if Phoenix-like levels of perchlorates were present in the Viking samples, the organic content of the Martian soil could have been as high as 0.1% and still would have produced the (false) negative result that the GCMS returned. Thus, while conventional wisdom regarding the Viking biology experiments still points to "no evidence of life", recent years have seen at least a small shift toward "inconclusive evidence"." According to a 2010 NASA press release: "The only organic chemicals identified when the Viking landers heated samples of Martian soil were chloromethane and dichloromethane -- chlorine compounds interpreted at the time as likely contaminants from cleaning fluids." According to a paper authored by a team led by Rafael Navarro-González of the National Autonomous University of Mexico, "those chemicals are exactly what [their] new study found when a little perchlorate -- the surprise finding from Phoenix -- was added to desert soil from Chile containing organics and analyzed in the manner of the Viking tests." However, the 2010 NASA press release also noted that: "One reason the chlorinated organics found by Viking were interpreted as contaminants from Earth was that the ratio of two isotopes of chlorine in them matched the three-to-one ratio for those isotopes on Earth. The ratio for them on Mars has not been clearly determined yet. If it is found to be much different than Earth's, that would support the 1970s interpretation." Biemann has written a commentary critical of the Navarro-González and McKay paper, to which the latter have replied; the exchange was published in December 2011. In 2021 the chlorine isotope ratio on Mars was measured by the Trace Gas Orbiter and found to be almost indistinguishable from the terrestrial ratio, leaving the interpretation of the GCMS results inconclusive. Gas exchange The gas exchange (GEX) experiment (PI: Vance Oyama, NASA Ames) looked for gases given off by an incubated soil sample by first replacing the Martian atmosphere with the inert gas helium. It applied a liquid complex of organic and inorganic nutrients and supplements to a soil sample, first with just nutrients added, then with water added too. Periodically, the instrument sampled the atmosphere of the incubation chamber and used a gas chromatograph to measure the concentrations of several gases, including oxygen, CO2, nitrogen, hydrogen, and methane. The scientists hypothesized that metabolizing organisms would either consume or release at least one of the gases being measured. In early November