Viking

Viking Lander

It was with the questions raised by the Mariner missions that the Viking mission was designed. Arguable one of the supreme achievements of the American robotic exploration spacecraft series, two identical craft were put into Mars orbit in 1976 and, after imaging the planet for suitable sites, the landers separated and soft-landed on the surface.

The primary mission objectives were to obtain high resolution images of the Martian surface, characterize the structure and composition of the atmosphere and surface, and search for evidence of life.

Viking Mission to Mars

Data from Hubble on Martian climate since Viking

Viking Biology Experiments

The Gas Exchange Experiment (GEX) was looking for changes in the makeup of gases in a test chamber, changes that would indicate biological activity. The results from this test were taken to counter-indicate biology.

The Labeled Release Experiment (LR) was set up to detect the uptake of a radioactively-tagged liquid nutrient by microbes. The idea was that gases emitted by these microbes would show the tagging. Initial results were in line with this prediction but in the end, the overall results were inconsistent.

The Pyrolytic Release Experiment (PR) involved "cooking" soil samples that had been exposed to radioactively-tagged carbon dioxide to see if the chemical had been used by organisms to make organic compounds. Seven of nine experimental runs seemed to show small concentration of micro-organisms but the results were later discounted.

The Gas Chromatograph -- Mass Spectrometer Experiment (GCMS) also heated a soil sample and revealed an unexpected amount of water but failed to detect organic compounds. This absence was so absolute that it seemed there must be some mechanism actually destroying carbon compounds on the surface.

It did appear that the surface of Mars was a fairly hostile environment due to the solar ultraviolent radiation.

At a NASA conference to discuss these results, the following sums up what was then the general consensus:

"Viking not only found no life on Mars, it showed why there is no life there.... the extreme dryness, the pervasive short-wavelength ultraviolet radiation... Viking found that Mars is even dryer than had previously been thought... The dryness alone would suffice to guarantee a lifeless Mars; combined with the planet's radiation flux, Mars becomes almost moon-like in its hostility to life."

However, there are others who maintain that the evidence was not so clear-cut. In fact, Gilbert Levin, who was the principal investigator for the Labeled Release (LR) experiment, maintains that the experimental results are easier to reconcile with the conclusion that life was detected rather than otherwise.

Another consideration is that all the Viking experiments used samples from the uppermost Martian surface - but current thinking, from the new facts of life in extreme environments on Earth, is that it is more likely that life would be extant in the deeper subsurface:

"We envision any microbes present in such a system to have been present from antiquity. As conditions on the Martian surface worsened, organisms that had migrated or been carried to deeper levels would enjoy a measure of protection not experienced by surface dwellers..."

re: Viking and the Search for Life on Mars

The following summarizes the experimental procedure and the possible and actual results:

To reduce the chance of false positives, the biology experiments not only had to detect life in a soil sample, they had to fail to detect it in another soil sample that had been heat-sterilized (the control sample). Had terrestrial life been tested with the Viking biology instrument, the following results would have been expected:

If life was completely absent from Mars, as the GCMS results suggested, these should have been the results from the biology experiments:

In highly simplified form, these were the actual results from Mars:

The fact that both the GEX and PR experiments produced positive results even with the control sample indicates that non-biological processes are at work. Subsequent laboratory experiments on Earth demonstrated that highly-reactive oxidizing compounds (oxides or superoxides) in the soil would, when exposed to water, produce hydrogen peroxide. Oxidized iron, such as maghemite, could act as a catalyst to produce the results seen by the PR experiment.

Only the LR experiment appears to have met the criteria for life detection, and it does this rather ambiguously. When the nutrient was first injected, there was a rapid increase in the amount of labeled gas emitted. Subsequent injections of nutrient caused the amount of gas to decrease initially (which is surprising if biological processes were at work) but then to increase slowly. No response was seen in the control sample sterilized at the highest temperature (160C, 320F.) While there is still some controversy, the consensus opinion is that the LR results can also be explained non-biologically.

from Life on Mars

Contrary Views and Data

from The Planets

How to build a 'mars jar'

The Viking Labeled Release Experiment and Life on Mars
The Labeled Release (LR) life detection experiment aboard NASA's 1976 Viking Mission reported results which met the established criteria for the detection of living microorganisms in the soil of Mars. However, a variety of reasons led to the consensus of involved scientists that the positive responses at both Lander sites were caused by a chemical agent in the soil and not by microorganisms. In the years since Viking, new information from Mars and Earth has come to bear on this issue. ... It is concluded that the Viking LR experiment detected living microorganisms in the soil of Mars."
-- Reprinted from: Proceedings of Spie, SPIE-The International Society for Optical Engineering, "Instruments, Methods, and Missions for the Investigation of Extraterrestrial Microorganisms. 29 July-1 August 1997, San Diego, California

But were the Viking probes looking for the wrong signs? The chemical reactions that drive biological activity are now known to be much more diverse than anyone suspected in Viking's day. It is now clear that reactions involving hydrogen, methane, sulphur or iron can sustain biological activity. Last year, Todd Stevens and James McKinley of the Pacific Northwest Laboratory in Richmond, Washington, discovered bacteria deep within basalt rock beneath the Columbia River that thrive only on hydrogen. The hydrogen is continually produced by chemical reactions between water and crushed basalt. An ecosystem like this would have been completely invisible to the Viking craft.

from Warm havens for life on Mars
"Hot springs could be just the right places for life on Mars"
--New Scientist, 5/4/96

Links