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Project Progress

Production of Mars methane by fluid-rock interaction in the crust

This work was a collaborative effort, involving J. Lyons, C. Manning, and F. Nimmo (now at UC Santa Cruz), to understand possible sources for the methane recently discovered in the Martian atmosphere. Detection of methane was reported by three different groups (Mumma et al. 2003; Formisano et al. 2004; Krasnopolsky et al. 2004). The observed methane volume fraction varied from 10 ppb (parts per billion) to about 100 ppb methane, with substantial (and surprising) variations with longitude. Given that terrestrial methane is primarily a product of methanogens, it is natural to consider Mars methane to also be biogenic. However, the methane flux implied by 10 ppb methane is only about 10⁷ moles per year, about one-millionth the terrestrial methane flux. Because many abiogenic sources of methane on Earth produce comparable flux levels, we chose to explore abiogenic geochemical sources on Mars.

An obvious possible methane source is serpentinization, in which Fe²⁺ in olivine is oxidized to Fe³⁺ during aqueous alteration. The redox couple is balanced by reduction of C in the fluid (e.g., as carbonate) to methane. Another possible methane source, and the one that we focused on, is thermochemical reactions in hydrothermal fluids. Previous work on the shergottite meteorites demonstrated that the oxygen fugacity in the Martian crust is no higher than that of a quartz-fayalite-magnetite (QFM) rock buffer, and may be as low as QFM-4 (in fO₂ log units). At these oxygen fugacities magamtic CO₂ in a water-dominated hydrothermal fluid will be converted to CH₄ at temperatures of about 400 °C and less, assuming thermochemical equilibrium. Phase separation of supercritical water and methane yields methane vapor, which can then diffuse through the crust and reach the atmosphere. We showed that a very small magmatic intrusion (~1 km wide x 10 km long) at a depth of 10 km could easily explain a 10 ppb methane fraction in the Mars atmosphere. This work has been accepted for publication in Geophysical Research Letters. (The thermochemistry and phase diagram calculations in this work were carried out primarily by C. Manning using methods he developed for abiogenic methane production on early Earth. Nimmo provided the geophysical context for the work. Lyons provided the atmospheric context and also initiated the project.)