P11C-0698
Oxidants: Chemical Energy for Life on Mars and in the Outer Solar System
Redox gradients are essential for life as we know it. Strong oxidants to retain these gradients are produced in a variety of planetary environments by UV and ionizing radiation. Houtkooper and Schulze-Makuch (2007) previously suggested hydrogen peroxide as an essential biological ingredient for putative Martian life to adapt to the challenging near-surface conditions on the Red Planet. On Earth, adaptation and use of oxidants is widespread. Examples are microorganisms that use or produce oxidants, and the microbe Acetobacter peroxidans, which uses the decomposition of H2O2 as its major metabolic pathway. However, oxidants may also be critical biogenic components on outer Solar System objects of high astrobiological potential such as Europa and Enceladus. Exothermic reactivity of oxidants additionally contributes heat for habitable environments and acceleration of chemical processes potentially supporting life. Oxidation chemistry produces volatile gases and other detectable species that may be diagnostic of recent and ongoing biochemistry. More reduced chemical environments like the Titan atmosphere, and more isolated liquid water habitats like the deep-lying subsurface oceans of Ganymede and Callisto, may be astrobiologically impacted by externally driven inputs of oxidants over billions of years. Houtkooper, J.M. and Schulze-Makuch, D. (2007) A possible biogenic origin for hydrogen peroxide on Mars: the Viking results reinterpreted. Int. J. of Astrobiology 6: 147-152. Cooper, J. F., P. D. Cooper, E. C. Sittler, S. J. Sturner, A. M. Rymer, and M. E. Hill. Radiolytic gas-driven cryovolcanism in the outer solar system, J. Geophys. Res., in review.
P11C-0699
Hydrogen Peroxide Production at the Rock-Water Interface
The theme "Follow the Oxidants" draws attention to the role of oxidants in planetary evolution. Earth, which acquired a progressively more oxidizing surface environment of the first 2+ Gyrs, provides a good example. The cause of Earth's slow oxidation is still not fully understood. Here we show that an electric current unlike any current previously described flows through igneous rocks (1, 2). The current arises when rocks are stressed. The charge carriers derive from oxygen anions in the minerals that have changed their valence from 2– to 1–. An O– in a matrix of O2– represents a defect electron or hole, also known as positive hole (3) or "phole" for short. Normally the O– occur in the structure of their host minerals in the form of pairs, O–-O–, equivalent to peroxy links. As such they are dormant and electrically inactive. When stresses are applied, dislocations move through the mineral grains, causing the peroxy links to break up, creating electrons and pholes. These charge carriers are capable of generating currents that flow for hours and days. In the laboratory the pholes propagate readily through 3 meter of dry granite. In the field they are expected to flow through kilometers of rocks. When the pholes reach a rock-water interface, they oxidize H2O quantitatively to H2O2. On the early Earth, which was certainly tectonically active, this mechanism represents a global source of H2O2, which must have been available over Gyrs. It has far-reaching implications for the oxidation of the early Earth and the evolution of early Life. 1. F. T. Freund, D. Sornette, Tectonophys. 431, 33 (2007). 2. F. T. Freund, A. Takeuchi, B. W. Lau, Phys. Chem. Earth 31, 389 (2006). 3. D. L. Griscom, Glass Sci. Technol. 4B, 151 (1990).
P11C-0700
Modeling the Sources of H2+ and O2+ in Saturn's Magnetosphere
The main rings, the icy satellites and the ice grains in the tenuous rings are all sources of neutral H2 and O2 in Saturn's magnetosphere due to the decomposition of ice (e.g., Johnson et al. 2005, 2006a) and Titan is a source of H2 via atmospheric escape (Yelle et al. 2006). These molecules are eventually ionized contributing the Saturn's trapped plasma. In addition, ion-neutral reactions in the narrow Enceladus neutral torus and the HST torus (Johnson et al. 2006b) can produce molecular hydrogen and oxygen ions and neutrals. Here we calculate the spatial distribution of neutrals ejected from satellites and rings and use these to estimate source rates for H2+ and O2+ for Saturn's magnetosphere. Johnson et al. J. Chem Phys. 123, 184715, 2005a Johnson et al. Icarus 180,393, 2006a Johnson et al. ApJ 644, L137, 2006b Yelle et al. Icarus 182, 567, 2006
P11C-0701
Optical Properties of Early Earth and Titan Haze Laboratory Analogs in the Mid-Visible
An organic haze layer, similar to the thick haze currently enveloping Titan, may have formed on early Earth. This haze could have had profound effects on the early Earth climate and the life existing and evolving on it. Climate models of early Earth that include this organic haze have had to rely on optical properties of Titan's haze. However, unlike Titan haze, early Earth organic haze likely formed in an atmosphere containing the oxidizer CO2 in addition to CH4 and N2. The presence of CO2 causes oxygen-incorporation into the haze aerosol. We investigate the differences in the chemical and optical properties between Titan and early Earth haze laboratory analogs that this oxygen-incorporation induces via the novel technique of cavity ring-down aerosol extinction spectroscopy at λ = 532nm. This technique allows the extinction of size-selected aerosols to be measured "on the fly," which is unprecedented for Titan or early Earth haze laboratory analog studies. Using the measured extinction values and applying Mie theory, we estimate a total refractive index at λ = 532nm of n = 1.55±0.05 and k = 0.025±0.005 for a Titan haze analog and compare our results to other workers who have used thin-film techniques. Studies of early Earth organic haze are ongoing. These experiments provide essential data for modeling early Earth's climate and understanding the conditions that early life experienced.
P11C-0702
The Degradation of Organic Compounds Under Mars Like Conditions
The Viking missions failed to detect organic compounds, of any origin, in the Martian regolith. The explanation for the lack of organics has centered on the presence of oxidants in the regolith and atmosphere. The interaction between UV light, the regolith and trace water is investigated here. In this study, organic compounds were mixed with JSC-1 Mars simulant and exposed in a Mars simulation chamber to study the effects of UV light and frost cycling on different compounds. Organic compounds studied ranged from polyaromatic hydrocarbons to alkanes with different functional groups. The Mars simulation chamber provides conditions that simulate the temperature, pressure, atmospheric composition and UV flux present on Mars. One day exposure in the simulation chamber provides about the same amount of UV exposure as three days on Mars. Samples were exposed under two sets of conditions for seven days: continuous exposure to UV at constant Mars temperatures and continuous exposure to UV with temperature cycling. The temperature cycling has the effect of generating a thin frost layer on the surface of the regolith. After exposure, the organic compounds were extracted from the JSC- 1 simulant and analyzed by GC/MS. Degradation of the organics was enhanced by the frost cycling and new compounds were identified in the samples exposed to frost cycling that were not present in the controls or in samples exposed at constant temperatures. These results will be compared to previous results of degradation due to glow discharge plasma, as may be present in Martian dust storms and devils.
P11C-0703
Assessing the Habitability and the Biogeochemical Output of Subsurface Biomes
The subsurface microbial biosphere has been estimated to represent the largest biome on Earth in terms of sheer volume of habitable space. Even if the magnitude of the subsurface biosphere is vastly overestimated, it could still represent a significant reservoir of nutrients, and harbor yet-to-be-discovered metabolic and physiological diversity. Subsurface environments have also been evoked as some of the most likely sites for extraterrestrial life as they are sheltered from harsh surface conditions, may retain liquid water, and may be linked to geothermal and geochemical energy sources. Despite the recognized importance of these environments, relatively little is known about factors constraining the habitability of subsurface ecosystems, or how these factors influence life detection through either in situ or remote measurements. We present a two part model in which we evaluate the habitability of subsurface environments based upon the physiological characteristics of specific metabolic groups of microorganisms (e.g. methanogens, sulfate reducers, etc.) then assess the potential for detection of their biosignatures through both direct and remote measurements. Easily parameterized values such as temperature, pressure, and pore space will be compared with free energy fluxes derived from models of subsurface environments. These data will be used to determine the rates, character, and magnitude of biosignatures produced. Since local influences such as the spatial and temporal scales of habitable niches may have important consequences for life, these factors will be evaluated. The geometry and composition of host materials will also be incorporated into the models, as this may either dampen, or record evidence of biological activities. This research provides a key framework for testing hypotheses based upon on observational data on Earth, and may lead to new insights into prospecting for life in subsurface environments on Mars, Europa, and rocky extrasolar planets.
P11C-0704
Organic and Inorganic Carbon in the Rio Tinto (Spain) Deep Subsurface System: a Possible Model for Subsurface Carbon and Lithoautotrophs on Mars.
The subsurface is the key environment for searching for life on planets lacking surface life. Subsurface ecosystems are of great relevance to astrobiology including the search for past/present life on Mars. Conditions on the Martian surface do not support biological activity but the subsurface might preserve organics and host subsurface life [1]. A key requirement for the analysis of subsurface samples on Mars is the ability to characterize organic vs. inorganic carbon pools. This information is needed to determine if the sample contains organic material of biological origin and/ or to establish if pools of inorganic carbon can support subsurface biospheres. The Mars Analog Rio Tinto Experiment (MARTE) performed deep drilling of cores i.e., down to 165-m depth, in a volcanically-hosted-massive-sulfide deposit at Rio Tinto, Spain, which is considered an important analog of the Sinus Meridiani site on Mars. Results from MARTE suggest the existence of a relatively complex subsurface life including aerobic and anaerobic chemoautotrophs, and strict anaerobic methanogens sustained by Fe and S minerals in anoxic conditions, which is an ideal model analog for a deep subsurface Martian environment. We report here on the distribution of organic (C-org: 0.01-0.3Wt% and inorganic carbon (IC = 0.01-7.0 Wt%) in a subsurface rock system including weathered/oxidized i.e., gossan, and unaltered pyrite stockwork. Cores were analyzed from 3 boreholes (BH-4, BH-7, and BH-8) that penetrated down to a depth of ~165 m into massive sulfide. Nearsurface phyllosilicate rich-pockets contain the highest amounts of organics (0.3Wt%) [2], while the deeper rocks contain the highest amount of carbonates. Assessing the amount of C pools available throughout the RT subsurface brings key insight on the type of trophic system sustaining its microbial ecosystem (i.e., heterotrophs vs. autotrophs) and the biogeochemical relationships that characterize a new type of subsurface biosphere at RT. This potentially novel biosphere on Earth could be used as a model to test for extant and extinct life on Mars. Furthermore, having found carbonates in an hyperacidic system (pH ~2.3) brings new insights on the possible occurrence of deep carbonates deposits under low-pH condition on Mars. [1] Boston, P.J., et al., 1992. Icarus 95,300-308; Bonaccorsi, Stoker and Sutter, 2007 Accepted with review in Astrobiology.
P11C-0705
Coupled laboratory experiments and numerical models for generating ice-depth profiles of steady-state hydrogen peroxide concentrations on radiolytically processed icy worlds.
The presence of hydrogen peroxide and condensed phase molecular oxygen on the surface of Europa is now well established [1,2] and laboratory experiments have repeatedly demonstrated the viability of various radiolytic processes for explaining the observations [see e.g. 3, 4]. To date, however, both the Europa observations and the laboratory work have been limited to only the upper few, or few tens of microns, of ice. The spectrum of charged particles incident on the surface of Europa penetrates deeper, and deposits energy over a much greater range, than any laboratory experiment has aimed to replicate [5, 6]. Here we present results from laboratory work on hydrogen peroxide production using energetic electrons (4 keV – 16 keV) and couple these results with a numerical model for the integrated steady-state density of hydrogen peroxide as a function of depth into the ice. Production rates and steady-state peroxide levels for a range of initial electron energies are used to generate functions for the number of peroxide molecules produced per initial electron as it penetrates through the ice. We examined the electron energy spectrum from 0.01 MeV to 10 MeV and accounted for electrons incident to the surface over the solid angle from cosine(theta) = 0.3-1.0, where theta is the angle from the normal to the surface. We found that, accounting for production and destruction as a function of energy deposition, steady-state hydrogen peroxide concentrations resulting from electron radiolysis likely increases by a factor of a few to an order of magnitude at a depth of a few hundred microns. In other words, the 0.13 percent by number abundance of peroxide observed by NIMS [1] may be a low-end value; at depth the peroxide concentration could increase to a few percent by number relative to water. [1] Carlson et al. 1999. [2] Spencer and Calvin, 2002. [3] Moore and Hudson, 2000. [4] Loeffler et al., 2006. [5] Cooper et al., 2001 [6] Paranicas et al., 2001.
P11C-0706
Aqueous Oxidation of Hydrogen Sulfide as a Cause of Acid Rock Alteration on Mars
The detection of ferric sulfates and silica-rich deposits with the Mars Exploration Rovers is consistent with mineral deposition in acid aqueous environments. By analogy with terrestrial volcanic settings (e.g. Yellowstone National Park), oxidation of dissolved H2S on Mars should have led to the production of sulfuric acid. Speciation models for martian volcanic gases reveal elevated H2S abundances in reduced mantle-derived emanations; and experimental data show efficient H2S oxidation by dissolved O2. On Mars, volcanic and impact-generated H2S could have been dissolved in cold/hot subsurface and spring waters, in atmospheric aerosols, and in surface water reservoirs away from volcanic/impact events. H2S could then be oxidized by O2 that forms rapidly in atmospheric photochemical processes and in strong impacts. We used kinetic-thermodynamic models to evaluate rates and chemical/mineralogical pathways of aqueous H2S oxidation on ancient Mars. The models consider dissolution rates of primary (basaltic) and secondary minerals, mineral grain sizes, supply of H2S and O2, kinetics of H2S oxidation [ Millero and Hershey, 1987], and mineral precipitation at variable water/rock ratios and atmospheric O2 pressures [cf. Zolotov and Mironenko, 2007, JGR-Planets, E07006]. The models were applied for volcanic springs, non-volcanic lakes, and atmospheric aerosols. Results show rapid sulfuric acid production even at the current O2 pressure and temperature of 273 K. Typically, H2S-bearing fluids oxidize faster than solution neutralizes through mineral dissolution. Although oxidation rate depends on several parameters, major oxidation occurs within a year. A continuous supply of H2S into near-surface fluids causes pervasive acid alteration of basalts. High water/rock ratios (e.g., due to discharge of solutions from springs) favor leaching of elements and cause deposition of relatively insoluble amorphous silica and Ti oxide. Subsequent freezing and/or evaporation of acid or neutralized spring fluids leads to deposition of sulfates. Degassing and oxidation of H2S could explain net addition of S to martian basaltic materials, and the formation of ferric sulfates and silica-rich deposits.