An insufficient methane budget for warming noachian and hesperian mars

Introduction: Mars is known to have warmed to above-freezing temperatures during the Noachian and Hesperian based on geomorphic evidence for fluvial channels [1] and paleolakes [2]. Wordsworth et al. [3] demonstrate that above-freezing temperatures can be achieved in the Noachian and Hesperian if there was a 1.25-2 bar CO2 atmosphere with 2-10% CH4 and H2 due to the greenhouse and collision induced absorption effects of these molecules with the CO2-rich atmosphere. These transient reducing greenhouse atmospheres (TRGAs) would last for 10-10 years, which is consistent with the expected timescales for delta formation and overall lack of chemical weathering that are observed on the surface [4]. Given the obliquity variations Mars is expected to have experienced throughout its history, release of CH4 to the atmosphere through depressurization of CH4 clathrate via latitudinal ice migration to form TRGAs during the Noachian and Hesperian is a hypothesis consistent with geomorphic observations [1,2], mineralogic observations [5], modeling results [4] and atmospheric paleopressure estimations [6-9]. Impacts and volcanism can also destabilize CH4 clathrate in the Noachian cryosphere. Here we demonstrate that while CH4 is thermodynamically stable throughout the Noachian crust, kinetic barriers to its formation via CO2-reduction make it difficult to form sufficient CH4 for a single TRGA given the expected amount of available H2 from radiolysis [10] and serpentinization [11] and our current understanding of abiotic methane formation on Earth. Methods: To estimate the amount of CH4 that could be produced abiotically in the Noachian crust via the Sabatier reaction, we first quantify the thermodynamic stability of CH4 with respect to depth in the crust using the CHNOSZ model [12], which uses the SUPCRT92 thermodynamic database [13]. Parameters fed into the thermodynamic database include the crustal temperature-vs-depth profile with a surface temperature boundary condition from climate models [14], the expected geothermal heat flux [15], and assuming heat transport by conduction [16], in addition to hydrostatic pressure, and dissolved H2 concentrations [10]. CH4 is thermodynamically stable throughout most of Earth’s crust [17]. However, its formation via reduction of CO2, the primary form of carbon input into the terrestrial crust, is kinetically inhibited, typically requiring metal catalysts on which H2 and CO2 can adsorb to facilitate the reaction. Awaruite [18] and chromite [19] are metal catalysts demonstrated to facilitate CH4 formation in natural systems. Abiotic CH4 formation on Noachian Mars may have occured in crustal regions that were rich in these metal alloys. On Earth, some serpentinites are mined for Ni due to the concentration of Ni in awaruite [20] formed in association with highly-reducing fluids involved in serpentinization reactions [18]. Lithologically similar crustal regions could have hosted abiotic CH4-forming environments on Mars, in addition to chromite-rich igneous rocks.