Organic molecules on Mars

You do not seem to be aware, or at least ignore the positive results for living organisms returned in situ from the Viking I and II landers in 1976 by the Labeled Release (LR) experiment.. This work has been summarized and published by the primary investigator, Dr, Gilbert Levin. Here is a recent summary from Professor Levin: https://www.youtube.com/watch?time_continue=53&v=Y4IIQPwxmwY

Sincerely,
Dr. George D. Nickas
Corpus Christi, TX

I would like to give my thanks to Inge Loes ten Kate for writing this excellent analysis named "Organic molecules on Mars" and I would also like to thank Science for publishing it. The author is correct when he says that the recent findings of Curiosity rover are "breakthroughs in astrobiology". But I have some disagreements when he states the following concerning NASA's 1976 Viking mission: "However, neither signs of life nor organic compounds were detected in the regolith samples analyzed during this mission".
Each of the two Viking Landers carried one Labeled Release (LR) Experiment, which tested the Martian soil for metabolic activity. The results from the experiment are ambiguous. So far the prevailing scientific consensus is that these results should rather be explained by non-biological sources, but not everyone agrees. I would like to remind that in the Science's article published in 1976 by Gilbert V. Levin and Patricia Ann Straat, it was stated that regardless of the fact Viking's GCMS couldn't detect organic compounds, no chemical experiment has quantitatively reproduced the LR Mars data, which is a reason why biological activity should be considered (1). Decades later, in 2016 G.V. Levin and P. A. Straat published another article (2) in which they once again affirmed that a non-biological agent cannot satisfy all Viking findings, thus biology should be considered as an explanation.
As a researcher I follow the principle that extraordinary claims require extraordinary evidence. Even with the latest discovery of organic molecules on Mars I agree that we need more experiments to unravel the mystery whether Martian life exists or not. The renewed search of organics, conducted by Phoenix mission in 2008 and Curiosity nowadays, is a step in the right direction. ESA's ExoMars mission, which will send the Pasteur rover in 2020s, also has a complicated molecule analyzer called MOMA, and Mars 2020 rover has the SHERLOC instrument. But even as Inge Loes ten Kate concludes that "the question of whether life might have originated or existed on Mars is a lot more opportune now that we know that organic molecules were present on its surface at that time", I am worried that these missions may not be able to give satisfactory answers. So far modern spacecraft have solved Viking's puzzles only partly – by sending improved gas chromatograph and mass spectrometers, we now know there are organic molecules on Mars. Conducting new experiments that can test the metabolic activity of the soil should, in my opinion, be considered once again by space agencies. Only then we could rule out non-biological or biological explanations for Viking's LR experiment. This would bring us a great step closer towards determining the existence of Martian life.

1. G.V. Levin, P.A. Straat, Viking labeled release biology experiment: interim results. Science. 194.4271, 1322-1329 (1976).
2. G.V. Levin, P.A. Straat, The case for extant life on mars and its possible detection by the viking labeled release experiment. Astrobiology. 16.10, 798-810 (2016).