Did we find life on Mars–and kill it?
September 7, 2010 2 Comments
Did we kill life on Mars?
The movie War of the Worlds may have it wrong. The collision between life forms from different planets may not involve fire, mutual bloody conflict, and pathogen exchange. Instead, it may already have happened, ending simply, quietly, and unilaterally, in water and heat at the microscopic level.
In the 1970s, we sent two Viking space probes to Mars on a mission to test for life on the red planet. The prevailing dogma among scientists was that water was required for life, anywhere in the universe. This viewpoint has evolved since then as we’ve discovered some Earthbound organisms, including the microbe Acetobacter peroxidans, relying on very different, non-water-based mechanisms for survival.
Organisms living alternative lifestyles
These discoveries of organisms living alternative lifestyles here at home led to the formation of the “weird life” panel, a U.S. group that addresses concerns that we may be too Earth-centric as we search for life beyond our planet. This narrow view may have led to a terrible accident on our Viking visits: we may have encountered Martian life and then drowned or baked it to death.
The Viking probes performed a series of experiments on samples from the Martian surface. The labeled release experiment involved exposing Martian soil samples to water and a nutrient source with incorporated radiolabeled carbon. To everyone’s excitement, the exposure elicited a spike in radiolabeled carbon dioxide (CO2), meaning that the nutrient source could have been metabolized, the carbon incorporated with oxygen to make CO2. However, the experiment seemed to fizzle as the CO2 levels dropped and plateaued.
Evidence of life–and death?
In the pyrolytic release experiments, radiolabeled CO2 appeared to have been incorporated into organic molecules by something in the Martian soil. This type of experiment would reflect activity similar to photosynthesis, which grabs CO2 from the air and incorporates it into the organic macromolecule glucose. In this Martian test, four of the seven soil samples showed significant production of organic molecules that had incorporated the radiolabeled carbon from the CO2. Interestingly, the sample treated with liquid water produced even less organic product than the control. In addition, the Viking probes found evidence of chemical oxidation (oxygen breaking down macromolecules), but we found no oxidative activity in the Martian soil on subsequent analyses.
The results seemed disappointing—they at first held promise for a life form metabolizing and catabolizing molecules. But as each experiment fizzled, expectations diminished, and everyone assumed that there was simply no life on Mars.
A review of the data from a different, less Earth-centric perspective, however, results in a potential confirmation of life on Mars. The scientists who devised this story point out that Mars is conducive to life based on hydrogen peroxide (HP). For example, the higher the HP content, the lower the freezing point of an aqueous HP solution. Even at freezing, HP does not form cell-destroying crystals, as water does. An HP also pulls water vapor from the air, an efficient way to grab water in a dry place like Mars.
Exposing an HP-based cell to water would drown it, and applying heat to the soil would bake it. These two things are exactly what the Viking probes did to Martian soil samples in executing the experiments. If HP-based life forms were present, they would have produced results exactly like those the probes found. For example, they could have taken the radiolabeled macromolecules and metabolized them, releasing CO2, but eventually dying as the water added to the samples overwhelmed them. Every single outcome of the Viking experiments appears to be consistent with a hypothesis that there might have been HP-based life on Mars.
We’ve now sent a new probe, the Phoenix, to the red planet. Data from the probe don’t answer all questions but do indicate the presence of “organics” on Mars. The next step is the Mars Science Laboratory, expected to launch next year with a Mars ETA in 2012. Will it settle the Life on Mars question once and for all?