A volcanic environment for bedrock diagenesis at Meridiani Planum on Mars

Exposed bedrocks at Meridiani Planum on Mars display chemical and mineralogical evidence suggesting interaction with liquid water. On the basis of morphological observations as well as high abundances of haematite and sulphate minerals, the rocks have been interpreted as sediments that were deposited in a shallow body of briny water with subsequent evaporation leaving behind the sulphate minerals. The iron-sulphur mineralization at Meridiani has also been inferred to be analogous to that produced during oxidative weathering of metal sulphide minerals, such as occurs at acid mine drainage sites. Neither of these interpretations, however, is consistent with the chemical composition of the rocks. Here we propose an alternative model for diagenesis of Meridiani bedrock that involves deposition of volcanic ash followed by reaction with condensed sulphur dioxide- and water-bearing vapours emitted from fumaroles. This scenario does not require prolonged interaction with a standing body of surface water and may have occurred at high temperatures. Consequently, the model invokes an environment considerably less favourable for biological activity on Mars than previously proposed interpretations.     

Geochemistry: biosignatures and abiotic constraints on early life

Ueno et al. contend that methane found in fluid inclusions within hydrothermally precipitated quartz in the Dresser Formation of western Australia (which is roughly 3.5 Gyr old) provides evidence for microbial methanogenesis in the early Archaean era. The authors discount alternative origins for this methane, suggesting that the range of delta(13)C(CH(4)) values that they record (-56 to -36 per thousand) is attributable to mixing between a primary microbial end-member with a delta(13)C(CH(4)) value of less than -56 per thousand and a mature thermogenic gas enriched in (13)C (about -36 per thousand). However, abiotic methane produced experimentally and in other Precambrian greenstone settings has (13)C-depleted delta(13)C(CH(4)) values, as well as Delta(13)C(CO(2)-CH(4)) relationships that encompass the range measured for the inclusions by Ueno et al. - which suggests that an alternative, abiotic origin for the methane is equally plausible. The conclusions of Ueno et al. about the timing of the onset of microbial methanogenesis might not therefore be justified.