The possibility of life on Mars has held human interest for hundreds of years and has recently become an obsession for NASA.
A number of atmospheric probes and surface craft have been sent to Mars to assess the planet’s habitability. The ultimate goal is to send future missions to Mars to directly look for evidence of life, both past and contemporary.
In the midst of all the excitement and anticipation, it’s easy to forget that there have already been missions to Mars specifically designed to detect life. Over thirty years ago in 1976 NASA sent the Viking 1 and 2 spacecraft to Mars. Two landers made it to the surface. These robotic systems harbored four life-detection experiments.
The Viking Biology Experiments
The Gas Exchange Experiment: Martian soil samples were incubated with a nutrient broth. A gas chromatograph monitored headspace samples for the generation of gases like oxygen, carbon dioxide, or methane. Gas production in the soil would indicate biological activity.
The Label Release Experiment: Martian soil samples were incubated with a nutrient broth. Some of the nutrients in the cocktail were labeled with carbon-14. If organisms were present in the soil, they would consume the labeled nutrients and generate radioactive gas. Detection of radioactivity in the headspace would indicate the presence of life.
The Pyrolytic Release Experiment: Martian soil samples were exposed to light, water, carbon-14-labeled carbon dioxide and carbon-14-labeled carbon monoxide. If photosynthetic life was present, the radioactive gases would become incorporated into the soil.
The Gas Chromatograph-Mass Spectrometer Experiment: This instrument was designed to detect and identify organic compounds (both from life and meteoritic infall) in the Martian soil. If life were present, organic materials would be abundant in the Martian soil.
Results of the Viking Biology Experiments
The Gas Exchange Experiment: Gas evolution from the soil was observed.
The Label Release Experiment: Radioactive gas was produced after the soil was incubated with radiolabeled nutrient broth.
The Pyrolytic Release Experiment: The results of this experiment were initially interpreted as evidence for extremely low levels of microbes in the soil. These results were later reinterpreted as a null result.
The Gas Chromatograph-Mass Spectrometer Experiment: No organic compounds were detected in the soil, not even at a trace level.
Interpretation of the Viking Biology Experiments
Even though the Gas Exchange and Label Release experiments gave positive results, the failure to detect organics in the soil was troubling. It is difficult to conceive of life on the Martian surface without organic compounds in the soil.
It appears that a highly oxidizing chemical species in the Martian soil was likely responsible for the release of gases after incubation with nutrients, many of them organic compounds. The oxidizing compounds in the Martian soil would rapidly break down any organic material, generating gases like oxygen and carbon dioxide as the by-products.
The highly oxidizing nature of the Martian soil and the intense exposure of the Martian surface to UV radiation explain why no organics exist in the Martian soil, not even organic materials from meteorite infall. UV radiation, like chemical oxidants, readily destroys organic materials.
The Viking landers looked for life on Mars and failed to detect it.
Revisiting the Interpretation
The interpretation of the Viking results is still discussed by astrobiologists. In fact, during the fall of 2006 a team of scientists published a paper questioning the design of the Gas Chromatograph-Mass Spectrometer Experiment. They argued that the experimental setup was fundamentally unable to detect low levels of organics in the Martian soil. They also maintained that if the organic materials were too refractory, the sample preparation procedure for the Gas Chromatograph-Mass Spectrometer Experiment would fail to release them from the soil, leaving the organics unavailable for detection and analysis. They also raised concerns about oxidation and, hence, destruction, of organics during sample preparation.
In short, these astrobiologists claimed that it was premature to discount the null results for the Gas Chromatograph-Mass Spectrometer experiments aboard the Viking lander. If organics are indeed present on the Martian surface, it means that the results of the Gas Exchange and Label Release experiments very well may be taken as an indication of life on Mars and, at minimum, could motivate future missions to Mars to look for life.
Not So Fast
A recent paper, however, discounts the criticisms leveled against the Gas Chromatograph-Mass Spectrometer Experiment. Klaus Biemann—a world renowned mass spectroscopist—demonstrated that the detection limit of the Viking Gas Chromatograph-Mass Spectrometer Experiment was 1-2 ppb (parts per billion). In fact, when on the surface of Mars, the Gas Chromatograph-Mass Spectrometer successfully detected and identified trace levels of organic contaminants introduced into the system while on Earth. Biemann also showed that the sample preparation procedure would not destroy organics and could readily detect refractory organic materials.
The bottom line: the null results of the Gas Chromatograph-Mass Spectrometer are valid. There are no organics, nor life, on the surface of Mars.
For a more detailed discussion of life on Mars, see the book I wrote with Hugh Ross, Origins of Life: Biblical and Evolutionary Models Face Off.