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| 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).
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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 led to positve results. However, when heated and the experiments repeated, (i) and (ii) still produced positive results. The heat did not change things suggesting the results were due to chemistry and not biology. The Labeled Release behaved as one would expect biology to behave; as the temperature was raised the activity decreased. A fourth experiment, the Gas Chromatograph Mass Spectrometer (GCMS), suggested that the Labeled Release result was not due to biology as the GCMS found no traces of organic molecules in the Martian soil. |
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Update 1:
A new look at the data from the Viking landers concludes that they
may have found the building blocks of life after all.
The surprise discovery of perchlorates
by the Phoenix
Mars mission 32 years after the Viking mission ended,
suggested that the way in which the
Viking experiment was set up actually 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
existed on Mars, but it does show us that we must be careful.
What happened was that
On Mars, the Viking lander scooped up some Martian soil,
put it in a tiny oven, heated the sample, and then tested it for
organic materials. The only organic chemicals identified in the Martian soil
were chloromethane and
dichloromethane, compounds which were
interpreted at the time as likely contaminants from the cleaning
fluids used on the spacecraft before they left Earth.
The new study found that when
a little perchlorate
was added to desert soil from Chile,
containing organics, and analyzed in the manner of the Viking tests, that the
only organics left were
chloromethane and
dichloromethane!
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; 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 . But for 30 years we were looking at a jigsaw puzzle with a piece missing. Phoenix provided the missing piece: perchlorate. Perchlorate, an ion of chlorine and oxygen, becomes a strong oxidant when heated. It could sit 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 rapidly destroys them. How will we know for sure? The Mars Science Lab mission, with the car-sized rover Curiosity answered the question. |
Update 2: The Sample Analysis at Mars (SAM) instrument on Curiosity made the first definitive detection of organic molecules at Mars. Organic molecules are the building blocks of all known forms of terrestrial life, and consist of molecules made primarily of carbon, hydrogen, and oxygen. Organic molecules can also be made by chemical reactions which don't involve life, and it is not known if the material found came from ancient Martian life or from a nonbiological process such as chemical reactions in water in ancient Martian hot springs or organic material brought to Mars by interplanetary dust or fragments of asteroids and comets.Curiosity with its suite of instruments including SAM was sent to Mars in 2011 to discover more about the ancient habitable Martian environment by examining clues in the chemistry of rocks and the atmosphere. Organic molecules were found in a drilled sample of the Sheepbed mudstone in Gale crater, the landing site for Curiosity.
The crater is thought to be the site of a lake billions of years ago, and rocks like mudstone formed from sediment in the lake. the mudstone was found to contain 20% smectite clays. On Earth, smectities are provide high surface area and optimal interlayer sites for concentration and preservation of organic compounds when rapidly deposited under reducing chemical conditions.
 The discovery shows that the ancient environment of Mars offered a supply of reduced organic molecules for use as building blocks for life and an energy source for life. Other analysis of the same mudstone revealed that water and chemical elements essential for life and a different chemical energy source. Life on Earth started ~3.8 billion years ago. There are sites on Mars that had the same conditions at that time; liquid water, a warm environment, and organic matter. The organic molecules also contained chlorine atoms, and include chlorobenzene and several dichloroalkanes, such as dichloroethane, dichloropropane and dichlorobutane. Chlorobenzene is the most abundant with concentrations between 150 and 300 parts-per-billion. Chlorobenzene is not a naturally occurring compound on Earth. It is used in the manufacturing process for pesticides (insecticide DDT), herbicides, adhesives, paints and rubber. Dichloropropane is used as an industrial solvent to make paint strippers, varnishes and furniture finish removers, and is classified as a carcinogen. It's possible that these chlorine-containing organic molecules were present as such in the mudstone. However, according to the team, it's more likely that a different suite of precursor organic molecules was in the mudstone, and that the chlorinated organics formed from reactions inside the SAM instrument as the sample was heated for analysis. Perchlorates (a chlorine atom bound to four oxygen atoms) are abundant on the surface of Mars. It's possible that as the sample was heated, chlorine from perchlorate combined with fragments from precursor organic molecules in the mudstone to produce the chlorinated organic molecules detected by SAM. In 1976, the Gas Chromatograph Mass Spectrometer instrument on NASA's Viking landers detected two simple chlorinated hydrocarbons after heating Martian soils for analysis (chloromethane and dichloromethane). However they were not able to rule out that the compounds were derived from the instrument itself, according to the team. While sources within the SAM instrument also produce chlorinated hydrocarbons, they don't make more than 22 parts-per-billion of chlorobenzene, far below the amounts detected in the mudstone sample, giving the team confidence that organic molecules really are present on Mars. |