New work by NASA researchers provides an explanation for two key life-detection experiments conducted by the Mars Viking landers in the mid 1970s. These two experiments yielded results that some have interpreted as indication that life exists on the Martian surface. The latest work, however, accounts for the results of the gas exchange and labeled release experiments based on the chemistry of the Martian soil. In spite of popular perception, there is no robust scientific evidence for the existence of life on Mars.
In response to the bandwagon popularity of country music in the early ’80s—brought on by the movie Urban Cowboy—Kye Fleming and Dennis Morgan wrote the song “I Was Country, When Country Wasn’t Cool.”
This hit, recorded in 1981 by Barbara Mandrell, reached number 1 on the charts, becoming one of Mandrell’s signature songs.
“I Was Country, When Country Wasn’t Cool” reminds me a bit about the recent mission to Mars and the search for life on the red planet. With the Curiosity rover currently on the Martian surface—characterizing the planet’s geology with the goal to determine if Mars can harbor microbial life—the search for life on Mars seems to be the “in” thing to do (at least from a scientific perspective).
In the midst of all the excitement surrounding this recent Martian mission, it is easy to forget the earlier voyages that sought to directly detect life on this planet—when looking for life on Mars wasn’t as cool. (Actually, looking for life on Mars has always been a cool thing to do. Looks like my lead just unraveled.)
The Viking 1 and Viking 2 missions in the mid 1970s were the first scientifically rigorous attempts to look for life on Mars. The two landers performed four life-detection experiments: (1) the gas exchange experiment; (2) the labeled release experiment; (3) the pyrolytic release experiment; and (4) the gas chromatograph-mass spectrometer experiment. In my view, these missions still have bearing on the question of whether or not we will find evidence for life on Mars.1 In fact, it might be surprising to learn that scientists continue to analyze the results of the Viking missions.
Recently, NASA scientists published a new assessment of the results from two of the more controversial life-detection experiments.2 Both the gas exchange experiment (GEX) and the labeled release (LR) experiment gave positive results. The GEX monitored gas production from Martian soil samples incubated in water. If microbes existed in the soil, they would flourish in the presence of water and in the process release gases like oxygen, carbon dioxide, or methane. When this experiment was performed, oxygen was released into the headspace.
The LR experiment involved incubation of Martian soil samples in a nutrient broth, with compounds in the broth tagged with carbon-14. Presumably microbes in the soil would consume the labeled nutrients and generate radioactive gas. Carbon-14 labeled gases were detected when the Viking landers performed the LR experiment.
Based on the failure of either lander to detect organics in the Martian soil (the outcome of the gas chromatograph-mass spectrometer experiment) and the null result obtained by the LR experiment when the soil sample was sterilized (killing off any microbes that might exist in the soil), some scientists conclude that the results of the GEX and LR experiment stem from the presence of hydrogen peroxide and superoxide in the Martian soil. Still, a minority of scientists insist that the GEX did, indeed, detect the presence of life on Mars.
Today, a team of NASA scientists, motivated by the recent detection of perchlorate in the Martian soil, proposed a new explanation for the results of the GEX and LR experiment. These researchers demonstrate that ionizing radiation impinging on the Martian surface will breakdown perchlorate in a carbon dioxide atmosphere (like that found on Mars) into hypochlorite, oxygen, and chlorine dioxide. The generation of oxygen explains the release of this gas in the GEX. Hypochlorite will react with the amino acid alanine (which was present in the nutrient broth of the LR experiment) to form chloralanine. This compound readily decomposes to yield radioactive gas.
At the end of the day, it looks as if the Viking Landers failed to detect any evidence for life on Mars. In my opinion, this result needs to be at the forefront as NASA continues to explore and characterize the surface of Mars with the Curiosity rover and plan future missions to the red planet.
So often, the existence of life on Mars is taken as a foregone conclusion—and it is only a matter of time before we find evidence of that life. Yet, the Viking missions failed to directly detect any evidence for this assumption. Until proven otherwise, the paradigm should remain: life does not exist on Mars.