nai

Executive Summary

The University of Colorado Center for Astrobiology member in the NASA Astrobiology Institute runs a multi-pronged program. Our research efforts span the entire range of disciplines that comprise astrobiology, and include components in the physical sciences, the biological sciences, and the humanities. In addition, we collaborate with leading players from the aerospace industrial community to provide leadership in developing technology for astrobiology. On the education side, our research program meshes with a graduate-education program in each of the sub-disciplines within astrobiology, and we provide a broad-based education to students. In addition, we offer courses and programs for undergraduates, as a way of involving them in astrobiology and, eventually, feeding into a graduate program either here or elsewhere. We also have a cutting-edge program in education and public outreach that provides significant activities at a number of different levels. Our education and outreach program will be described elsewhere, and we focus here on our astrobiology research program.

Our research activities divide up into two science themes, a humanities theme, and a technology theme. Each of these will be discussed below.

Origin and evolution of life

Chiral determinism and the origin of translation. An important part of understanding the origin of the genetic code is understanding the chirality of the molecules comprising life. Why does the ribose backbone in RNA utilize the D orientation, and amino acids the L orientation? Mike Yarus has been carrying out experiments designed to understand the relationship between the chirality of these, by looking at the ability of randomized RNA molecules to preferentially grab one chirality rather than the other of amino acids. The experiments show a remarkable preference for L histidine. However, similar experiments show the exact opposite preference, for D phenylalanine. Are the results different for different amino acid side chains (thereby giving us a clue about which amino acids entered biology via RNA activity) or is this difference the anticipated confusion arising from a chiral support? Future experiments will help us to choose.

A mechanism for the association of amino acids with their codons and the origin of the genetic code. In the origin of the genetic code, an explanation of why certain amino acids are assigned to certain codons has not been developed. Shelley Copley has been developing a model for origin of the genetic code. The genetic code has many regularities, some of which include explanation in terms of tRNA function or robustness against deleterious effects of mutations in translation. For example, there is a strong correlation between the first bases and the biosynthetic pathways of the amino acids they encode. Codons beginning with C, A, and U encode amino acids synthesized from x-ketoglutarate, oxaloacetate, and pyruvate, respectively. This and other regularities can be explained if, before the emergence of macromolecules, simple amino acids were synthesized in covalent complex of dinucleotides with alpha deto acids originating from the reductive tricarboxylic acid cycle or reductive acetate pathway. These processes could have created an association between amino acids and the first two bases of their codons that was retained when translation emerged later in evolution.

The biogeochemical evolution of sulfur on the early Earth. Rock samples retain a chemical remnant of the environmental conditions extant at the time that the rocks were formed or altered, and provide the most detailed information on the history of the Earth’s habitability. Steve Mojzsis and his group have been utilizing this type of information to understand the ancient Earth. They have been examining mass-independent sulfur isotope effects as a way of tracing the transformation of the surface zone to an oxygen-rich environment between 2.5 and 1.8 b.y.a. They have been examining oxygen and sulfur isotopes in supracrustal sequences in West Greenland as a way to corroborate a sedimentary origin for Akilia iron-rich quartzitic enclaves. In collaboration with other groups, they have expanded the record of nitrogen isotopic fractionations by life into the oldest sediments from West Greenland. And, in a complementary analysis, they have been examining the detailed phylogenetic character of living stromatolites in Shark’s Bay, Western Australia, in order to make inferences about early Archean habitats and the “classical” interpretation of the origin of stromatolites in the geological record.

Molecular survey of microbial diversity in hypersaline ecosystems. In ongoing work related to the microbial diversity in naturally occurring extremophile ecosystems, Norman Pace and his group have been working in hypersaline and hyperthermophilic environments. In the hypersaline environments, ~4000 new rRNA sequences were isolated and described and, therefore, organisms were described. Many new bacterial and archaeal clades were discovered, including ~15 new bacterial phylogenetic divisions (“kingdoms”). Surprisingly, cyanobacteria do not constitute the main biomass (by rRNA gene content) but, rather, green nonsulfur bacteria dominate numerically. In the high-temperature environments, the group has completed analysis of Yellowstone systems at temperatures above 70°C. Notably, organisms identified molecularly as hydrogen-metabolizing constitute the most common life in those hot springs. As well, the hydrogen abundance in a number of hot spring systems has been measured, and sufficient hydrogen to support metabolism has been identified (15-300nM).

Origin of multicellularity and complex land-based ecosystems. Understanding the symbiotic relationships between organisms at the surface of the Earth, and the distribution between above- and below-ground phases of a life cycle, is important for understanding how ecosystems evolve in response to major trauma such as that accompanying extinction-event-inducing impacts. Under William Friedman, we have been investigating the relationship between land plants and fungi. 80-90 % of all land plants have mutualistic symbiotic associations with fungi where the fungal symbionts provide increased access to essential minerals and the fungal symbionts gain access to fixed carbon. Fungal symbionts in early lineages of land plants can have a life cycle where one phase is above ground and photosynthetic and another is completely underground for as long as fifteen years. These early lineages represent a potential ancient life cycle mode that succeeded based on its fungal dependent subterranean phase. Given the diversity of fungal species capable of translocating carbon between photosynthetic plants and subterranean plants, it is possible if not probable that carbon transfer between different plant species through fungal networks may well extend beyond underground plants.

Origin and evolution of habitable planets

Biogeochemical cycling and resources on Mars. Our approach to exploring the biological potential of Mars has been to examine the environmental conditions necessary to support life and to understand their availability, accessibility, and abundance on Mars. Bruce Jakosky and his group have been focusing primarily on the availability of liquid water (and the connections to the history of climate and volatiles) and the availability of geochemical energy that can support metabolism. On the liquid water side, they have been trying to synthesize the wide range of results from the five ongoing spacecraft missions that pertain to volatiles and climate on timescales of the seasonal cycles, the obliquity cycles, and the four-billion-year history of volatiles. The synthesis differs from previous ones in significant ways and represents a change in our understanding of the driving forces behind Mars climate evolution. On the issue of available geochemical energy, we have modeled the geochemical reactions between water and rock in Mars-like environments at ambient conditions, in order to determine the availability of energy to support metabolism. We are focusing on ambient conditions given the increasingly compelling evidence for long-lived liquid water that has not been heated to hydrothermal temperatures. Our results suggest that ambient-temperature conditions provide significantly more energy from weathering reactions than do hydrothermal conditions, due to the greater energy change at low temperatures; however, the energy may be made available at a slower rate.

Untangling Europa’s evolution. Significant progress has been made by Bob Pappalardo toward determining the stress history of Europa’s lineaments to understand whether the satellite is in a steady-state or if its activity level has changed over time, with implications for its capability to support life. We have succeeded in modeling surface stresses on Europa from orbital (diurnal) stressing and from nonsynchronous rotation of the ice shell using gravitational potential theory. We find that potential theory significantly improves upon previous models of surface stresses on Europa, modifying previous predictions for the extents of zones of compression, extension, and strike-slip across the surface. We have found that combined diurnal and nonsynchronous stresses predict patterns that are a compelling match for the patterns of lineaments on Europa’s surface. We are in the process of developing and testing “graph theory” algorithms for quantitative comparison of stress orientations and styles to mapped lineaments, to constrain the temporal evolution of the surface.

The impact of atmospheric particles on life. Brian Toon and his group have been working in part with the NASA/Ames team in the NAI to understand the forces that drive the climate and atmosphere on terrestrial planets and satellites. This has included examining processes related to atmospheric evolution on the early Earth, potential external forcing of terrestrial snowball Earth episodes, the evolution of oxygen in the Earth’s ancient atmosphere using sulfur isotopes, the record of biological extinction following the K/T impact event and the implications for climate, the geomorphological processes responsible for martian gullies and their implications for near-surface and surface liquid water, and the formation of hazes in the atmospheres of Titan and the early Earth. The largest common thread is radiative forcing of the atmosphere, via interactions with the surface, with atmospheric gases, and with atmospheric aerosols. A paper on the first of these topics appeared in Science, with a demonstration that hydrodynamic escape could have left the Earth (or an extrasolar planet) with an atmosphere enriched in H₂, at perhaps the tens of percent level, and with an H₂/CO₂ ratio greater than 1. This would mean that the prebiotic atmosphere might have been a good source of organic molecules for the origin of life.

Geochemical-microbe interactions in chemolithotrophic communities on the Earth. Tom McCollom, along with Brian Hynek, has been using geochemical modeling and remote sensing data to evaluate the geologic history of bedrocks exposed on Meridiani Planum that have been observed by the Mars Exploration Rover Opportunity. The MER science team has interpreted the bedrock to be volcanic sediment deposited in a standing body of briny water that later evaporated to leave behind sulfate salts, implying prolonged clement conditions and a relatively benign environment that might be conducive to life. However, this interpretation appears to be incompatible with the chemistry and geologic setting of the rocks, and we have inferred that the bedrocks are, instead, volcanoclastic airfall deposits subsequently altered by sulfur-rich volcanic gases in a solfatara-like environment. This scenario would imply and environment much less conducive to life.

Star and planet formation. We have been examining the early phases of star formation, using both theoretical and observational approaches, in order to understand the conditions surrounding planet formation and the conditions under which planets do form and under which they do not form. John Bally is leading this effort. Theoretical approaches include modeling of the gas loss induced by photoablation by ultraviolet light. In particular, removal of H and He by this process increases the dust/gas ratio and enhances the formation of kilometer-sized planetesimals that are a precursor to planet formation. Simulations of the interactions between protoplanetary disks reveal that spiral shock waves can be generated; such shocks may lead to significant chemical processing, and possibly may explain the properties of chondrules found in some primitive meteorites. Observationally, we have continued our recent studies of the nearest star-cluster forming clouds such as Perseus and Orion. Results include the discovery of many proto-planetary disks surrounding young stars and thermal-infrared emission from warm dust from Orion’s “naked” stars (those not surrounded by photo-evaporating disks).

Philosophical and societal issues.

The historical character of astrobiology and circumventing the problem of defining life. Carol Cleland continues to lead the field in discussing astrobiology from the perspective of philosophy of science. Two areas have been emphasized recently. The first one relates to the definition of life and the fact that, in absence of a general theory of living systems, it is unlikely that scientists will recognize genuinely strange life if they come across it. We explored the possibility of anomalous forms of microbial life on Earth today. Showing how preconceptions based on familiar Earth life can blind us to the possibilities for life in general underscores the importance and difficulties involved in searching for extraterrestrial life. Second, we explored the development of a theory of biology that encompasses all life, not just known terrestrial life; we emphasized the issue of Darwinian evolution as a component of a definition of life. We conclude that this is not necessarily a requirement for life; as software simulations of Lamarckian evolution suggest, it is possible that organisms with a different molecular architecture could utilize different physical mechanisms for achieving biological change.

Astrobiology technology development.

Astrobiotechnology development. Although astrobiology technology development is proceeding through several different programs, they are not integrated well into a single coherent program. Our goal is to provide some oversight from the community and synthesis and integration within the community to the program as a whole, and thereby to stimulate the development of astrobiology technology in a time in which there is increasing scientific focus in these areas. We have done this by creating a consortium involving academia, national research laboratories, and industry. Our first effort was to organize a workshop on Mars astrobiotechnology development, in order to bring the various science and technology communities together into a single forum for the exchange of ideas. The workshop was held in September 2004 and was a tremendous success. Planning is ongoing for our second workshop, focusing on providing background and support for instrument development by astrobiologists who do not have much history or experience in the flight programs.