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Executive Summary

Ames Research Center NAI team has maintained a coordinated research program that links the formation, evolution, and climates of habitable planets; the roles of interstellar chemistry in supplying potential biological precursors to these worlds; the origins and nature of metabolism in the first cells; the impact of established biospheres on planetary climate and crustal and atmospheric chemistry, emphasizing the formation of detectable biosignatures; the response of vegetation to regional climate change; and, finally, the potential for life to transcend planetary boundaries through transfer between habitable worlds. Our program for education and public outreach captures these themes to develop an engaging and informative package that is being disseminated to national- and international-scale audiences. This is being achieved through partnerships with the California Academy of Sciences (CAS), Yellowstone National Park (YNP), New York Hall of Science, and several K-14 educational organizations. Strong conceptual and functional links to multiple NASA missions provide context, motivation, and funding leverage for our research component, along with resource- and audience-sharing opportunities for our education and public outreach component.

Our investigations of the formation, evolution, and climatology of habitable planets have focused on terrestrial (rocky) planets where liquid water is stable at the surface. We chose this focus because extrasolar planets that host surface biospheres are the most likely to be detected by remote spectroscopic searches. Simulations of planetary accretion have shown that the amount of mass accreted by Earth from the asteroid region depends sensitively on the formation time and the early orbital evolution of the giant planets. In particular, the mass of volatile materials delivered to Earth depends critically on the orbital eccentricities of giant planets, because these control the width and strength of the unstable resonances in the asteroid region. Our team has shown that methane could have provided a maximum of 30 degrees C of surface warming on early Mars. Higher methane concentrations would have resulted in an anti-greenhouse effect that would have caused surface cooling. We proposed a novel explanation for the Martian rivers, namely that they originated as the result of impacts. This view greatly restricts the environments where life might have originated and suggests that the only likely place might be in the subsurface. For the first time, we created particles that may form in methane-rich atmospheres that also contain carbon dioxide. Such atmospheres might resemble those of early Earth.

We have studied the formation of complex and potentially protobiological organics under simulated interstellar conditions. We also have helped to trace, spectroscopically and chemically, the cosmic evolution of organic molecules from the interstellar medium to protoplanetary disks, planetesimals, and finally onto habitable bodies. We have made progress in tracing the link between ice processes and the organic molecules in meteorites.

Figure 1. Results from this past year’s effort support the hypothesis that molecules that are important for the origins of life, might originate in the interstellar medium.

First, we have demonstrated that the ultraviolet (UV) photolysis of presolar ices can produce the amino acids alanine, serine, and glycine, as well as hydroxy acids, and glycerol, all of which have been extracted from the Murchison meteorite. Thus, some of the meteoritic amino acids glycine, alanine and serine, as well as structurally related hydroxy acids, may have been synthesized in ice before our solar system formed. Also, the amino (NH₂), carboxylic acid (COOH), cyano (CN), ether (O-CH₃), hydroxy (OH), keto (>C=O), and methyl (CH₃) groups that decorate the aromatic compounds in meteorites and interplanetary dust particles (IDPs) can be formed by energetic processing of low temperature ices. This pathway is consistent with observed deuterium enrichments and is the first proposed mechanism that makes predictions on a molecular level.

We also address the origin of metabolism in the earliest ancestors of cells by testing the hypothesis that proteins might have arisen and initially evolved in the absence of a genome. We selected four new adenosine triphosphate (ATP)-binding proteins from a population of >10¹² random polypeptides. Proteins from one family have been characterized. They are highly selective and do not bind either guanosine triphosphate (GTP) or deoxyadenosine triphosphate (dATP). They do not contain any known ATP-binding motifs, but they do require zinc ions and contain conserved cysteine residues, suggesting a possible structural similarity to zinc finger proteins. In order to test this possibility, and more generally to determine if these de novo evolved proteins resemble biological protein, we have evolved mutants that are suitable for biophysical studies by continued selection for ATP binding in the presence of guanidine. As expected, more of the newly selected proteins stay soluble in the presence of ATP, presumably because ATP binding stabilizes the folded state of the protein. We have established that some proteins remain stable as monomers. To simulate self-organization of protobiological proteins into metabolic networks capable of evolution towards increasing complexity we developed a computational approach for describing systems having many species and reaction channels. This method is not limited to chemical reactions, and it allows for incorporation of other cellular processes such as channel-mediated transport and cell growth and division.

We investigated living photosynthetic microbial mat ecosystems because they allow us to examine those microbiota and ecological processes that participated in early evolution, modified the early environment, and created biosignatures. Photosynthetically active cyanobacterial mats were studied in a hypersaline pond near Guerrero Negro, Mexico. We quantified the substantial quantities of volatile fatty acids (VFA) produced by these mats. We measured fluxes of dimethylsulfide (DMS) and methanethiol (MT) under a range of conditions. These two gases are probably formed by reactions that occur between low molecular weight organic carbon compounds and biogenic hydrogen sulfide. In an early anoxic global environment, DMS and MT can escape to the atmosphere and serve as biosignatures of these microbial communities. Our microscopic, genetic, biomarker and biogeochemical observations indicated that hypersaline mats can be maintained in a greenhouse at Ames for more than a year without sustaining major changes. This greenhouse will allow astrobiologists to explore the microbiological effects of environmental conditions that were widespread on early Earth but that are rare today. We documented a high diversity of uncultivated green nonsulfur bacteria (GNS) in photosynthetic microbial mats in the hot, alkaline Mushroom Spring, Yellowstone National Park. Some GNS were observed in close association with cyanobacteria, and diverse members of the GNS group assimilated acetate.

Figure 2. Recent experiments indicate that the Ames greenhouse facility enables us to study microbial mats under environmental conditions that were important on early Earth but are either rare or absent today.

We are extending the ecosystem-level studies of photosynthetic microbial mats to a planetary scale by creating quantitative models that simulate energy relationships, biogeochemical cycling, trace gas exchange, and biodiversity. We developed a simulation model called MBGC (Microbial BioGeoChemistry) to infer effects of major environmental controllers on microbial community structure and function. Already the model reproduces major diel fluctuations in trace gases in hypersaline subtidal mat layers and into the overlying water column. We are testing various approaches to simulating the net hourly emissions of O₂, CO₂, and H₂S gases to the atmosphere above the mat ecosystem, over the course of several days during which actual measurements of these same fluxes have been made.

Figure 3. Schematic representation of the MBGC model (Decker and Potter, 2002), including bacterial pools (squares), light (PAR, NIR) and temperature input as model drivers, and gas concentration pools (circles).

We have developed, for community use, a new Global Atmospheric Integrator for Astrobiology (GAIA). It is unique in that it can consistently leap from simulations of a “typical” day of varying atmospheric chemistry to periods of geologic time. GAIA incorporates fully modern descriptions of atmospheric photolysis and UV shielding, photochemical reactions, thermal reactions, a simple water cycle, and lower and upper driving fluxes, i.e., biogeochemical interchange and hydrogen escape. GAIA performs stably as fluxes of O₂ and reduced (CH₄, H₂) gases are increased, thus allowing a full study of composition accompanying the rise of aerobic life.

We are examining the effects of climate variability on a vegetation-rich biosphere over intermediate time scales, using South American ecosystems as a model. Previously, we demonstrated that a strong correlation exists between vegetation changes at 32 South American sites and variations in sea-surface temperature (SST) over a 12-year period, related to the El Niño Southern Oscillation (ENSO). This past year, our map of remote-sensing-based estimates of vegetation indices (advanced very-high-resolution radiometer – normalized difference vegetation index (AVHRR-NDVI)) for the subtropical growing season of South America (October through January) on monthly data from 1981 through 2000 validates earlier ground-level studies by other investigators. Our graphic model also has modified current knowledge regarding the areas affected by “El Niño” Southern Oscillation in South America. We identified areas that are affected differently by El Niño, and La Niña, as well as the “normal” conditions. We should eventually be able to link SST to tree-ring widths. Finally, our reconstructed “paleo”-SST simulation correlates well with volcanic phenomena, the Little Ice Age, and other events. The simulation also complements, often with greater detail, reconstructions made with other proxy records, such as paleomicroplankton.

Figure 4. Summary description of South America’s AVHRR-NDVI-deviations from normal (1981-2000) under “El Niño”, and “La Niña” conditions.

We are assessing the potential for life to move beyond its planet of origin, as a potentially important component in the evolution of life in our own solar system. We address natural transport where survivors must withstand radiation, desiccation, and time in transit. Observations both in the laboratory and in nature indicate that low levels of radiation and oxidative damage on cells can result in a mitogenic effect, meaning that responses to radiation are non-linear and cannot be predicted from simple models. Experiments conducted in the space simulation facility at the Deutsche Forschungsanstalt für Luft- und Raumfahrt (DLR) in Cologne, Germany reveal that the cyanobacterium Lyngbya can survive exposure to both UV radiation and desiccation. The number of organisms that might be able to survive exposure to the space environment might be far greater than originally recognized.

The strong mission relevance of our research, combined with the direct participation of several team members in the planning and execution of NASA missions, places the Ames team in a position to influence strongly the astrobiology content of ongoing and future missions. Studies of planet formation and habitability currently benefit the Space Infrared Telescope Facility (SIRTF), Kepler, Eddington and Terrestrial Planet Finder (TPF) missions. Giant planets orbiting nearby stars can be detected using current technology, and the masses and orbits of these objects can be calculated. By combining these observations with simulations by our group, it will be possible to determine promising candidate stars for future missions and observing programs designed to detect Earth analogues, such as Kepler and TPF. We address the effects of methane on the Martian climate evolution. Our work concerning CO2 clouds and the effects of impacts is relevant for interpreting data from Mars Odyssey and future Mars missions, such as the 2009 Mars Science Laboratory rover. Our team’s studies of cosmic ices and organics are synergistic with the Stratospheric Observatory for Infrared Astronomy (SOFIA), SIRTF and Stardust missions. The work with microbial ecosystems strengthens the systematics for interpreting the microbial fossil record and thereby enhances astrobiological studies of Martian samples. A team member serves as interdisciplinary scientist for astrobiology on the 2003 Mars Exploration Rover (MER) Mission. Models of biogenic gas emissions will enhance models of atmospheres that might be detected on inhabited extrasolar planets. One of us is first author on a major white paper that addresses the feasibility of searching for biosignatures during the proposed TPF mission. This white paper was just published in the peer-reviewed literature.

We have served the needs and interests of the nation’s educators, students and public through a high-impact education and public outreach program. Specifically, we partner with the California Academy of Sciences (CAS), Yellowstone National Park (YNP), and the New York Hall of Science to develop new astrobiology workshops, activities, exhibits, and other products for the public. CAS has chosen to utilize astrobiology to link its natural history museum, planetarium, and aquarium under the theme, “Earth and its Place in the Universe.” Ames personnel serve directly on the CAS design and exhibit development teams. Ames and CAS also facilitate interactions between researchers and educators in order to develop inquiry-based programs and activities for K-14 students. Our partnership with YNP combines a large annual visitation with a highly effective venue for conveying astrobiology-related content. With material input from the Ames team, YNP is introducing astrobiology content into trailside interpretive signs, brochures, and the Yellowstone Resources and Issues Guide. Ultimately, astrobiology will be integrated into permanent exhibits for the major visitor centers.

Figure 5. The Trailside Sign Project represents the E/PO collaboration between the NAI Ames Team and Yellowstone staff, to allow park visitors to learn about Astrobiology by experiencing Yellowstone’s natural features.

We have extended this impact to the professional level by engaging graduate students and postdoctoral associates in the proposed research activities, through the teaching of undergraduate courses in astrobiology at Stanford University and at local community colleges.