Recent results from the Viking Labeled Release experiment on Mars

The Labeled Release (LR) life detection experiment seeks detection of heterotrophic metabolism by monitoring radio-active gas evolution following the addition of a radioactive nutrient containing seven C-14 labeled organic substrates to surface material. LR results obtained on Mars prior to conjunction showed rapid evolution of radioactive counts upon addition of the nutrient to a fresh surface sample. The responses at both landing sites were quite similar. The additional results summarized in the present paper provide further information on the gas kinetics following a second injection of nutrient and on the effect of 'cold sterilization' of the Martian surface material. Specifically, it is shown that the production of gas from the LR nutrient is remarkably uniform; after reaction approaches completion, addition of more nutrient results in a net loss of the radioactive gas; the reactant in the Mars soil is completely inhibited by heating the soil to 160 C, and is largely inhibited by heating to 46 C. In contrast, exposure to 18 C for 2 sol (1 sol = 24.6 hr) does not appear to inhibit the reaction.