Most people interested in Solar System exploration know that in 1976, two craft in NASA’s Viking mission landed at two sites on Mars. Most of these people also know that the Viking landers carried out experiments that sought evidence for life in Martian soils at those landing sites. However, surprisingly few people can say with accuracy what data the Viking experiments actually collected. Fewer have understood how our understanding of Viking results has changed in light of information gathered since.
And worrisomely, still fewer have freed themselves from an incorrect “community consensus” that began to be formulated in 1976, that “Viking failed to find signs of extant life on Mars“. This “consensus” has persisted long after 2010, when it was shown that it was based on mistakes made in the early interpretation of the Viking data. The fact that the “consensus” has persisted so long is a window into the culture of the Mars exploration community.
In an effort to correct this, I have written a book entitled: Meet the Neighbors. Life on Mars and How to Find It. It will be released by Penguin Press in July on the 50th anniversary of the Viking landings (Benner, 2026, Meet the Neighbors. Life on Mars and How to Find It).
However, since “To Long Didn’t Read” is the mantra for today, I am posting here a series of short Single Scoops for those who are crushed for time. This, the first, summarizes all of the data from Viking that is directly relevant to life detection.
This summary will be followed by sets of Single Scoops, one set for each of the Viking experiments.
1. The first Scoop in each set will explain how and why the individuals who created each Viking experiment designed their experiments. This was the intention of the experiment.
2. The second will describe what experimental data were actually observed when the experiment was run on Mars, and how they were interpreted, immediately and then in light of other data that led to the incorrect “consensus”.
3. The third will discuss how we reinterpret those observations today in light of what we have learned since 1976 about the Martian environment.
4. Jan Spacek and his team have reintroduced the concept of Mars penetrators collected in a distributed rideshare system where the cost of each is a few tens of thousands of dollars (International Mars Prospector Ride Share, IMPRESS). The fourth Scoop in each set will describe experiments that might be done on such penetrators to help resolve the remaining Viking uncertainties, and Earth-based experiments to help design them.
5. The last Scoop in each set will comment on the sociology of this science, how the community “consensus” sees the same data, and what students in astrobiology can learn from the mistakes of their elders.
For now, here is what you need to know about the Viking experiments, with interpetations of Viking results presented in the terms of the experiments’ original design, without more.
• The Viking experiments that sought evidence for Martian photosynthesis saw it. Martian soils fixed radioactive 14CO2 in amounts corresponding to ~1000 Earth-like cells per cubic centimeter of soil. The amount of carbon fixation may be 100x larger depending on how much perchlorate was present in the soil, the ratio of products arising from the oxidation of organics by perchlorate and nitrate, and how much 14CO2 was absorbed unfixed in the soil as carbonates. This inferred Martian bio-load is compatible with bio-loads seen in resource limited environments here on Earth (Carrier et al. 2020). No reasonable mechanism has been proposed to explain the carbon fixation without biology. However, non-fixed 14CO2 absorbed on the soil as inorganic carbonates stable at 120 °C might offer a false positive.
• The Viking experiments seeking Martian respiration saw it. Martian soils released radioactive 14CO2 from 14C-labeled “food” in amounts of 14CO2 also consistent with results from Earth soils in resource limited environments. Perchlorate cannot directly offer a non-biological explanation for these results. An alternative non-biological oxidant might explain these results, but so far, none of the oxidants proposed fit the data in terms of rates.
• The Viking experiments seeking exchanges of gasses in and out of the Viking soils saw them. Soils harboring life were expected to exchange gasses agnostic of their metabolic survivl strategy. CO2 exchange was seen, together with an unexpectedly large amount of O2 released upon soil humidification. The unexpected result is compatible with both biologically stored O2 and abiologically stored O2. The “consensus” interpretation, that these data show that the soils must contain a “highly reactive oxidant”, has little experimental support.
• The Viking gas chromatograph-mass spectrometer (GC-MS) detected large amounts of organic molecules in the Viking soils. These were detected indirectly, in the form of methyl chloride and other chlorinated species that were created by their oxidation by perchlorate, also present in the Viking soils (Navarro-Gonzalez et al., 2010). This interpretation has been confirmed by the Curiosity rover (see Pavlov et al., 2025 and references therein).
To read more, we have posted an e-letter correcting the 1976 paper in Science that incorrectly interpreted methyl chloride as a “terrestrial contaminant” rather than a product of perchlorate oxidation of indigenous organics (Benner et al., 2025). A peer-reviewed paper in Astrobiology last month lays out these facts in more detail (Benner et al., 2026).
And two recent important papers from the Perseverance and Curiosity rover teams relevant to past Martian life are here: Hurowitz et al., 2025 and Pavlov et al., 2025.
References
Benner, S. A., Schulze-Makuch, D., Spacek, J., Abraham, C. A. (2025) The mistaken assignment in this paper of Martian methyl chloride as a “terrestrial contaminant” obstructed Mars exploration for a half century, continuing to today [eLetter]. Science https://www.science.org/doi/10.1126/science.194.4260.72#elettersSection
Benner, S. A., Schulze-Makuch, D., Spacek, J., Abraham, C. A. (2026) Viking Mars, now 50 years old, still needs a scientific analysis. Astrobiology doi:10.1177/15311074251404929
Carrier, B. L., Beaty, D. W., Meyer, M. A., Blank, J. G., Chou, L., Dassarma, S., des Marais, D. J., Eigenbrode, J. L., Grefenstette, N., Lanza, N. L., Schuerger, A. C., Schwendner, P., Smith, H. D., Stoker, C. R., Tarnas, J. D., Webster, K. D., Bakermans, C., Baxter, B. K., Bell, M. S., Benner, S. A., Bolivar Torres, H. H., Boston, P. J., Bruner, R., Clark, B. C., Dassarma, P., Engelhart, A. E., Gallegos, Z. E., Garvin, Z. K., Gasda, P. J., Green, J. H., Harris, R. L., Hoffman, M. E., Kieft, T., Koeppel, A. H. D., Lee, P. A., Li, X., Lynch, K. L., MacKelprang, R., Mahaffy, P. R., Matthies, L. H., Nellessen, M. A., Newsom, H. E., Northup, D. E., O’Connor, B. R. W., Perl, S. M., Quinn, R. C., Rowe, L. A., Sauterey, B., Schneegurt, M. A., Schulze-Makuch, D., Scuderi, L. A., Spilde, M. N., Stamenković, V., Torres Celis, J. A., Viola, D., Wade, B. D., Walker, C. J., Wiens, R. C., Williams, A. J., Williams, J. M., Xu, J. (2020) Mars Extant Life: What’s Next? Conference Report. Astrobiology 20, 785–814.
Chandler, D. (1977, February). Life on Mars. The Atlantic Monthly, 239(2), 18–24.
Hurowitz, J. A., Tice, M. M., Allwood, A. C., Cable, M. L., Hand, K. P., Murphy, A. E., … & Wolf, Z. U. (2025). Redox-driven mineral and organic associations in Jezero Crater, Mars. Nature, 645(8080), 332-340.
Navarro‐González, R., Vargas, E., de La Rosa, J., Raga, A. C., & McKay, C. P. (2010). Reanalysis of the Viking results suggests perchlorate and organics at midlatitudes on Mars. Journal of Geophysical Research: Planets, 115(E12).
Pavlov, A. A., Freissinet, C., Glavin, D. P., House, C. H., Stern, J. C., McAdam, A. C., … & Gomez, F. (2025). Does the Measured Abundance Suggest a Biological Origin for the Ancient Alkanes Preserved in a Martian Mudstone?. Astrobiology, 15311074261417879.
Spacek, J., Benner, S.A. (2022) Agnostic Life Finder (ALF) for large-scale screening of Martian life during in situ refueling. Astrobiology 22, 1255-1263. doi.org/10.1089/ast.2021.0070
Spacek, J. (2024)IMPRESS to Deliver Art and Science to Mars. Primordial Scoop, e20240920. https://doi.org/10.52400/PCFY7624
Posted with minor revisions by Jan Spacek 2/18/2026.
