Researchers submit 23-page reanalysis of 1976 Viking carbon-assimilation experiment

Researchers Jan Spacek, Clay A. Abraham and Steven A. Benner submitted a 23-page reanalysis of the 1976 Viking Carbon Assimilation-Pyrolytic Release experiment to the journal Astrobiology. The manuscript carries arXiv identifier 2610.30368, and version 1 of the preprint appeared on Oct. 21, 2026 at 19:29:31 UTC at 476 kilobytes.

The Viking Carbon Assimilation-Pyrolytic Release experiment, called CAPyR, tested whether radioactive carbon dioxide and carbon monoxide entered the Martian soil during a 120-hour incubation, looking for carbon that survived a 120-degree Celsius purge but was released by pyrolysis, or heating to break down material, at 635 degrees Celsius. Radioactivity not caught by a trap was recorded as Peak 1, while material held in an organic vapor trap was oxidized and recorded as Peak 2, intended to indicate synthesized reduced organic carbon.

The reanalysis says the results match a biological carbon-assimilation system that kept working after C5’s 120-degree Celsius/90-degree Celsius pretreatment but was sharply weakened by C2’s 175-degree Celsius pretreatment. The authors say CAPyR and the Labeled Release experiment suggest separate soil portions displayed oxidative chemistry and reductive chemistry drawn from common Martian acquisitions. No non-biological process has been shown to match every CAPyR observation under Mars-relevant conditions, and Hubbard’s 1979 mineral-substrate experiments produced abiotic Peak 2 signals without matching the Viking data’s heat sensitivity.

Seven of the nine usable tests on Chryse Planitia and Utopia Planitia yielded significant net Peak 2 signals, including the dark U1 run. A gas leak on Viking Lander 2 that was not widely reported leaves runs U1 through U3 in doubt, and researchers put the C1 signal at roughly 10 or 30 picomoles of carbon, varying with whether the label came entirely from carbon monoxide or carbon dioxide.

Earlier work on CAPyR left out this perchlorate-specific effect. The new analysis says possible perchlorate salts in Viking soil might oxidize fixed organic radiocarbon during pyrolysis, moving the radioactive label from Peak 2 to Peak 1.

In 2008, NASA’s Phoenix lander found high perchlorate concentrations in Martian soil. When heated, perchlorate can chemically destroy organic compounds, and researchers viewed it as the probable reason the molecular analysis instrument did not find organics.

Viking 1 touched down on Mars on July 20, 1976, and Viking 2 followed on September 3, 1976, marking the first two times working spacecraft soft-landed on another planet. Only the Labeled Release experiment gave results that hinted at a biological source, while the carbon assimilation test and the gas exchange test both pointed toward chemical rather than biological reactions. The lead researchers concluded that the Viking landers did not make an unambiguous discovery of life, although life cannot be ruled out. Viking’s atmospheric measurements suggested Mars once possessed a far thicker atmosphere before losing it, and the mission observed wind lifting dust particles that color the sky permanently pink.

Astrobiologist Dirk Schulze-Makuch of Technical University Berlin proposed that Viking’s life-detection tests exposed Martian microbes to deadly amounts of liquid water. Schulze-Makuch said NASA might have uncovered Martian life nearly 50 years ago and then accidentally destroyed it before realizing the discovery.

A 2018 Scientific Reports study of severe flooding in Chile’s Atacama Desert found that sudden water killed up to 85 percent of native microbes. Cornell University astrobiologist Alberto Fairén, who co-authored the 2018 research, said he fully agrees that adding water during the Viking tests may have killed potential Martian organisms.

Schulze-Makuch’s hypothesis does not prove that Viking found life, but it shows that life-detection experiments can fail if they assume alien organisms will respond like terrestrial microbes. He points to Atacama Desert microbes that live inside hygroscopic rocks drawing moisture from the air, and suggests that if Martian organisms evolved to rely on trace atmospheric moisture, a sudden soaking could overwhelm them.Future Mars missions may require tests that preserve natural conditions instead of adding water immediately or using strong heat, and instruments could study samples at various moisture levels. The discussion supports using multiple independent methods because a single chemical reaction can look like metabolism and one destructive preparation step may wipe out organic evidence before instruments detect it.

The Foundation for Applied Molecular Evolution is led by Steven A. Benner, who built a model for current Martian life using measurements from the three Viking biology experiments. Under the Benner model, microbes might consume the radioactive carbon nutrient as food and release radioactive carbon dioxide, and at night they could take in oxygen and emit carbon dioxide. The model raises the possibility that living microorganisms exist on Mars’s surface, though confirming this would require further research and measurements.