Radiolytic hydrogen and microbial respiration in subsurface sediments

Radiolysis of water may provide a continuous flux of an electron donor (molecular hydrogen) to subsurface microbial communities. We assessed the significance of this process in anoxic marine sediments by comparing calculated radiolytic H(2) production rates to estimates of net (organic-fueled) respiration at several Ocean Drilling Program (ODP) Leg 201 sites. Radiolytic H(2) yield calculations are based on abundances of radioactive elements (uranium, thorium, and potassium), porosity, grain density, and a model of water radiolysis. Net respiration estimates are based on fluxes of dissolved electron acceptors and their products. Comparison of radiolytic H(2) yields and respiration at multiple sites suggests that radiolysis gains importance as an electron donor source as net respiration and organic carbon content decrease. Our results suggest that radiolytic production of H(2) may fuel 10% of the metabolic respiration at the Leg 201 site where organic-fueled respiration is lowest (ODP Site 1231). In sediments with even lower rates of organic-fueled respiration, water radiolysis may be the principal source of electron donors. Marine sedimentary ecosystems may be useful models for non-photosynthetic ecosystems on early Earth and on other planets and moons, such as Mars and Europa.     

A reappraisal of the habitability of planets around M dwarf stars

Stable, hydrogen-burning, M dwarf stars make up about 75% of all stars in the Galaxy. They are extremely long-lived, and because they are much smaller in mass than the Sun (between 0.5 and 0.08 M(Sun)), their temperature and stellar luminosity are low and peaked in the red. We have re-examined what is known at present about the potential for a terrestrial planet forming within, or migrating into, the classic liquid-surface-water habitable zone close to an M dwarf star. Observations of protoplanetary disks suggest that planet-building materials are common around M dwarfs, but N-body simulations differ in their estimations of the likelihood of potentially habitable, wet planets that reside within their habitable zones, which are only about one-fifth to 1/50th of the width of that for a G star. Particularly in light of the claimed detection of the planets with masses as small as 5.5 and 7.5 M(Earth) orbiting M stars, there seems no reason to exclude the possibility of terrestrial planets. Tidally locked synchronous rotation within the narrow habitable zone does not necessarily lead to atmospheric collapse, and active stellar flaring may not be as much of an evolutionarily disadvantageous factor as has previously been supposed. We conclude that M dwarf stars may indeed be viable hosts for planets on which the origin and evolution of life can occur. A number of planetary processes such as cessation of geothermal activity or thermal and nonthermal atmospheric loss processes may limit the duration of planetary habitability to periods far shorter than the extreme lifetime of the M dwarf star. Nevertheless, it makes sense to include M dwarf stars in programs that seek to find habitable worlds and evidence of life. This paper presents the summary conclusions of an interdisciplinary workshop (http://mstars.seti.org) sponsored by the NASA Astrobiology Institute and convened at the SETI Institute.