Quantifying Low Temperature Production of Methane on Mars

Introduction: Methane was first detected in the Matian atmosphere in 2003 [1-3], and its implications have been widely debated [e.g., 4-8]. Methane can be generated in terrestrial biological systems, and so is a potential indicator for past or present life (see e.g., [4,5]). However, methane can be produced anorganically (without intervention of life) in several sorts of terrestrial environments, some of which are relevant for Mars [4-7,9-13]. On Earth, anorganic methane can be produced by redox reactions associated with highand low-temperature alteration of Fe-bearing rocks, so long as carbon is present. Methane production is common in mid-ocean ridge hydrothermal systems, and wherever serpentine is forming [6,7]. In fact, the anorganic production of methane on Earth is estimated to be 50–70 Mt y [15]. Methane-producing reactions, including serpentinitization, can also provide significant energy that can be utilized by living organisms [14]. Thermochemical modeling of rock-water chemical reactions shows that, in addition to methane, molecular hydrogen and other reduced gas species can be produced. The alteration phases formed by those processes include hydrous silicates. Here, I quantify methane production and alteration minerals from three Martian rock compositions and pure olivine and compare them to the amounts released and observed in 2003. Method: The CHILLER code [16] is used to evaluate gas phases and mineral assemblages that are likely to form from Martian rocks in low-temperature alteration environments. Earlier, Schwenzer and Kring [17,18] modeled LEW 88516 whole rock composition, LEW 88516 olivine and Chassigny at 1 °C at 1 bar (= near Mars’ surface); here I add models with the composition of the Martian rock Fastball [19] at those conditions and at 13 °C and 110 bar (~ 1 km depth [20]). CO2 concentration in the solution is adjusted to that in equilibrium with current Martian atmosphere: 0.2x10 mole/L. The system is closed to atmosphere and no further supply of C-species is available during consumption of CO2 through the reaction. For comparison, I ran a model with higher CO2 concentration to explore the effects of a potentially denser atmosphere on early Mars or subsurface CO2 sources. Results are in Figure 1-4, and Table 1.