Ultraviolet-radiation-induced methane emissions from meteorites and the Martian atmosphere

Abstract

Almost a decade after methane was first reported in the atmosphere of Mars there is an intensive discussion about both the reliability of the observations--particularly the suggested seasonal and latitudinal variations--and the sources of methane on Mars. Given that the lifetime of methane in the Martian atmosphere is limited, a process on or below the planet's surface would need to be continuously producing methane. A biological source would provide support for the potential existence of life on Mars, whereas a chemical origin would imply that there are unexpected geological processes. Methane release from carbonaceous meteorites associated with ablation during atmospheric entry is considered negligible. Here we show that methane is produced in much larger quantities from the Murchison meteorite (a type CM2 carbonaceous chondrite) when exposed to ultraviolet radiation under conditions similar to those expected at the Martian surface. Meteorites containing several per cent of intact organic matter reach the Martian surface at high rates, and our experiments suggest that a significant fraction of the organic matter accessible to ultraviolet radiation is converted to methane. Ultraviolet-radiation-induced methane formation from meteorites could explain a substantial fraction of the most recently estimated atmospheric methane mixing ratios. Stable hydrogen isotope analysis unambiguously confirms that the methane released from Murchison is of extraterrestrial origin. The stable carbon isotope composition, in contrast, is similar to that of terrestrial microbial origin; hence, measurements of this signature in future Mars missions may not enable an unambiguous identification of biogenic methane.

Earth as a Tool for Astrobiology—A European Perspective

“…Among these parameters, the use of a Xenon arc lamp as the radiation source has proven to better reproduce the energy and relative abundance of the UV photons (190 to 400 nm) supposed to reach the surface of Mars (Schuerger et al 2003). Moreover, temperature and pressure should also be representative of the Martian ones for several reasons: First, because temperature can influence the kinetics of the chemical reactions occurring in the simulation reactor (ten Kate et al 2006), as pressure may also do (Keppler et al 2012). Second, because the chemical stability and physical state (solid or gas) of the products is largely dependent of these parameters.…”

Astrobiology and the Possibility of Life on Earth and Elsewhere…

“…Zahnle et al 2011). New measurements taken by the Sample Analysis at Mars (SAM) instrument suite onboard the Curiosity rover at the Martian surface indicate a baseline value of 0.07 ppb, which is consistent with the expected source of meteoritic organic material falling onto the surface of Mars (Flynn and McKay 1990) and degrading to release methane (Keppler et al 2012;, which is then photochemically destroyed with a 300-year lifetime (Wong et al 2003). An unexpected result, however, is the detection of a 7 ppb methane plume that lasted 60 days (Webster et al 2014), which has been ruled out to be an instrument artefact.…”

Methane on Mars

“…Other proposed sources of Martian methane include volcanic/ magmatic degassing [33,34], exogenous delivery [35][36][37][38][39] and release from clathrates [40]. It would be premature to adopt or dismiss any of these hypotheses, or to suppose there can be no others.…”

“…The transport from the source region to the planetary surface is a steady diffusive process and no model has produced a localized pulse. Similarly, the action of UV on organics is a diffuse process [37][38][39] and is not expected to prefer a particular location or time.…”