Gil Levin, one of the scientists involved with the 1976 Viking mission to Mars, has claimed and continues to claim that there is life on Mars, based off the positive results of the Labeled Release (LR) experiment.

This was the only one of the landers' four life-detecting instruments that returned positive results, but prior to the mission it was expected that because of the instruments' radically different approaches, actual Martian organisms might still only return positive results in one of them. Furthermore, there is no evidence (to my knowledge) that the LR instrument was faulty, nor that LR's strong positive results could be explained by any non-biological process. (Levin details his fairly comprehensive explanation and argument for life on Mars in this paper).

Despite these findings, scientific consensus still posits that no life on Mars has ever been detected, by LR or any other experiment. My question is simply: Why?

Can anyone point me to a credible source, statement, or publication that addresses LR's results and explains why they don't constitute sufficient evidence for life on Mars?

  • $\begingroup$ Extraordinary claims require extraordinary evidence. Viking did not provide that. You should ask the other question: why, with such poor and ambiguous evidence, would anyone believe there is life on Mars? $\endgroup$ Oct 15, 2019 at 11:22
  • $\begingroup$ This kind of attitude is exactly what I'm talking about. Where is the rationale? Yes, there's the extraordinary claims/evidence argument, but in this case, the extraordinary evidence is that an instrument specifically designed to test for life in Mars performed multiple successful tests without error and produced positive results every time, with no plausible alternative explanations. At what point do we say, "We set out to test for this thing, and our test says we found it"? $\endgroup$
    – ETL
    Oct 16, 2019 at 2:22
  • $\begingroup$ What attitude? You mean scientific rigor? The experiment was NOT designed to find life, none so far is. It was designed to find released $^{14}CO_2$ gas and it did, albeit in a confusing and incoherent manner (your statement "produced positive results every time" is wrong, why would you lie on the internet), that is inconsistent with what we'd expect from life. The other experiments on the Vikings gave negative results. I don't see any strong evidence for life here. Also only parroting one guy that has a history of overselling his results is not a very credible approach to science. $\endgroup$ Oct 16, 2019 at 9:50
  • $\begingroup$ You're right, I should've said "produced positive results every time, except in control scenarios where negative results were expected and further supported the hypothesis." Beyond that, your logic is circular and your evidence nonexistent. Welcome to StackExchange, please provide sources for the "confusing and incoherent" results you claim were produced. I started this question by stating that I couldn't find sources that reliably refuted "the one guy." If you have one, please share and post an answer. Otherwise, you're just parroting what YOU have been told. Where's the scientific rigor? $\endgroup$
    – ETL
    Oct 16, 2019 at 17:51
  • 1
    $\begingroup$ Short follow-up: The Yen+2000 paper science.sciencemag.org/content/289/5486/1909.full.pdf+html seems to explain the experiment+control well. It has been cited by a paper with Levin on it only once: koreascience.or.kr/article/JAKO201213549741763.page which I haven't read yet. Is that the explanation which you've referred to as 'disproven' in your comment to @MSalters answer? $\endgroup$ Oct 16, 2019 at 20:40

1 Answer 1


The Search for Life on Mars (Christopher P. Mckay, 1996) quotes

  1. Klein, H. P.: 1978, Icarus 34, 666–674.
  2. Klein, H. P.: 1979, Rev. Geophys. Space Phys. 17, 1655–1662.
  3. Horowitz, N. H.: 1986, To Utopia and Back: The Search for Life in the Solar System, W.

Also by Mckay:

4 .McKay C. P. et al. 1998. The Mars Oxidant experiment (MOx) for Mars '96. Planet. Space Science 46:769–777.

In short, it's the common scientific problem. The Labeled Release Experiment is not evidence against life. The result outcomes are compatible with multiple mechanisms, including but not limited to microbial life.


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