Chemical model for Viking biology experiments: implications for the composition of the martian regolith

Abstract

THE 1976 Mars Viking biology experiments were designed to detect life by observing the products of biochemical reactions. In the labelled-release (LR) experiments^1–4, about 25 nmol of ¹⁴C-labelled gases evolved when regolith samples were moistened with nutrient solution. About 22% of the products reabsorbed upon second injection. As a biological test the LR results were positive, although the reabsorption was not readily explained. In the gas-exchange (GEX) experiments, up to 800 nmol of O₂ gas was evolved when samples were humidified^5,6, suggesting that the martian regolith might contain a strong chemical oxidant which caused the LR results. Several chemical models have been proposed^7,8 but no self-consistent explanation of all of the observations has been achieved. Here we propose a chemical model for these biology experiments in which the reactants are an inorganic nitrate salt, which has been partly photolysed by ultraviolet light, and a sparingly soluble metal carbonate such as calcite. The model reproduces the main effects seen, indicating that nitrates are present in the martian regolith as well as calcite (or some other carbonate with similar solubility).