity. Jastrow and several other scientists feel that this weakens the argument that peroxides were responsible for the results of the first gas exchange experiment and leaves biology as the logical alternative.

To Gilbert Levin, principal Viking investigator in the microbe experiment, the evidence from the lander experiments strongly suggests biology: "Certainly whatever they have shown cannot easily be explained otherwise." Chief Project Scientist Gerald Soffen is less certain. "It is possible," he notes, "to formulate chemical equations to describe all the results we've had." Whether these are the right equations, though, remains to be determined. Adds Soffen: "It's not a question of finding a chemical explanation; we must find the chemical explanation, and we need a natural as opposed to a blackboard explanation."

There is good reason for Soffen's --and NASA's--caution; the implications of discovering any form of life on Mars are so staggering that no scientist can afford to be wrong. But the ambiguities surrounding the Viking biology tests may soon be resolved. Now that Mars has re-emerged from behind the sun--which blocked transmissions between Viking and the earth for more than a month late last year--scientists have "reawakened" the sleeping laboratories and instructed them to run a new round of experiments.

These tests will allow Martian soil samples to incubate longer in order to give any microbes present a better chance to grow. The experiments will be run at temperatures closer to the frigid levels that prevail on Mars. Also, in a search for life that may have burrowed deeper into the Martian soil to escape ultraviolet radiation bombarding the surface, one of the landers will try to dig 30.5 centimeters (1 foot) deep for a soil sample. Explains Klein: "We want to play out our whole set of cards before we make our best judgment on the question of life on Mars."

Upholding Einstein. In other new experiments not concerned with biology, one of the landers will pick up and analyze a pebble (only soil has been examined to date) to get a better idea of the planet's geology. Scientists will also continue monitoring Viking 2's seismograph (the one aboard Viking 1 is disabled), which earlier picked up what may have been the only Marsquake to have occurred so far during the mission. They also plan to maneuver one of the Viking orbiters to within 50 kilometers (31 miles) of Phobos in order to get high-resolution pictures of the tiny, potato-shaped Martian moon.

As Viking emerged from its conjunction with the sun, a team headed by Physicist Irwin Shapiro of Massachusetts Institute of Technology measured the time it took for radio signals to make the round trip between the earth and the Viking orbiters, 400 million kilometers (248.5 million miles) away. The scientists were making the most accurate tests yet of one of the tenets of Einstein's theory of relativity, which holds that radio waves passing close to a massive body like the sun should be slowed down by its gravitational field. The signals to and from the Viking orbiters have further strengthened Einstein's case. Their round trip required 200 microseconds (200 millionths of a second) longer than they would have taken had they not been affected by the sun's gravity.

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