Is there methane on Mars?

Abstract

a b s t r a c tThere have been several reports of methane on Mars at the 10-60 ppbv level. Most suggest that methane is both seasonally and latitudinally variable. Here we review why variable methane on Mars is physically and chemically implausible, and then we critically review the published reports. There is no known mechanism for destroying methane chemically on Mars. But if there is one, methane oxidation would deplete the O 2 in Mars's atmosphere in less than 10,000 years unless balanced by an equally large unknown source of oxidizing power. Physical sequestration does not raise these questions, but adsorption in the regolith or condensation in clathrates ignore competition for adsorption sites or are inconsistent with clathrate stability, respectively. Furthermore, any mechanism that relies on methane's van der Waals' attraction is inconsistent with the continued presence of Xe in the atmosphere at the 60 ppbv level. We then use the HITRAN database and transmission calculations to identify and characterize the absorption lines that would be present on Earth or Mars at the wavelengths of the published observations. These reveal strong competing telluric absorption that is most problematic at just those wavelengths where methane's signature seems most clearly seen from Earth. The competing telluric lines must be removed with models. The best case for martian methane was made for the 12 CH 4 m 3 R0 and R1 lines seen in blueshift when Mars was approaching Earth in early 2003 (Mumma, M.J., Villanueva, G.L., Novak, R.E., Hewagama, T., Bonev, B.P., DiSanti, M.A., Mandell, A.M., Smith, M.D. [2009]. Science 323, 1041-1045). For these the Doppler shift moves the two martian lines into near coincidence with telluric 13 CH 4 m 3 R1 and R2 lines that are 10-50Â stronger than the inferred martian lines. By contrast, the 12 CH 4 m 3 R0 and R1 lines when observed in redshift do not contend with telluric 13 CH 4 . For these lines, Mumma et al.'s observations and analyses are consistent with an upper limit on the order of 3 ppbv.Published by Elsevier Inc.

Bayesian analysis of the astrobiological implications of life’s early emergence on Earth

“…Recent observations have tentatively suggested the presence of methane at the level of approximately 20 ppb (24), which could potentially be indicative of biological activity. The case is not entirely clear, however, as alternative analysis of the same data suggests that an upper limit to the methane abundance is in the vicinity of approximately 3 ppb (25). If, in the future, researchers find compelling evidence that Mars or an exoplanet hosts life that arose independently of life on Earth [or that life arose on Earth a second, independent time (26,27)], how would this discovery affect the posterior probability density of λ (assuming that the same λ holds for both instances of abiogenesis)?…”

NOMAD, an Integrated Suite of Three Spectrometers for the ExoMars Trace Gas Mission: Technical Description, Science Objectives and Expected Performance

“…The reports of methane observations from ground-based observatories (Krasnopolsky et al 2004;Mumma et al 2009), from space-borne instruments (Fonti and Marzo 2010;Geminale et al 2011), and in-situ (Webster et al 2015) pose fundamental challenges to our understanding of the chemistry taking place in the Martian atmosphere (Zahnle et al 2011). These studies showed high spatial and temporal variability, which cannot immediately be explained by models (Lefèvre and Forget 2009).…”

A Conspicuous Clay Ovoid in Nakhla: Evidence for Subsurface Hydrothermal Alteration on Mars with Implications for Astrobiology

“…Evidentially, methane has been detected in the martian atmosphere at scattered locations on Mars (Formisano et al, 2004;Krasnopolsky et al, 2004), although disputes over this finding do exist (Zahnle et al, 2011). More recently, the MSL Curiosity rover on the martian surface did not detect methane (Webster et al, 2013).…”