The amounts of methane and δ13CCH4 evolving from pyrolysis of the Sample Analysis of Mars evolved gas analysis (SAM-EGA) instrument suite as reported in ref. 1 are inconsistent with fundamental observations of thermogenic formation of methane from organic matter precursors. In pyrolysis, samples with organic carbon are heated, whereby the carbon isotopic composition of the evolving methane (δ13CCH4) is depleted in 13C compared to the organic carbon precursor and increases systematically with increasing temperature (e.g., figure 11 in ref. 2 and figure 5 in ref. 3). The pyrolysis setup of the SAM-EGA instrument suite is that of the so-called open-system pyrolysis for which quantitative data are available (3).
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