A Technical Reanalysis of the Viking Carbon Assimilation-Pyrolytic Release Experiment
The 1976 Viking Carbon Assimilation-Pyrolytic Release (CAPyR) experiment tested whether carbon from $^{14}$CO$_2$ and $^{14}$CO entered Martian soil during a 120-hour incubation in a form retained during a 120 $^\circ$C purge but released by pyrolysis at 635 $^\circ$C. Untrapped radioactivity was measured as Peak 1; material retained in an organic vapor trap was oxidized and measured as Peak 2. Net Peak 2 was intended to represent synthesized reduced organic carbon. Seven of nine usable incubations at Chryse and Utopia Planitia yielded significant net Peak 2 signals, including the dark U1 run. An underreported Viking Lander 2 gas leak introduces uncertainty into U1-U3. The C1 signal corresponds to approximately 10 or 30 pmol of carbon, assuming derivation entirely from CO or CO$_2$, respectively. We reassess these results in light of the possible presence of perchlorate salts in Viking soils. Oxidation of fixed organic radiocarbon during pyrolysis could have shifted label from Peak 2 to Peak 1, a perchlorate-specific effect not considered in previous CAPyR analyses. Hubbard's (1979) mineral-substrate experiments produced abiotic Peak 2 signals without reproducing the heat sensitivity. No demonstrated abiotic mechanism has reproduced all CAPyR observations under Mars-relevant conditions. The observations remain consistent with a biological carbon-assimilation system whose relevant activity survives C5's 120 $^\circ$C/90 $^\circ$C pretreatment but is strongly impaired by C2's 175 $^\circ$C pretreatment. Together with the Labeled Release experiment, CAPyR suggests oxidative and reductive chemistry in separate aliquots from common Martian soil acquisitions. Metabolically active life combines these processes; a quantitative abiotic explanation of their combined heat-labile response patterns remains to be demonstrated.