P42B-01 INVITED
Earth Analogs for Potential Martian Biomes and the Search for Life on Mars
Planetary history, especially the history of the hydrosphere of Mars, can provide important constraints for the search for potentially habitable environments. Our team has focused on factors that influence the distribution of water over Mars history, including hydrothermal processes in the Martian crust and mantle convection. The phenomena that sculpt the planetary surface also constrain environment types that may be targets for life detection analyses. We have analyzed the mobility of granular flows for evidence of water and explored processes that give rise to geomorphological features on Earth considered to have formed by the same processes as those that formed amphitheater headed canyons on Mars. These analyses challenge the idea that the Mars canyon heads are spring incised, altering our perspective on these locations as sites of long term water availability. In geomicrobiological studies we have focused on sites that are likely to serve as partial analogs for potential habitats on Mars. We have selected systems in which the biomes are driven by chemical energy, especially those in which iron and sulfur serve key roles. These constituents are of particular interest, given that they occur in reasonable abundance at, and near, the Martian surface in a range of redox states. In addition to exploring the chemistry, microbiology, and biosignatures of environments shaped by interaction between water and basaltic rock and fault-related spring systems, we have investigated subsurface systems in which the microbiology is driven by iron and sulfur oxidation. These extreme acid environments are populated by self sustaining communities that are of low enough complexity to allow detailed metabolic analysis, as well as investigation of the interplay between environmental chemistry and biochemistry. Spectroscopic methods applied to the chemical signatures of these systems show characteristics with some similarity to those of Mars surface materials, increasing the possibility that acid sulfate systems may be appropriate targets for biosignature investigation.
P42B-02
Exploration of a Subsurface Biosphere in a Volcanic Massive Sulfide: Results of the Mars Analog Rio Tinto Drilling Experiment
Biological systems on Earth require three key ingredients-- liquid water, an energy source, and a carbon source, that are found in very few extraterrestrial environments. Previous examples of independent subsurface ecosystems have been found only in basalt aquifers. Such lithotrophic microbial ecosystems (LME) have been proposed as models for steps in the early evolution of Earth's biosphere and for potential biospheres on other planets where the surface is uninhabitable, such as Mars and Europa.. The Mars Analog Rio Tinto Experiment (MARTE) has searched in a volcanic massive sulfide deposit in Rio Tinto Spain for a subsurface biosphere capable of living without sunlight or oxygen and found a subsurface ecosystem driven by the weathering of the massive sulfide deposit (VMS) in which the rock matrix provides sufficient resources to support microbial metabolism, including the vigorous production of H2 by water-rock interactions. Microbial production of methane and sulfate occurred in the sulfide orebody and microbial production of methane and hydrogen sulfide continued in an anoxic plume downgradient from the sulfide ore. Organic carbon concentrations in the parent rock were too low to support microbes. The Rio Tinto system thus represents a new type of subsurface ecosystem with strong relevance for exobiological studies. Commercial drilling was used to reach the aquifer system at 100 m depth and conventional laboratory techniques were used to identify and characterize the biosphere. Then, the life search strategy that led to successful identification of this biosphere was applied to the development of a robotic drilling, core handling, inspection, subsampling, and life detection system built on a prototype planetary lander that was deployed in Rio Tinto Spain in September 2005 to test the capability of a robotic drilling system to search for subsurface life. A remote science team directed the simulation and analyzed the data from the MARTE robotic drill. The results of this experiment have important implications for the strategy for searching for life on Mars.
P42B-03
Multiple Reaction Pathways during Radiolytic Oxidation of Pyrite
Passage of ionizing radiation through groundwater produces a complex mixture of short-lived ions, free radicals, and excited molecules that participate in a wide range of chemical reactions and accelerate water-rock interaction. Radiolysis of groundwater in contact with sulfide minerals or elemental sulfur can produce plumes of partially to fully oxidized sulfur species, thereby stimulating microbial metabolism in unexpected subsurface environments. In order to study fractionation of sulfur isotopes during radiolysis, initial experiments were performed using sealed quartz tubes that contained pyrite and millimolar solutions of hydrogen peroxide (H2O2) that were reacted at temperatures from 4 to 150 C over time periods of days to week. Mineralogical, chemical, and stable isotopic date from H2O2 experiments reveal multiple pathways for pyrite oxidation and distinct assemblages of products at difference temperatures. Sulfur isotopic signatures of oxidized products are enriched in 32S by 0.5 to 2 per mil compared to source sulfate. Radiation experiments were carried out using a 60Co gamma sources at the Radiation Laboratory of the University of Notre Dame. Sealed quartz tubes that contained pyrite and deoxygenated DI water were irradiated from 1 to 14 hours with a dose rate of 11.3 krad/min (113 Gy/min). Initial experiments produced oxidized sulfur as gaseous (e.g., SO2) and aqueous (e.g, SO4) species at concentrations directly correlated to the volume of water and total irradiation dose. Radiolysis proves to be an effective mechanism for the production of oxidizing species in geologically long-lived oxidizing systems. Iron sulfide minerals are decomposed and iron oxide/hydroxide minerals and sulfate ions are produced. Recognizing geochemical signatures of radiolytic oxidation is particularly important for understanding biotic and abiotic reaction pathways in environments where molecular oxygen is negligible and for assessing potential sources of chemical energy for microbial metabolism in the deep subsurface of Earth and Mars.
P42B-04
Microbial Adaptations to Biosustainabilitiy in Deep-Subsurface Environments on Earth
Exploration for life on Mars and icy moons in our solar system necessitates development of innovative techniques for life-detection followed by field testing in analogue environments on Earth. A collaborative international effort is underway to drill and sample within regions of persistent permafrost in northern Canada for the purpose of characterizing microbial ecosystems adapted to long-term cold conditions. In 2001 and 2002, Finnish and Canadian scientists installed an instrumented borehole array in a commercial gold mine with sampling valves at 890 and 1130 meters below the surface. Numerous water and gas samples from the Lupin borehole array have been analyzed for molecular and isotopic compositions of organic and inorganic chemical constituents. Boreholes with the lowest concentration of methane and largest $^{34}$S fractionation between dissolved sulfate and sulfide are the focus of microbiological sampling. Microbial diversity at Lupin is being assessed by culturing, sequencing, and direct detection of microbial reactions. Cell counts indicate a low biodensity, ranging from 100 to100,000 cells/ml. Phylogenetic analysis using 16S rDNA indicates low biodiversity with the planktonic biota dominated by a distinctive new phlyotype having 95-97% similarity to Thiohalobaccili. Similarly, the subsurface brines sampled at depths of 1500 to 3500 meters in the Witwatersrand basin of South Africa yield low biodensity and biodiversity with the dominant phylotype being a Desulfotomaculum-like organism that appears to represent a new species and new family. Microbes sampled in fracture water at kilometer depths below the surface are significantly different from surface extremophiles and show specific genetic adaptations to biosustainability in deep-subsurface environments.
P42B-05
Arsenophilic Bacterial Processes in Searles Lake: A Salt-saturated, Arsenic-rich, Alkaline Soda Lake.
Searles Lake, located in the Mojave Desert of California, is essentially a chemically-similar, concentrated version of Mono Lake, but having a much higher salinity (e.g., 340 vs. 90 g/L) and a greater dissolved inorganic arsenic content in its brine (e.g., 3.9 vs. 0.2 mM). The source of all this arsenic ultimately comes from hydrothermal spring inputs, thereby underscoring the importance of volcanic and fluvial processes in transporting this toxic element into these closed basin lakes. Nonetheless, the presence of microbial activities with regard to respiration of arsenate oxyanions under anaerobic conditions and the oxidation of arsenite oxyanions under aerobic conditions can be inferred from porewater profiles taken from handcores retrieved beneath Searles Lake's salt crust. Sediment slurry incubations confirmed biological arsenate respiration and arsenite oxidation, with the former processes notably enhanced by provision of the inorganic electron donor sulfide or H2. Hence, arsenic-linked chemo-autotrophy appears to be an important means of carbon fixation in this system. Subsequent efforts using 73As-arsenate as radiotracer detected dissimilatory arsenate reduction activity down the length of the core, but we were unable to detect any evidence for sulfate-reduction using 35S-sulfate. An extremely halophilic anaerobic bacterium of the order Haloanaerobiales [strain SLAS-1] was isolated from the sediments that grew via arsenate respiration using lactate or sulfide as its electron donors. These results show that, unlike sulfate-reduction, arsenic metabolism (i.e., both oxidation of arsenite and dissimilatory reduction of arsenate) is operative and even vigorous under the extreme conditions of salt-saturation and high pH. The occurrence of arsenophilic microbial processes in Searles Lake is relevant to the search for extant or extinct microbial life on Mars. It is evident from surface imagery that Mars had past episodes of volcanism, fluvial transport, and most likely brine concentration reactions (e.g., evapo- and cryo-concentration) occurring in its early Noachian/Hesperian epochs. We speculate that these processes may have created arsenic-rich, dense brines on the Martian surface or even within its underlying regolith. Whether such brines persisted long enough for prokaryotic life to evolve in them, and if so, was such life capable of adapting to and exploiting arsenic redox reactions for the purpose of generating metabolic energy remain tantalizing, but still hypothetical questions.
P42B-06
Pavilion Lake, British Columbia, Canada - An investigation of potentially unique freshwater microbialites, and their application to Mars exploration.
Pavilion Lake, B. C., Canada has become the first target site of an on-going NASA effort to investigate the Mars analogue potential of terrestrial lacustrine carbonates. A combination of hypothesis and exploration driven research activities are underway to study the unusual freshwater microbialite structures found in this lake. These structures are of interest in terms of models of Precambrian reefs and may also be relevant to carbonate formation in ancient lakes on Mars. Laval et al. (2000) provides an overview of the morphological characteristics of the microbialites, and explores the physical limnology of Pavilion Lake. Several key hypotheses and questions related to the role of biology in the formation of the microbialites, and the effect of varying light levels on the microbialite morphologies have since resulted from Laval et al., but to date remain unanswered. In August 2004, the Pavilion Lake Research Project (PLRP) was established to commence a new round of investigations into Pavilion Lake, to test hypotheses concerning the factors controlling carbonate formation, and to collect further exploration data related to understanding the lake's limnology and development. Laval et al. classified the differing microbialite structures into four depth categories: shallow to intermediate (5-10m), intermediate (\~20m), intermediate to deep (20-30m), and deep facies (30-35m). There is a variation of the morphology and mechanical strength of the structures with depth, which may reflect a change in the relative role of biotic and non-biotic precipitation with lowering light levels. Recent field investigations (August 2005) revealed carbonate structures at depths beyond what was reported in Laval et al. (2000). These new structures appear at 46-55m, and are morphologically distinct from the previously described microbialites. Here we present our current research activities at Pavilion Lake, along with recent data collection results. Excursions to the lake have included the use of SCUBA to sample collect and to install a variety of sensors including a thermistor chain and a Licor light meter. Seepage meters have also been placed in strategic regions of the lake to collect incoming lake groundwater for future chemical limnological analyses and hydrological mapping of the area. In addition, conventional Conductivity/Temperature/Depth (CTD) profiles and water sampling has been conducted during a winter and summer period, and will be presented here. Microbialite samples recovered from each of the discernible morphological depth transects have been investigated for relative variations in d13C isotopic signatures, and the results will presented here.
P42B-07
Relic and Active Cold Permafrost Springs in the Arctic as Martian Analogs
The discovery of presumably geologically recent gully features on Mars (Malin and Edgett 2000) and the revelation by the MER rover Opportunity that Meridiani Planum may have once been the site of a significant body of water (or groundwater) at or near the surface of Mars (Squyres et al. 2004) has significantly increased the prospects of locating sites capable of hosting microbial life. Future exploration of potential habitats for extant life or evidence of past life on Mars will require fundamental knowledge of life and habitable environments on Earth particularly those found in regions with a mean annual temperature well below freezing. We describe a series of perennial springs and associated periglacial features (seasonal icings, frost mounds and pingos) along with their physical, chemical and biological components that occur in a region of thick continuous permafrost in the Canadian High Arctic at latitudes of nearly 80° N and higher. A recently discovered paleospring site located on Axel Heiberg Island will also be described and discussed as an analog to relic martian springs.
P42B-08
Microbial Ecology and Resultant Biomarkers Preserved in a Terrestrial Analog of a Martian Spring System
On Mars, groundwater discharge, heated by geological processes at depth, represents a likely late-stage reservoir of liquid water available for biological activity. Photo-geological observations of the Martian surface support geologically, relatively young groundwater discharge via sapping and/or fault-controlled springs. Our approach to the investigation of the possible biological potential of such reservoirs has been to characterize analogous, terrestrial spring systems. Our study site is a fault-driven, mesophilic, sulfur spring system between the Hayward and Calaveras faults in California. We have examined hydro-geological variables, nutrient availability for microbial metabolism, differences in extant community structure, and the seasonal changes associated with these variables. The springs under study also precipitate calcite and form large mounds, offering the potential to evaluate the preservation of biosignatures. The geochemistry and isotopic composition (2H/18O) of spring waters indicate that the various springs discharge waters represent differing amounts of mixing between deeper, connate water with shallow meteoric inputs. Clone libraries of 16S rDNA and fluorescence {\it in situ} hybridization experiments suggest that oxidation of sulfur compounds by {\it Epsilon-} and {\it Gammaproteobacteria} is a significant process occurring in the springs, and lipid analyses support these observations. While the studied springs undergo seasonal shifts in their respective geochemistries, only the microbial community at one of the springs elicits a commensurate seasonal variation. During the dry season, the community at this spring shifts to a red, plaque-like biofilm and iron-cycling organisms from the {\it Alphaproteobacteria} class increase significantly in their relative abundance within the community. Preliminary chemical analysis of the calcite accretions indicates abundant organic carbon, and thus, suggests a possible record of prior microbial ecosystems. On-going investigations of recalcitrant lipid species such as bacteriohopanepolyols (BHPs), in both extant biology as well as the accreted calcite, is underway and should provide insight to the taphonomic processes affecting the viability of lipid biosignatures. Results emphasize the role of local geophysical history in spring microbial community structure and productivity.