Investigating the contribution of methane produced by ablating micrometeorites to the atmosphere of Mars

https://doi.org/10.1016/j.epsl.2009.09.041Get rights and content

Abstract

The presence of methane in the atmosphere of Mars has been suggested as evidence of life, as methane has a short lifetime in the atmosphere of Mars of just a few hundred years, requiring replacement by mechanisms, continuous or episodic, such as biology, volcanism, serpentinization of ultramafic crust or large cometary or asteroidal impacts. The potential of meteoritic infall to deliver significant quantities of methane is restricted by the low abundance of free methane in carbonaceous meteorites. However, the delivery of meteoritic material to the surface of a planet is an energetic process, and the ability of carbonaceous meteorites to generate methane upon ablation during atmospheric entry has not been previously considered. Here, we use analytical pyrolysis to simulate the ablation and pyrolysis of carbonaceous micrometeorites upon atmospheric entry, and Fourier-transform infrared spectroscopy to quantify the subsequent yield of methane. We show that ablation produces a yield of methane that is approximately two orders of magnitude greater than the measured free methane present in the CM2 carbonaceous chondrite, Murchison, but that this previously overlooked source of methane can only account for less than 10 kg of methane annually, a mass far below that required to maintain the abundance of methane observed in the atmosphere of Mars. Our data support attempts to search for life and explore subsurface chemical processes on the Red Planet.

Section snippets

The methane of Mars

Methane has been detected in the atmosphere of Mars, at an average concentration of 10 ± 3 ppbv as observed by an Earth-based telescope (Krasnopolsky et al., 2004), at 10 ± 5 ppbv (Formisano et al., 2004) and 14 ± 5 ppbv (Geminale et al., 2008) by the Planetary Fourier Spectrometer on the orbiting Mars Express spacecraft. The lifetime of this methane has been estimated to be around 300–600 yr (Wong et al., 2003, Formisano et al., 2004, Krasnopolsky et al., 2004, Krasnopolsky, 2006, Atreya et al., 2007).

Production of methane upon pyrolysis of organic matter in micrometeorites

However, estimates of the contribution of micrometeoritic organic matter to the martian methane budget based on the free methane content of 0.14 ppm do not consider the possibility of the production of methane upon atmospheric entry, and indeed assume that organic matter subjected to intense heating upon atmospheric entry is oxidized and destroyed (Formisano et al., 2004, Krasnopolsky et al., 2004, Atreya et al., 2007). Infalling meteoritic material can be split into two groups, depending on

Determining the methane yield from carbonaceous meteorites using pyrolysis-FTIR

We have investigated the potential of micrometeorites to generate methane during ablation upon atmospheric entry using analytical flash pyrolysis to simulate the heat of atmospheric entry and ablation, with the products of pyrolysis being characterised by Fourier-transform infrared spectrometry (FTIR). This approach has been used to quantify the gas yields from organic-rich materials, where this technique is described in full (Court and Sephton, 2009a, Court and Sephton, 2009b). Briefly,

Acknowledgements

This work was supported by STFC. We are grateful for the constructive comments of two anonymous reviewers.

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