Ultrastructural study of iron oxide precipitates: implications for the search for biosignatures in the Meridiani hematite concretions, Mars

Two terrestrial environments that have been proposed as analogs for the iron oxide precipitation in the Meridiani Planum region of Mars include the Rio Tinto precipitates and southern Utah marble concretions. Samples of two typical Utah iron oxide concretions and iron oxide precipitates in contact with biofilms from Rio Tinto have been studied to determine whether evidence could be found for biomediation in the precipitation process and to identify likely locations for fossil microorganisms. Scanning electron microscopy, energy dispersive X-ray, and gas chromatography-mass spectrometry (GC-MS) were used to search for biosignatures in the Utah marbles. The precipitation of iron oxides resembles known biosignatures, though organic compounds could not be confirmed with GC-MS analysis. In contrast, textural variations induced by biological activity are abundant in the modern Rio Tinto samples. Although no compelling evidence of direct or indirect biomediation was found in the Utah marbles, the ultrastructure of the iron oxide cement in the concretion suggests an inward growth during concretion precipitation from an initially spherical redox front. No indication for growth from a physical nucleus was found.     

A combinatorial approach to biochemical space: description and application to the redox distribution of metabolism

Redox chemistry is central to life on Earth. It is well known that life uses redox chemistry to capture energy from environmental chemical energy gradients. Here, we propose that a second use of redox chemistry, related to building biomass from environmental carbon, is equally important to life. We apply a method based on chemical structure to evaluate the redox range of different groups of terrestrial biochemicals, and find that they are consistently of intermediate redox range. We hypothesize the common intermediate range is related to the chemical space required for the selection of a consistent set of metabolites. We apply a computational method to show that the redox range of the chemical space shows the same restricted redox range as the biochemicals that are selected from that space. By contrast, the carbon from which life is composed is available in the environment only as fully oxidized or reduced species. We therefore argue that redox chemistry is essential to life for assembling biochemicals for biomass building. This biomass-building reason for life to require redox chemistry is in addition (and in contrast) to life's use of redox chemistry to capture energy. Life's use of redox chemistry for biomass capture will generate chemical by-products-that is, biosignature gases-that are not in redox equilibrium with life's environment. These potential biosignature gases may differ from energy-capture redox biosignatures.     

Project Icarus: A review of local interstellar medium properties of relevance for space missions to the nearest stars

I review those properties of the interstellar medium within 15 light-years of the Sun, which will be relevant for the planning of future rapid (v≥0.1c) interstellar space missions to the nearest stars. As the detailed properties of the local interstellar medium (LISM) may only become apparent after interstellar probes have been able to make in situ measurements, the first such probes will have to be designed conservatively with respect to what can be learned about the LISM from the immediate environment of the Solar System. It follows that studies of interstellar vehicles should assume the lowest plausible density when considering braking devices, which rely on transferring momentum from the vehicle to the surrounding medium, but the highest plausible densities when considering possible damage caused by the impact of the vehicle with interstellar material. Some suggestions for working values of these parameters are provided. This paper is a submission of the Project Icarus Study Group.     

Thermal drilling in planetary ices: an analytic solution with application to planetary protection problems of radioisotope power sources

Abstract Thermal drilling has been applied to studies of glaciers on Earth and proposed for study of the martian ice caps and the crust of Europa. Additionally, inadvertent thermal drilling by radioisotope sources released from the breakup of a space vehicle is of astrobiological concern in that this process may form a downward-propagating "warm little pond" that could convey terrestrial biota to a habitable environment. A simple analytic solution to the asymptotic slow-speed case of thermal drilling is noted and used to show that the high thermal conductivity of the low-temperature ice on Europa and Titan makes thermal drilling qualitatively more difficult than at Mars. It is shown that an isolated General Purpose Heat Source (GPHS) "brick" can drill effectively on Earth or Mars, whereas on Titan or Europa with ice at 100 K, the source would stall and become stuck in the ice with a surface temperature of <200 K. Key Words: Planetary protection-Planetary environments-Ice-Titan. Astrobiology 12, 799-802.