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Executive Summary

An Overview of Astrobiology at Arizona State

During our five years as an NAI charter member, Arizona State University sponsored a broadly based program of research and training in Astrobiology to address the origin, evolution and distribution of life in the Solar System. The following paragraphs provide an overview of the specific research areas pursued by the Arizona State University (ASU) Astrobiology team during Year 5. With such a large and diverse team, it is not possible in the space available to cover all details of the research progress made during the past year. For a more complete review, the reader is referred to the individual detailed reports for ASU that accompany this Executive Summary.

Section I: Origins of the Basic Builidng Blocks of Life

At ASU, the origins of living systems has focused primarily on understanding 1) the exogenous origins of carbon compounds through studies of the cosmochemistry of carbonaceous meteorites and 2) the endogenous abiotic synthesis of organics in deep sea hydrothermal vent environments.

A. Meteorite Cosmochemistry.
During Year 5, Co-I Laurie Leshin and her group continued studies of the chemistry of carbonaceous meteorites. By coupling accretion modeling of the early Solar System with D/H estimates obtained from meteorites, the group produced consistent water delivery scenarios for both the Earth and Mars (Lunine et al., 2003). In addition, Astrobiology Postdoctoral fellow Michele Minitti showed that the high D/H values of Martian rocks probably did not result from shock (Minitti et al. in preparation).

Research into nature of hydrous alteration environments on carbonaceous meteorite parent bodies was published by NAI Astrobiology Postdoc Gretchen Benedix in Geochemica et Cosmochemica Acta (Benedix et al. 2003). Using a combination of oxygen isotopic, electron microprobe and petrographic methods, this study established the nature and timing of aqueous alteration processes on the meteorite parent body. In addition, C isotope data provided new constraints on magmatic and surficial carbon reservoirs in the Martian carbon cycle (Goreva et al., 2003, Niles et al., in prep.)

B. Abiotic Synthesis of Organics in Hydrothermal Environments
Co-I’s John Holloway and Peggy O’Day lead an effort to understand the endogenous (terrestrially based) synthesis of prebiotic organic compounds at high temperatures, specifically within seafloor black smoker hydrothermal systems. During Year 5, the group advanced a corollary to their previously published hypothesis that the degassing of seafloor basaltic magmas can provide hydrogen and carbon dioxide for abiotic organic syntheses (Holloway & O’Day, 2000), showing that the same processes can operate in conjunction with silicic magmas (Holloway, submitted). The team also continued to explore the synthesis of organic compounds from hydrogen and carbon dioxide in the presence of mineral catalysts under seafloor hydrothermal conditions. In this work they showed that smectite clay minerals, common in seafloor hydrothermal deposits, provided reactive sites for synthesis of complex organic compounds such as hexamethylbenzene and long-chain methyl esters from aqueous methanol solutions (Williams, et al., 2002; Canfield, et al., 2003).

The Holloway-O’Day group also applied synchrotron computed microtomography to characterize hydrothermal microhabitats by imaging the physical structure of young hydrothermal chimneys from 9°N East Pacific Rise. This analysis showed that the internal structure of sulfide chimneys consisted of a loosely connected network of euhedral mineral grains (predominantly sulfide by bulk X-ray diffraction analysis) and is dominated by diffuse flow (i.e., unchannelized). Imaging established that the physical structure of sulfide chimneys can indeed serve as an effective molecular sieve for the adsorption of organic compounds and bacteria from vent fluids and seawater. This provided the basis for formulating a model to show how organic compounds can accumulate on catalytic mineral surfaces under pre-biotic conditions.

Section II: Early Biosphere Evolution

The early evolutionary history of living systems on Earth has been addressed in a variety of ways at ASU, including ecological and molecular studies aimed at understanding the origins of photosynthesis, studies of microbial fossilization in modern terrestrial environments, with applications in interpreting the biosignatures preserved in ancient terrestrial (and extraterrestrial) geological materials, by novel approaches for reconstructing the paleoenvironmental conditions on the Archean Earth and early Mars and by understanding the role played by of impacts in shaping the early biosphere.

The appearance of oxygenic photosynthesis is regarded as one of the pivotal evolutionary events in the development of Earth’s biosphere in that the accumulation of photosynthetic oxygen at the Earth’s surface and in the atmosphere ultimately paved the way for the origin of advanced multicellular life forms and human intelligence. Continuing controversy over the paleoenvironments and biogenicity of the oldest known terrestrial fossils, as well as the origin of putative fossil biosignatures in Martian meteorite ALH 84001, have emphasized the need for developing more robust chemical and morphological criteria for the recognition of ancient environments and fossil biosignatures in ancient rocks. At ASU, an interdisciplinary effort has been underway to address important aspects of the nature of Precambrian paleoenvironments and to define new approaches to fossil biosignature capture, preservation and detection in ancient terrestrial and extraterrestrial materials.

A. Origins of Oxygenic Photosynthesis:
During Year 5, Co-I Robert Blankenship led a group to study the origin and evolution of photosynthetic systems. This effort involved a broadly based interdisciplinary collaboration between a half dozen partnering universities. Over the past year, the Blankenship team continued their search for novel phototrophs by exploring the non-solar environments around hydrothermal vents. Deep sea hydrothermal vents emit a small amount of fluorescent light from both thermal and non-thermal sources. This emitted light has been suggested as a potential energy source for photoautotrophic organisms. Organisms collected during a 2001 field expedition to hydrothermal vents of the Nine North vent system of the East Pacific Rise are now in laboratory culture and being evaluated for their phylogenetic affinities and metabolic capabilities. Additional light measurements taken at vents along the Mid-Atlantic Ridge gave results similar to vents in the Pacific (White et al. 2003). This work on alternative energy sources holds important implications for the exploration for habitable environments elsewhere in the Solar System (e.g. deep ocean floor environments of Europa).

Field and laboratory studies of photosynthetic organisms in iron-rich environments showed that some photosynthetic organisms utilize reduced iron as an electron donor. This has important implications for understanding the origin of banded iron formations in the Precambrian and for interpreting the rise of atmospheric oxygen. The Blankenship team isolated and analyzed mutant proteins that are potential candidates for Fe-oxidizing complexes.

B. Origins and Preservation of Fossil Microbial Biosignatures
1. Microbial Fossilization Processes in Alkaline Lakes and Hotsprings:
The interpretation of biosignatures in ancient terrestrial and extraterrestrial materials depends critically upon an understanding of the processes of microbial fossilization, inherent biases in biosignature capture and preservation and the effects of diagenesis (post-burial alteration) on biosignature retention. The most relevant systems for study are those extreme environments where active mineralization processes operate and where environmental conditions provide good analogs for the ancient Earth, or other planetary settings. During Year 5 of NAI membership, a group led by Co-I Jack Farmer completed a study of the fossilization processes associated with oncoids (spherical stromatolites) found in the Rio Mesquites, Cuatro Cienegas Basin, Central Mexico. Results were reported at the NAI general meeting and a manuscript is in preparation. This work was done in collaboration with James Elser’s group, who have been studying the ecological aspects of the same system (see below). The work established that surface biofilms have a bi-layered community structure, with the upper zone being dominated by several groups of larger, mostly erect-growing filamentous cyanobacteria and many species of diatoms. Microelectrode studies showed that these surface species effectively control precipitation of calcium carbonate through photosynthesis. However, most mineralization is occurring at a depth of 1-2 mm below the biofilm surface in associaition with a subsurface community that is dominated by smaller filamentous species. Pervasive carbonate precipitation at this depth entombs the deeper community, preserving numerous cellular remains. Interestingly, species of the surface community are rarely preserved, creating a strong taphonomic bias toward preservation of the subsurface community.

Farmer’s group also began a study to charaterize bioflims and microbial fossil assemblages associated with low temperature carbonate-precipitating springs on the floor of Mono Lake, an alkaline-saline lake located in eastern California. (This comprises the topic of a PhD dissertation by Mike Thomas). The goal of the study is to trace the fate of biosignatures during carbonate precipitation, through early diagenesis. The results of the work in Mono Lake will be compared with ancient carbonate spring deposits elsewhere in the Mono Basin that were formed during the last glacial period when the lake was much deeper and colder. The work in the Mono Basin involves a collaboration with the Woods Hole team, who are focusing on the eukaryotic biodiversity of the lake biota.

2. Modes of Preservation in Precambrian Cherts:
Evaluating the nature of microbial biosignature preservation at the cellular scale is an important line of inquiry that may eventually lead to robust criteria for the recognition of biogenic signatures in ancient materials. During Year 5, Co-I Sharp (Dept. of Geological Sciences) continued to apply Analytical Transmission Electron Microscopy (ATEM) methods to characterize the nanometer scale microstructures and composition of kerogenous microbial biosignatures preserved in the 2.0 Ga Gunflint Iron Formation. Electron Energy-Loss spectroscopy (EELS) confirmed that microfossils consist of amorphous kerogen concentrated along grain boundaries of micro-quartz. Kerogen is amorphous, with little evidence for graphitization. In coccoidal microfossils, kerogen forms cell-wall like features around cores of granular microquartz, whereas in filamentous forms, the kerogen is disseminated along needle-shaped grain boundaries that separate submicron-sized fibers of chalcedonic quartz. Sharp’s team are also applying similar methods to the study of putative biosignatures in the controversial 3.5 Ga Apex Chert from Western Australia. The goal is to characterize the composition, crystallinity and distribution of the kerogen to determine if this material is of biogenic or inorganic hydrothermal in origin. This work is ongoing and forms the basis for a PhD dissertation by Brad DeGregorio.

3. Origin of Magnetite in Martian Meteorite, ALH84001:
The hypothesis of a biogenic origin for magnetite grains preserved in carbonates of Martian meteorite ALH84001 is the most compelling line of evidence for the discovery of Martian life by McKay et al (1996). During Year 5, Co-I Peter Buseck and colleagues continued their work with electron tomography and holography to evaluate the biogenicity of putative magnetite biosignatures in Martian meteorite ALH845001 (Buseck et al. 2002). The ultimate goal of the research is to detemine whether the characteristics of crystal size distributions (CSDs) and shape factor distributions (SFDs) of magnetite obtained from magnetotactic bacteria can be used as mineral biosignatures. The group found that CSDs of magnetite obtained from 16 uncultured strains of bacteria showed both similarities and differences among crystals from bacteria from distinct localities and environments. Using a numerical method, the group sorted magnetite crystal populations based on features of the SFD of all particles and found that the numerical methods are useful for identifying bacterial magnetite in rocks. Basic conclusions may be summarized as follows: 1) Using electron tomography, there is too much uncertainty in any TEM results to date to confirm a biogenic origin for the magnetite in meteorite ALH84001; 2) Magnetite crystals in bacterial strain MV-1, the standard for comparison for ALH84001 magnetite, show variations within a chain and rounding of faces, which are not as well developed, or as crisp as in published interpretations; and 3) Progress has been made in developing automated procedures for electron tomographic data acquisition, reconstruction, and visualization.

C. Inferring Paleoenvironmental Conditions on the Early Earth (and Mars):
The initial colonization of the land surface marked an important event in the evolution of the biosphere. This opened a whole new dimension for biological habitation and dramatically expanded microbial involvement in global biogeochemical cycles.

During Year 5, Co-I Paul Knauth (Dept. of Geological Sciences) submitted a manuscript that describes the geologic context of the world’s oldest land-based microbial fossil assemblage, dated at 1.2 Ga (Apache Group paleokarst, north-central Arizona). A second manuscript is in preparation that describes cave deposits associated with the Apache Group paleokarst and the putative microfossils entombed within these deposits. In addition, a paleontological study of modern caliche developed by the surface weathering of basaltic lava flows is also nearing completion. This particular study also demonstrates the potential of caliche as a host medium for capturing and preserving microfossils on Mars. (Caliche is a predicted weathering product on Mars, provided the planet once had a warm, wet climate).

E. Role of Impacts in Early Biosphere Evolution:
Asteroid and cometary impacts have been identified as an important environmental and evolutionary agent throughout the history of Earth’s biosphere.

During Year 5, Co-I David Kring’s group determined the distribution of impact-generated wildfires for the Chicxulub (end Cretaceous) impact event (~65 Ma) and illustrated how impact parameters (e.g., trajectory) can influence the distribution of wildfires (Kring and Durda, 2002). An invited review paper outlined the environmental and biologic effects of large impact events, like Chicxulub, throughout Earth history (Kring, 2003). The Kring group also continued their analysis of lunar impact melts, to determine the flux of impact cratering events in the Earth-Moon system, particularly during the first billion years of Earth’s history (Daubar et al. 2002; Cohen et al. submitted). In an invited review paper, the group also explored the impact delivery of Earth’s water, a key ingredient for life’s origin (Campins et al., 2003) and initiated new directions in the study of impact-generated hydrothermal systems, habitats that may have been a particularly widespread and important during the early evolution of life on Earth.

F. Microbially-based Ecosystems, Cuatro Cienegas Mexico: Windows for Lower Cambrian Ecosystem Structure and Function.
The sudden appearance of complex, well-skeletonized invertebrates at the base of the Cambrian marks a singular event in the history of our biosphere. In an incredibly short interval of geologic time (<10 million years), representatives of all of the modern animal phyla appeared. With the addition of large herbivores and predators, a new global ecology emerged, replacing the microbial mat-dominated benthic ecosystems that had previously been the norm. The rise of large, multicelled bottom dwellers is broadly correlated with the decline of stromatolites, the fossilized biosedimentary structures produced by microbial mats. The disappearance of stromatolites has been attributed to competitive exclusion by algae and/or disturbance by invertebrate grazers. Whatever the cause, the nature of simple microbe-based/grazer ecosystems, the role of environmental and genomic factors in the evolution of such ecosystems and the nature of nutrients and energy flows provide an important context for evaluting the ecological and evolutionary context of the Cambrian explosion.

To better understand the ecological interactions that may have prevailed during the Cambrian transition, an interdisciplinary team of ASU and Mexican scientists, led by Co-I James Elser (Dept. of Biology) has been studying the ecology of simple, microbially based (fish-snail-microbial mat) ecosystems found in modern desert spring environments in the Cuatro Cienegas Basin, Central Mexico. Goals of the study include an improved understanding the energy flow within such simple ecosystems and an improved understanding of the ecological factors that contribute to explosive evolution within highly endemic clades, the nature of ecological interactions between grazers and stromatolite-producing microbial mat communities and mechanisms of microbial fossilization. Ultimately this highly interdisciplinary study aims to test specific hypotheses about the ecological mechanisms that contributed to the Proterozoic decline of stromatolite-producing microbial ecosystems, as well as the ecological and evolutionary factors (particularly stoichiometric constraints on evolution, arising from disparities in C:N:P) that could have contributed to the Cambrian explosion that followed.

During the last year the team tested the hypothesis that the metazoan grazers of stromatolitic microbes face a stoichiometric constraint that results from consuming microbial biomass with elevated C:P ratio. This would lead to microbes having a high biomass C:P ratio due to extreme limitation of their growth by PO4 in the well-illuminated, but oligotrophic spring fed environments that typify the study area. Sampling this past year confirmed that environmental levels of PO4 at CC are indeed extremely low (<3 µg/L usually; levels of inorganic N are high) while organic matter in the surficial layers of various mats and oncoid stromatolites exhibits an extremely high C:P ratio, greatly exceeding values seen for other autotrophic communities in aquatic ecosystems and resembling the extremely nutrient-poor autotroph biomass observed in terrestrial ecosystems. The group also determined that PO4 added to CC waters containing algal mats or stromatolites is removed extremely rapidly and results in a significant lowering of biomass C:P in the mat. Does such P-enrichment of microbial biomass improve the performance of metazoan grazers, as we have hypothesized? The first experiment to answer this question was performed during summer 2001, when a 2-week P-enrichment treatment lowered mat biomass from >3000 (by atoms) to ~900. Snails (Mexithauma sp.) grazing on P-enriched mats had higher RNA:DNA ratios than those feeding on unenriched control stromatolites (generally RNA:DNA ratio is considered a good indicator of growth rate). The encouraging results from 2001 inspired a larger, longer-term (2-month) experiment performed during summer 2002 when we performed a factorial experiment in which both PO4 and the presence / absence of snails were manipulated. Data are preliminary but indicate that PO4 enrichment lowered microbial C:P from ~1000 to ~100. However, in contrast to the 2001 experiment, snails on P-enriched stromatolites had lower RNA:DNA ratios and experienced high rates of mortality and significantly lower rates of tissue and shell growth. Thus, P enrichment in 2002 appears to have “poisoned” the snails. This outcome was perplexing but is comprehensible in light of recent findings related to another herbivore known to be often P-limited, Daphnia, which grows slowly when food C:P ratios are reduced to unnaturally low levels. It was hypothesized that Mexithauma has evolved in the presence of consistently low P availability and experiences routine P-limitation in nature; however, when exposed to unnaturally P-rich food, it suffers a growth penalty. Thus, the simple food webs at CC may be poised on a stoichiometric “knife’s edge” with regard to phosphorus. Could the same have held true for the simple microbially based ecosystems of the lower Cambrian?

The team also conducted a series of studies using microelectrode profiling and bulk incubations to quantify calcification rates of stromatolites. This appears to be the first time this type of experiment has been accomplished on modern stromatolites. Measured areal rates of calcification were very high (~150 mg CaCO₃ cm^-2 yr^-1), similar in magnitude to those measured in tropical coral reefs. However, rates of bioerosion by invertebrates in the same oncolites, measured by analyses of fecal pellet production, are only slightly lower (~106 mg CaCO₃ cm^-2 yr^-1). This indicates that the system is in a precarious balance between net carbonate accumulation and destruction by consumers. This delicate balance could explain why stromatlites are so rare in grazer-dominated ecosystems, while also providing support for the grazer hypothesis as an explanation for the Proterozoic decline of stromatolites. The results of this work were submitted to Science and are presently in review (Garcia-Pichel et al. submitted).

Another goal of the Cuatro Cienegas study is to characterize the genetic and morphological diversity of its understudied biota (esp. microbes, cyanobacteria, and snails) in order to advance our understanding of the evolutionary forces that have affected species, especially the desert pupfish and snail species which exemplify extraordinarily rapid diversification under environmental extremes of temperature and salinity.

With regard to microbial diversity, Garcia-Pichel (et al, 2002) described associations of cyanobacteria that exhibit exceptionally novel adaptations for buoyancy involving calcite as ballast. In addition, a new form of extremely rare fresh-water red alga is in the process of formal description. Collaborator Valeria Souza (UNAM) isolated nearly 3500 separate strains of eubacteria and archaea from various habitats (benign to extreme) at Cuatro Cienegas and characterized the DNA for 350 strains using RFLP with sequencing of the 16S rDNA in selected cases. Sequenced strains from cultivable bacteria include Gram-positive taxa (Bacillus and Staphylococcus) and an ample suite of Gram-negative forms (Pseudomonas, Aeromonas, Aquaspirilum, Vibrio and Halomonas predominate, while clones of uncultivable DNA show marine Archaea and taxa previously isolated at hydrothermal vents). Preliminary analysis of TRFLP patterns of CC microbes shows a moderate alpha diversity with few species dominating the community (within a site) and a very large beta diversity (between sites) where each site has its own species.

The high levels of morphological diversity were also demonstrated for the hydrobiid snail Mexipyrgus by Co-Is Carol Tang and Peter Roopnarine. Morphometric analysis of samples of Mexipyrgus populations have revealed an extremely high level of morphological diversification and differentiation among even closely adjacent habitats. A goal of future work is to examine the ecogenomic basis of this diversification and to determine if the high degree of morphometric variation documented is controlled by dominantly environmental or genetic factors.

Section III. Exploring for Life in the Solar System.

The active involvement of ASU astrobiologists in NASA missions to Mars and Europa has provided ongoing opportunities for research and training in the exploration for life elsewhere in the Solar System. Data from the Mars Global Surveyor (MGS) and Odyssey instruments have continued to provide the ASU team with basic mineralogical information needed to explore the past distribution of water on Mars and for identifying high priority landing sites for future Mars missions for Astrobiology. Orbital data from the Galileo spacecraft has allowed critical testing of the hypothesis of liquid water environments on Europa, Ganymede and Callisto and provided a basis for the selection of high priority landing sites for Astrobiology. Involvement of ASU astrobiologists in various Mars Program missions and mission planning efforts during Year 5 have continued to strengthen the NAI’s contribution to NASA’s missions.

Co-I Philip Christensen is Principal Investigator for three instruments on either current or planned missions to Mars, including the Thermal Emission Spectrometer (TES) instrument (onboard the Mars Global Surveyor (MGS) orbiter), the THEMIS instrument (onboard the Odyssey orbiter) and two mini-TES instruments (to be launched with the Mars Exploration Rover (MER) mission in 2003). Co-I Greeley is a former Chair of the Mars Exploration Payload Assessment Group (MEPAG), the primary community-based science strategy group for the Mars Program. Co-I Farmer was MEPAG Chair during 2002-03, was acting Chair of MEPAG’s Astrobiology Science Steering Group during 2002-03 and is the current Chair of the NAI Mars Focus Group. Greeley and Farmer were members of the Mars Exploration Review Team (MERT) and MAST (Mars Ad Hoc Science Team) the past year, (each is an external oversight committee for the Mars Exploration Program). Greeley and Farmer are also participating scientists on the Mars Exploration Rover Mission and Phil Christensen is PI for the mini-TES instrument on MER. Co-I Laurie Leshin was PI for a Mars Scout mission proposal (SCIM).

A. Astrobiological Exploration of Mars.
Mars Global Surveyor Thermal Emission Spectrometer (TES) data continued to provide new information about the role of aqueous processes in shaping the history of Mars. Although no large-scale carbonate deposits have yet been detected on Mars, spectral evidence was obtained for the presence of H₂O-bearing minerals within Martian dust. Based upon spectral details of the dust, it appears that zeolites are a possible candidate for the aqueous mineral component.

TES data were also used to refine the candidate landing sites for the 2003 MER mission. Along with Gusev Crater, the specular hematite deposit at Meridiani Planum, discovered with TES data in 2001, was a designated as a primary landing site for one of two Mars Exploration Rovers launched in 2003. On Earth, coarse-grained (specular) hematite deposits only form in the presence of large amounts of water and most are hydrothermal in origin.

Newly acquired images from the Thermal Imaging System (THEMIS) on the Odyssey orbiter were used to characterize the candidate landing sites at Merdiaini Planum and Gusev Crater in much greater detail. Many new geologic features of these sites were revealed with stunning clarity using THEMIS data, helping to further refine the geologic context of the MER landing sites.

Masters student Alice Baldridge developed detailed mineralogical ground truth for remotely sensed analog sites for Mars located in the Badwater Basin of Death Valley. MASTER (mid-infrared spectral) data were used to identify the locations of mineralogically pure, end-member pixels (carbonate, sulfate and silicates) within the basin. To establish ground truth, end member pixels were located on the ground and sampled for detailed laboratory analysis of mineralogy. Laboratory methods included X-ray Diffraction, electron microprobe, electron microscopy, thin section petrography and point counting, lab and ground-based spectral analysis (using TES analog spectrometers). To aid spectroscopic identifications, a mid-IR spectral library was developed for evaporate minerals and added to ASU’s spectral data base for use by the TES and THEMIS project teams who are presently mapping Mars. These spectra will also be used by the MER project team. Results of this study were used to establish abundance thresholds (for natural mixtures in the Badwater Basin) necessary for the detection of discrete evaporite deposits (especially carbonates, sulfates and silicates, including zeolites). Results suggested that at the coarse spatial of the TES instrument (3 km/pixel), the detection of carbonates and sulfates is unlikely. However, at the enhanced spatial resolution of THEMIS (100 m/pixel), both carbonates and sulfates should be easily detected, provided they are present at abundances exceeding ~15%. This work was submitted to the Journal of Geophysical Research-Planets and is presently in review (Baldridge et al. in review). The next phase of this research will examine the spectral resolution thresholds for the same data sets.

A second Masters study was also completed during Year 5 that explored the margins of the North polar remnant ice cap of Mars to search for sites of possible magma-cryosphere (volcano-ice). This work comprised a Masters thesis by Meredith Payne who has now entered the PhD program at ASU. The study commenced with broad photogeological reconnaissance using Viking data to identify potential water-formed geomorphic features. This was followed by detailed studies at four sites, selected to cover a range of potential aqueous processes. Hypotheses posed for the origin of geomorphic features were tested using Mars Orbiter Laser Altimeter (MOLA) data and Geographic Information System (GIS) tools (e.g. Digital Elevation Models) and comparisons to terrestrial analogs. In the course of this work, it was discovered that MOLA data are sensitive to subglacal topography in areas that have been recently deglaciated, but are presently covered by snow. The ability to “see through the ice” broadens our access to polar geological history based on MOLA topography (Payne and Farmer, submitted, Icarus). In addition, the hypothesis of a pseudocrater origin for a small field of cinder cone-like features was tested using MOLA data. It was determined that they are more likely be of a subglacial origin, but still formed by a process involving liquid water (Payne and Farmer, submitted, JGR-Planets).

A highlight of Year 5 was the publication of a paper by Co-I Christensen in Science that provided an alternative hypothesis for the origin of the numerous seep sites identified previously by Malin and Edgett (2001) based on Mars Orbiter Camera (MOC) data (Christensen 2003). The Christensen paper was based on THEMIS visible imaging that suggests that the seeps could have formed beneath snow-pack that accumulated during a recent period of low obliquity, and not by outflows of subsurface water (e.g. hydrothermal brines) as previously suggested.

Reconnaissance studies of Astrobiology Martian landing sites for the Mars Exploration Rover mission (successfully launched in June), as well as detailed studies of the Gusev Crater landing site, provided another major focus for the project during Year 5. Gusev was selected as the site for the first MER landing, scheduled to occur in January 2004, while the hematite site was chosen for the second MER landing a few weeks later. Results of these landing site studies were presented at community workshops, to the Mars Exploration Payload Analysis Group (MEPAG) and to the MER project team.

In addition to planning for NASA missions, Ronald Greeley has also participated extensively in planning for the Mars Express mission, which was also successfully launched by the European Space Agency in 2003. Greeley is a Co-Investigator on the High Resolution Stereo Camera System (HRSC) imaging team and provided a list of high priority imaging targets for Astrobiology to the imaging team for use with that instrument. Similarly, Co-I Farmer provided a similar list of high priority Astrobiology targets to the CRISM instrument team (CRISM is a hyperspectral near-IR spectrometer that will be launched to Mars in 2005).

B. Astrobiological Studies of Europa.
Co-I Ron Greeley’s group completed a study of the “mitten” feature on Europa, which represents the extrusion of ice onto the surface from a subsurface source (Figueredo et al. 2002). As such, the mitten structure comprises a high priority target for the future exploration of Europa to search for past or present life. Pole-to-pole geological mapping of Europa was also completed for strips representing the leading and the trailing hemispheres of Europa. This mapping was to explore for potential latitudinal or hemispheric asymmetries in ice fracture patterns. Studies of ice deformation in another region of Europa provided evidence for crustal fore-shortening, important for understanding deformation processes in Europa’s crust. Each of the above activities helped to further characterize the nature and evolution of surface-near-surface environments on Europa needed to further assess the potential for habitable zones of subsurface liquid water. It was concluded from an analysis of domes and other features on Europa that they are geologically young and appear to have brought material to the surface from beneath the ice crust. Additional global geological mapping of Europa was also initiated during Year 5 as a first step toward identifying key sites for future surface exploration of Europa. Current data for Europa obtained by the Galileo project are also being analyzed to further understand the potential for europan environments conducive for life (Figueredo et al. in press).