VIKING Landers

The current environment of Mars is harsh, surface pressure = 0.006 - 0.010 mBars, surface temperature from -90 C to -3 C and an atmosphere dominated by CO2. The current atmosphere is too thin to allow standing water on its surface today. It was to this environment that the Viking landers were sent in 1974 (reaching Mars in 1976). To the right is shown a Viking lander next to Carl Sagan.

The two Viking landers settled in the regions shown to to the left. The landers among other things, performed tests designed to search for Life As We Know It (LAWKI).

Using their arms, the Viking Landers obtained soil samples and sampled the atmosphere of Mars.

The landers performed three experiments: (i) Gas-Exchange Experiment: Martain soil was mixed with a broth of organic nutrients brought from Earth and looked for the release of gases from respiration; (ii) Pyrolitic Release Experiment: Martian soil was mixed with carbon monoxide and carbon dioxide brought from Earth to see if the carbon would be incorporated into the soil; (iii) Labeled Release Experiment: Martian soil was mixed organic nutrients to see if products of respiration and metabolism appeared.

All three experiments intitially were thought to yield positive results, however, only the labelled release (gas release) experiment is still encouraging. When heated and the experiments repeated, experiments (i) and (ii) still produced positive results. The heat did not change things suggesting that was found was a result of chemistry and not biology. The Labeled Release expereiment behaved as one would expect biology to behave; as the temperature was raised the observed activity decreased.

The labelled release experiment detected an initial burst of oxygen (Plant life?) followed by a slower release (over days) of carbon dioxide (Animals and/or microbes?) when a Martian soil sample was studied. The burst of oxygen quickly subsided (within hours), however. If biology was responsible it was not immediately clear why life would behave in this manner. Later work, however, discovered an interesting thing, Solar ultraviolet radiation striking the Martian soil can produce hydrogen peroxide, H2O2, which then adheres to the soil. If the soil is moistened, the hydrogen peroxide breaks up very quickly releasing a burst of oxygen and forms other compounds. Although, on the face of it this sounds bad as it then suggests that the detected release of oxygen by Viking was due to chemistry, not biology. It is interesting, however, in that the burst of oxygen was hard to understand, if it was due to biology.

A fourth experiment suggested, however, that the Labeled Release result was not due to biology as analysis of the soil found that there were no traces of organic molecules in the Martian soil. The only organics found were chloromethane and dichloromethan compounds, thought to arise as contaminants from the cleaning fluids used to sterilize the spacecraft and instruments. The lack of organic compounds was perplexing.

Update: A new look at data from the Mars Viking landers concludes that the two landers may have found the building blocks of life after all. The surprise discovery of perchlorates by the Phoenix Mars mission on 32 years later could mean the way the Viking experiment was set up actually would have destroyed any carbon-based chemical building blocks of life, what the experiment set about to try and find. This doesn't say anything about the question of whether or not life has existed on Mars, but it could make a big difference in how we look for evidence to answer that question, said Chris McKay of NASA's Ames Research Center. McKay coauthored a study published online by the Journal of Geophysical Research Planets, reanalyzing results of Viking's tests for organic chemicals in Martian soil.

The Viking lander scooped up some soil, put it in a tiny oven and heated the sample. The only organic chemicals identified in the Martian soil from that experiment chloromethane and dichloromethane compounds interpreted at the time as likely contaminants from cleaning fluids used on the spacecraft before it left Earth. But those chemicals are exactly what the new study found when a little perchlorate the surprise finding from Phoenix was added to desert soil from Chile containing organics and analyzed in the manner of the Viking tests. Our results suggest that not only organics, but also perchlorate, may have been present in the soil at both Viking landing sites, said the study's lead author, Rafael Navarro-Gonzalez of the National Autonomous University of Mexico, Mexico City.

The Viking experiment results have been rather controversial over the years. There are some scientists who say the experiment actually did find evidence for life, and others who say the results were inconclusive. McKay said that organics can come from non-biological or biological sources. Many meteorites raining onto Mars and Earth for the past 5 billion years contain organics. Even if Mars has never had life, scientists before Viking anticipated that Martian soil would contain organics from meteorites.

The lack of organics was a big surprise from the Vikings, McKay said. But for 30 years we were looking at a jigsaw puzzle with a piece missing Phoenix has provided the missing piece: perchlorate. The perchlorate discovery by Phoenix was one of the most important results from Mars since Viking. Perchlorate, an ion of chlorine and oxygen, becomes a strong oxidant when heated. It could sit there in the Martian soil with organics around it for billions of years and not break them down, but when you heat the soil to check for organics, the perchlorate destroys them rapidly, McKay said.

This interpretation proposed by Navarro-Gonzalez and his for co-authors challenges the interpretation by Viking scientists that Martian organic compounds were not present in their samples at the detection limit of the Viking experiment. Instead, the Viking scientists interpreted the chlorine compounds as contaminants.

How will we know for sure? The Mars Science Lab mission, with the car-sized rover called Curiosity could help resolve this question.