APRIL 20, 2013 - MARS - Proclamations about life on Mars are typically cloaked in mystical language, sketched with broad qualitative strokes that cause reflexive aversion among scientists used to the realm of mathematical rigor. Carol Stoker, a staff planetary scientist at NASA Ames Research Center, is hoping to change this. She's come up with a four-component equation that calculates the probability that a particular site is habitable. The relevant factors are 1) Pw, the probability that liquid water is present, 2) Pe, the probability of a biologically usable energy source, 3) Penv, the probability of a non-injurious environment amenable to life, and 4) Pc, the probability that the chemical building blocks of life are present.A habitable environment? The surface beneath the Phoenix Mars Lander 97 sols into the mission. (Image credit: NASA/JPL-Caltech//University of Arizona/Max Planck Institute)Multiply these probabilities together - they all must be present at the same time at the same place, after all - and voila, a quantitative probability of habitability pops out. In a presentation at the Present-Day Habitability of Mars Conference, Stoker broke it down further, describing each parameter with data from the Phoenix Mars Lander's landing site. Pw: Photographs taken of the soil below the spacecraft showed agglomerations of material that seemed to disappear, leading many observers to invoke liquid brine droplets. Pe: Biologically usable energy could come in the form of sunlight or particular chemicals. Both are promising. Sunlight is abundant, noted Stoker, and "though UV radiation is sterilizing, there could be transparent glassy mineral grains that are capable of blocking the UV and admitting photosynthetically active wavelengths of light." Chemical energy - possibly utilizing perchlorate as an electron acceptor and hydrogen, methane, sulfide, or iron as an electron donor - is also available. Penv: There are a lot of factors at play when determining if a martian environment is sufficiently welcoming to life. There must be enough available water, an acceptable pH (Phoenix measured 7.7), mechanisms of protection (i.e., a rock-based niche to avoid UV damage), and a temperature within the known biological growth range (for most of the year, Phoenix's site was well below the necessary 253 K). Pc: The six elements most integral to life - carbon, hydrogen, nitrogen, oxygen, phosphorous, and sulfur - were all detected by Phoenix. The only complication: nitrogen was not detected in a "fixed", or biologically usable, form.Ultimately, Stoker's equation indicated that the probability that the Phoenix landing site is habitable is approximately 40%. "No other site has comparably complete measurements of all of these factors," she explains, "so this is the best place to make this full calculation." The probabilistic treatment is reminiscent of the Drake equation, a formula that estimates the number of detectable extraterrestrial civilizations in our galaxy. These approaches are inherently very noisy, given our poor resolution on the quantitative values, but Stoker's equation is a move in the right direction, a well-reasoned enactment of a scientist's instinct to treat problems quantitatively. In the process, she's helping to move the discussion of Mars' habitability from the realm of misleading simplicity to compelling, if incomplete, statistical rigor. "Establishing habitability," says Stoker, "requires more than just following the water." - WIRED.
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