The Viking missions showed the martian soil to be lifeless and depleted in organic material and indicated the presence of one or more reactive oxidants. Here we report the presence of Mars-like soils in the extreme arid region of the Atacama Desert. Samples from this region had organic species only at trace levels and extremely low levels of culturable bacteria. Two samples from the extreme arid region were tested for DNA and none was recovered. Incubation experiments, patterned after the Viking labeled-release experiment but with separate biological and nonbiological isomers, show active decomposition of organic species in these soils by nonbiological processes.
[[RP_SECTION:atacama-soil-and-viking|Atacama soil and Viking]]
Alex: [measured, clear] Soil from the hyper-arid core of the Atacama Desert can produce gas-release signals that look like metabolism, yet appear to be purely chemical. That's according to Rafael Navarro-Gonzalez and colleagues in Science, who propose this may explain the puzzling Viking lander data.
Sam: [curious, leaning in] That's a strong thing to say about soil. How did they establish that the reactions were abiotic, and not just very sparse microbial activity? [[RP_SECTION:chiral-isomers-and-biology|Chiral isomers and biology]]
Alex: [steady, analytical] They used chiral isomers of alanine and glucose in labeled-release experiments, the same style of assay Viking used. Biology is homochiral, so a living soil should process the L- and D-forms at different rates.
Sam: [processing] So if the CO2 release differed between isomers, you'd infer biology. If it didn't, you'd infer chemistry.
Alex: [nodding] And in the hyper-arid Yungay region, the release was the same regardless of isomer. That points to a chemical oxidation reaction. It's a clean discriminating test because it doesn't depend on detecting cells at all.
Sam: [thoughtful] What's the mechanism for the soil ending up that way? [[RP_SECTION:oxidant-mechanism-and-paradox|Oxidant mechanism and paradox]]
Alex: [affirming] Photochemical processes at the surface create a strongly oxidizing environment that degrades organic matter. Extreme dryness limits biological input, so nothing replenishes what the oxidation removes. The proposal is that this erases the biological signature.
Sam: [probing] And that connects to the Viking paradox? The landers saw reactivity, but the mass spectrometry didn't detect organics.
Alex: [measured] That's the link they draw. The Viking instruments operated at lower temperatures, which likely failed to volatilize refractory, oxidized organic salts in the soil.
Sam: [skeptical] A referee would ask whether they tested that, or just proposed it.
Alex: [clear] They ran pyrolysis gas chromatography-mass spectrometry at both 500 and 750 degrees Celsius. The higher temperature recovered significantly more organic fragments.
Sam: [nodding] So the organic material was there, just thermally stubborn. Though that's an analog-soil comparison. It shows the mechanism is plausible, not that Viking's specific hardware missed organics for exactly this reason.
Alex: [even pace] That's a fair distinction. It's supporting evidence for the temperature explanation, and the isomer test is what carries the abiotic claim.
Sam: [curious] Did they identify the oxidant? [[RP_SECTION:identifying-the-soil-oxidant|Identifying the soil oxidant]]
Alex: [measured, slightly slower] That's the primary limitation. They ruled out hydrogen peroxide and superoxides, because their measured concentrations were too low to account for the reactivity.
Sam: [thoughtful] So a strong oxidant is doing the work, but nobody has pinned down what it is. Is there a candidate at all?
Alex: [even pace] They suggest nitrate-derived peroxonitrites, but they acknowledge that the concentrations required would be much higher than what they measured in the Atacama. So the candidate doesn't yet fit the numbers.
Sam: [sitting back] Then what is the study actually delivering? A functioning sterile analog of the Martian surface, with the oxidant chemistry still a black box. [[RP_SECTION:implications-for-mars-exploration|Implications for Mars exploration]]
Alex: [reflective] The value is largely in calibration. The Atacama soil gives a reference for distinguishing biological signatures from abiotic photochemical artifacts. That shifts the question from why the Viking experiments were ambiguous to how to design experiments that account for photochemical oxidation.
Sam: [probing] Does that change what we should look for on Mars? Biomarkers that resist oxidation, perhaps?
Alex: [measured] That's one implication. If the surface is this oxidizing, subsurface samples, where the photochemical influence is attenuated, become a more sensible priority. The paper supports that as a direction, not as something it tested.
Sam: [thoughtful] It also suggests the dry limit of life isn't only about water availability. It may be a redox threshold, where chemistry overtakes biology.
Alex: [affirming] That's a useful way to frame it, with the Atacama as empirical support. I'd add one caution: it's one terrestrial analog, and the unresolved oxidant limits how far the analogy can be pushed.
Sam: [reflective] Which is a reminder that our instruments don't just measure an environment. They interact with it, and that interaction has to be calibrated.
Alex: [concluding] Yes, and the Atacama work is a step toward doing that. It moves the Viking data toward a geochemical explanation that fits the analytical evidence, though not a fully closed one.
Sam: [steady] If you want the figures and the method choices we skipped, you can generate a deep dive of this paper. The paper has the rest either way.
Alex: Thanks for listening.