Chemical weathering and the Viking Biology Experiments on Mars

Abstract

The activity observed by the three Viking lander life detection experiments is reviewed, and the active agents are characterized. It is proposed that the activity in all three experiments may have been caused by a single active agent in the Martian soil, resulting from contemporary chemical weathering of mafic silicates by low‐temperature frost and adsorbed H2O. Earlier laboratory studies indicated that chemisorbed H2O2 may be produced by frost weathering, and it was shown that the active cycle results of the Viking gas exchange (GEx) and labeled release (LR) experiments could be adequately simulated. Those studies were recently supported by LR test standards module (TSM) tests. The TSM tests further supported the prediction that the sterilization and long‐term storage results of the LR and GEx experiments may have been consequences of the frost‐weathering process as well. In addition,those tests now permit the model to be extended to include the activity in the pyrolytic release (PR) experiment. The activity is consistent with the presence of ∼1019 molecules cm−3 of chemisorbed H2O2 in the soil, and its lifetime is estimated to be ∼104–106 years. Decomposition is predicted to result in the production of ∼1013–1015 O2 molecules cm−2 yr−1 per centimeter depth. A comparable loss rate for O2 would be required to maintain the currently observed O2/CO2 abundance ratio of 1.3×10−3 Contemporary surface photooxidation is proposed as a possible candidate loss process. Over geologic time, frost weathering and surface oxidation may have resulted in the loss of ∼1 m to 1 km of ice and produced significant variations in atmospheric and regolith oxidation states. The present‐day loss rates indicate that regolith vapor sources may be replenishing atmospheric H2O at mean annual rates of 10−4–10−1 km3 (equivalent ice) yr−1.