Abiotic Production of Methane in Terrestrial Planets

Abstract

On Earth, methane is produced mainly by life, and it has been proposed that, under certain conditions, methane detected in an exoplanetary spectrum may be considered a biosignature. Here, we estimate how much methane may be produced in hydrothermal vent systems by serpentinization, its main geological source, using the kinetic properties of the main reactions involved in methane production by serpentinization. Hydrogen production by serpentinization was calculated as a function of the available FeO in the crust, given the current spreading rates. Carbon dioxide is the limiting reactant for methane formation because it is highly depleted in aqueous form in hydrothermal vent systems. We estimated maximum CH 4 surface fluxes of 6.8 · 10 8 and 1.3 · 10 9 molecules cm -2 s -1 for rocky planets with 1 and 5 M 4 , respectively. Using a 1-D photochemical model, we simulated atmospheres with volume mixing ratios of 0.03 and 0.1 CO 2 to calculate atmospheric methane concentrations for the maximum production of this compound by serpentinization. The resulting abundances were 2.5 and 2.1 ppmv for 1 M 4 planets and 4.1 and 3.7 ppmv for 5 M 4 planets. Therefore, low atmospheric concentrations of methane may be produced by serpentinization. For habitable planets around Sun-like stars with N 2 -CO 2 atmospheres, methane concentrations larger than 10 ppmv may indicate the presence of life.

Constraints on the Early Terrestrial Surface UV Environment Relevant to Prebiotic Chemistry

“…Kasting and Brown (1998) estimate a methane abundance of 0.5 ppm (mixing ratio of 5×10 −7 ) for a 1-bar N 2 -CO 2 atmosphere assuming conversion of 1% carbon flux from mid-ocean ridges to CH 4 prior to the rise of life. Kasting (2014) echo this estimated mixing ratio for a CH 4 source of serpentinization of ultramafic rock with seawater at present-day levels, while Guzmán-Marmolejo et al (2013) estimate serpentinization can drive CH 4 levels up to 2.1 ppmv. However, Kasting (2014) also note that methane production from impacts could have outpaced the supply from serpentinization multiple orders of magnitude, and could have sustained abiotic CH 4 levels up to 1000 ppmv.…”

The Pale Orange Dot: The Spectrum and Habitability of Hazy Archean Earth

“…, on extremely cold worlds like Titan with reducing atmospheres), on a planet like Archean Earth, the presence of hydrocarbon haze may require a higher level of methane production than is possible from abiotic sources alone. The maximum abiotic methane production rate from serpentinization, its primary nonbiological source, has been estimated as 6.8 × 10 8 and 1.3 × 10 9 molecules/cm 2 /s for rocky planets of 1 and 5 Earth masses, respectively (Guzmán-Marmolejo et al , 2013), although there has been earlier speculation of higher abiotic production rates (Kasting, 2005; Shaw, 2008), especially if ancient seafloor spreading rates were faster or the amount of iron-rich ancient seafloor rock was greater. Based on their calculations, Guzmán-Marmolejo et al (2013) suggested that an atmospheric CH 4 concentration greater than 10 ppmv is suggestive of life.…”

“…The maximum abiotic methane production rate from serpentinization, its primary nonbiological source, has been estimated as 6.8 × 10 8 and 1.3 × 10 9 molecules/cm 2 /s for rocky planets of 1 and 5 Earth masses, respectively (Guzmán-Marmolejo et al , 2013), although there has been earlier speculation of higher abiotic production rates (Kasting, 2005; Shaw, 2008), especially if ancient seafloor spreading rates were faster or the amount of iron-rich ancient seafloor rock was greater. Based on their calculations, Guzmán-Marmolejo et al (2013) suggested that an atmospheric CH 4 concentration greater than 10 ppmv is suggestive of life. At the range of pCO 2 allowed by Driese et al (2011), we find that the CH 4 flux needed to initiate haze formation ranges between about 1 × 10 11 and 3 × 10 11 molecules/cm 2 /s, broadly consistent with estimates for the biological Archean methane flux after the origin of oxygenic photosynthesis (Kharecha et al , 2005; Claire et al , 2014).…”

“…In addition, several abiotic processes including serpentinization (the hydration of ultramafic rocks, mainly olivine and pyroxenes) can form methane (Kelley et al , 2005; Etiope and Sherwood Lollar, 2013; Guzmán-Marmolejo et al , 2013), but the biotic flux of methane to the Archean atmosphere was likely much higher than the abiotic flux (Kharecha et al , 2005), as it is on Earth today. While the climatic effects of this haze have been studied ( e.g.…”

Biosignature False Positives

“…While life produces the overwhelming majority (>99%) of CH 4 in the Earth's atmosphere (Kasting 2005), some geologic processes emit small amounts of CH 4 (e.g., Etiope and Sherwood Lollar 2013). Additionally, serpentinization, the hydration of ultramafic (e.g., basaltic) seafloor, releases substantial amounts of H 2 , which can (in the presence of CO 2 ) result in CH 4 production (Guzmán-Marmolejo et al 2013;Etiope and Sherwood Lollar 2013). However, there is wide disagreement on the fraction of CH 4 from serpentinizing systems on Earth that is biological, rather than geochemical in nature; this is an important area of future work for this false-positive source.…”