Mars: Life Pinned on Viking Horns?
The milestone launch of NASA's latest Mars mission--called Spirit--along with the scheduled end-of-June launch for its twin--Opportunity--together, provide the impetus to revisit the remarkable journey of the earliest martian missions. Excerpts from the lively debates that took place prior to the 1976 Viking missions give immediacy and perspective on both the rewards and challenges that the Red Planet offers. In this and forthcoming issues, Astrobiology Magazine is pleased to commemorate the descriptions offered in the words of then mission contemporaries. NASA historians have compiled these notes in their five-hundred page edition of: On Mars: Exploration of the Red Planet. 1958-1978 (NASA HQ SP-4212).
At the 1:30 pm news briefing on 31 July 1976 (sol 11), Jim Martin made an announcement. Prefacing his remarks with. "I wanted to state that it's been project policy for seven years to make data available to the media when we have [them]," Martin noted that this day was no exception.
|Viking Lander-1 (1976) showed dramatic changes during dust storm activity. The appearance of the sky changes with the atmospheric dust content. Although the colors shown here are processed, not real, they do show relative changes. There was a dust storm covering the VL-1 site on Martian day (1742) or sol 1742 (1 Martian year=669 Earth days). High winds on Mars create more dust than any hazard one might expect from a Kansas-like tornado--the air pressure is small and atmosphere so thin (<1% or 1/150th of Earth's) that more likely difficulties are the mess a dust storm creates than the net force other than upon descent. Even a 100-mile per hour gale on Mars packs the gentle push of a 10 mile per hour breeze on Earth In 1971, Mariner 9 and 2 USSR missions all arrived during a dust storm.
"We have received biology data that we believe to be good data," he continued. Engineering telemetry indicated that the biology instrument was performing "extremely well," perhaps too well, since early reactions from the gas-exchange and labeled-release experiments were very positive. That could possibly be the consequence of biological activity, but Martin was cautious: "I think Chuck Klein will continue to caution you that the biology experiment is a complex one. We've seen that Mars is a complex planet. There are many things that we do not understand." The scientists were proceeding systematically and methodically.
|The robotic arm of the Viking 2 Lander extends to collect a sample of soil for analysis. Click to enlarge.
Biology Team Leader Harold P. Klein and his colleagues had already conducted a number of tutorials for the news people covering the Viking mission, and at each session where they presented analytical details they took time to explain the experiment in question. The biologists started with the basics.
Each Viking Lander carried an integrated biology instrument, which contained three experiments designed to detect the metabolic activity of microorganisms should they be present in the soil sampled. First, the gas-exchange experiment would determine if changes caused by microbial metabolism occurred in the composition of the test chamber atmosphere. Second, the labeled-release experiment, also known as Gulliver, would determine if decomposed organic compounds were produced by microbes when a nutrient was added. Third, the pyrolytic-release experiment would detect, from gases in the chamber, any synthesis of organic matter in the Martian soil. A change could be the result of either photosynthetic or nonphotosynthetic processes.
[In the gas-exchange experiments] after ruling out all other possible causes, the scientists concluded that the oxygen had to be coming from the soil itself. While one possible explanation for the increase was biological activity, other explanations were possible, too.
A possible alternative answer to why the initial amount of oxygen had been released lay in the desert area of landing site; the Martian samples contained peroxides and superoxides, which when exposed to abnormal (non-Marslike) humidity in the instrument quickly released oxygen. The related release of carbon dioxide suggested that the samples had an alkaline core. Although such reactions had not been witnessed on Earth, the scientists believed that the intense ultraviolet radiation bombarding the surface of the Red Planet could have produced unique photocatalytic effects. Still, there was much to be explained, including the reactions observed from the labeled-release investigation.
As in the gas-exchange experiment, [so too in the labeled-release study] there was a possibility that the soil itself contained catalysts, minerals, inorganics that produced some breakdown of the radioactive compounds. "The effect of water introduced into the dry Mars soil may cause violent chemical reactions that would disintegrate a portion of our medium," said Gil Levin.
By l August, the production of oxygen in the gas-exchange experiment had decreased considerably, thus supporting the belief that the release was the function of oxides in the soil. In a 2 August update on the labeled-release experiment, Levin noted that they had examined the radioactivity curve very carefully. They had found no evidence of any doubling of cells. No growth appeared to be taking place, but the curve did not seem to behave as scientists would have expected it to for chemical reactions either. "We find that the chemical reaction took place at a very rapid rate initially, and then uncharacteristically slowed down and took a long time to plateau." The curve detected with the labeled-release experiment did not agree with known responses for either chemical or biological reactions.
Data returned by the pyrolytic-release experiment and reported by Norman Horowitz on 7 August were equally confounding. Once again, the specialists had detected a reaction, but they did not know what it meant. "There's a possibility that this is biological," Horowitz said, but "there are many other possibilities that have to be excluded."
Norman Horowitz told the press: "We hope by the end of this mission to have excluded all but one of the explanations, whichever that may be. I want to emphasize that if this were normal science, we wouldn't even be here-we'd be working in our laboratories for three more months-you wouldn't even know what was going on and at the end of that time we would come out and tell you the answer. Having to work in a fishbowl like this is an experience that none of us is used to."
The scientist's caution was prompted by his knowledge that "we well might be wrong in anything we say. Anyone who has carried out a scientific investigation knows that the pathway of science is paved not only with brilliant insights and great discoveries, but also with false leads and bitter disappointments."
Later in a November 1977 Scientific American article, Horowitz was able to speak more authoritatively about the results that had been observed in all three experiments. In the gas-exchange experiment, "the findings of the first stage of the experiment were both surprising and simple." Immediately following the addition of the moisture to the sample chamber-the soil sample was not directly wetted-carbon dioxide and oxygen were released. The evolution of gases was short-lived, but the pressure in the chamber increased measurably. At the Chryse site, the amount of carbon dioxide increased by about 5 times, and the amount of oxygen increased by about 200 times in little more than one sol. At the landing site in Utopia, the increases were smaller but still "considerable."
|Water seeps from between layers of rock on the wall of a cliff, crater, or other type of depression. The alcove forms above the site of seepage as water comes out of the ground and undermines the material from which it is seeping. The erosion of material at the site of seepage causes rock and debris on the slope above this area to collapse and slide downhill, creating the alcove.
Upon reflection, Horowitz stated that "the rapidity and brevity of the response recorded by both landers suggested that the process observed was a chemical reaction, not a biological one."
|Scientists now hypothesize that liquid water burst out from underground, eroded the gullies, and pooled at the bottom of the Newton Crater (shown above) as it froze and evaporated. If so, life-sustaining ice and water might exist even today below the Martian surface.
Horowitz felt that the appearance of the carbon dioxide was readily explainable: "Carbon dioxide gas would be expected to be adsorbed on the surface of the dry Martian soil; if the soil was exposed to very humid atmosphere, the gas would be displaced by water vapor." The presence of the oxygen was logical but harder to account for, since so much oxygen would seem to require an oxygen-producing substance, not just the physical release of preexisting gas. There was just not that much oxygen available in the atmosphere-past or present-to account for the quantities measured.
Horowitz argued that it was "likely that the oxygen was released when the water vapor decomposed an oxygen-rich compound such as a peroxide. Peroxides are known to decompose if they are exposed to water in the presence of iron compounds, and according to the X-ray fluorescence spectrometer....the Martian soil is 13 percent iron."
Viking looked at only two samples at each of the two landing sites from depths of 5 to 10 centimeters. If organic materials were produced millions or hundreds of millions of years ago, they could be present at greater depths and protected there from the damaging ultraviolet radiation. The Viking spacecraft could be sitting on an area containing a deposit of organic material a few meters down. There could also be other areas on the planet where the surface material is more protected or where organic material is now being synthesized and not destroyed.
While Jerry Soffen believed that it was possible for life to have developed on Mars, he also thought it likely that the biology instrument, for a host of reasons, had not been designed properly to detect it. However, he was also very confident that if organic compounds had been present, the [gas chromatograph] GCMS would have detected them. For that reason, he had fought for the instrument throughout the evolution of the Viking project. Soffen could have accepted a negative biology result, if there had been a positive measurement of organic compounds. But positive biology results could not be interpreted as indicating the existence of life in the absence of organics.
Others have argued that perhaps Viking landed at the wrong places on the planet. Nearer the poles where there was a higher moisture content in the soil and atmosphere, life might exist.
Or perhaps, as suggested by Carl Sagan and Joshua Lederberg, there are Martian microenvironments where in small oasislike areas life has evolved and survived. Soffen thought this unlikely since the homogenizing effects of wind and dust storms would have likely distributed any organic material all over the planet.
Related Web Pages
Evidence for Snow on Mars - and Perhaps an Abode for Life?
Mars Odyssey web site (with new images)
Mars by Stories
Impact Crater Landing Sites for the 2003 Mars Exploration Rovers
Mars Exploration Rover Homepage
2003 Mars Exploration Rover Mission