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Executive Summary

The SETI Institute (SI) NASA Astrobiology Institute (NAI) team is conducting a suite of coupled research projects in the co-evolution of life and its planetary environment. These projects address three of NASA’s Astrobiology Roadmap fundamental questions: (1) how does life begin and evolve; (2) does life exist elsewhere in the universe? and (3) what is the future of life on Earth and beyond? These projects begin by examining specific fundamental ancient transitions that ultimately made complex life possible on Earth. They conclude with a synthesis that brings many of the team’s investigations together into an examination of the suitability of planets orbiting M dwarfs for either single-celled or more complex life.

The astrobiology roadmap calls for a strategy “for recognizing novel biosignatures” that “ultimately should accommodate a diversity of habitable conditions, biota and technologies in the universe that probably exceeds the diversity observed on Earth.” Some of our results, especially those concerning abiotic mechanisms for the oxidation of planetary atmospheres, speak to the interpretation of extrasolar planet atmospheric spectra (and in particular, the role of oxygen as a potential biosignature) in terms of the presence of photosynthesizing life. The team’s M-star project addresses the roadmap’s observation that “although technology is probably much more rare than life in the universe, its associated biosignatures perhaps enjoy a much higher signal-to-noise ratio. Accordingly, current methods should be further developed and novel methods should be identified for detecting electromagnetic radiation or other diagnostic artifacts that indicate remote technological civilizations.” As the roadmap recognizes, there is a continuum of investigations that comprise astrobiology, from prebiotic evolution to the evolution of technology.

The SETI Institute’s NAI team’s research emphasizes the elucidation of the co-evolution of life and its planetary environment, investigating global-scale processes that have shaped, and been shaped by both. Throughout, the team recognizes the importance of pursuing the planetary evolution aspects of this research in the context of comparative planetology: since laboratory experiments are impossible over many (but not all) of the time and spatial scales relevant to early Earth, we supplement laboratory data with insights gained by exploring extraterrestrial environments that provide partial analogs to the early Earth environment and its processes.

The SETI Institute team is pursuing two investigations into the oxidation of early Earth’s environment. While the biological aspects of this “oxygen transition” have been emphasized, our team is exploring non-biological contributions to this transition. Dr. Friedemann Freund and Dr. Lynn Rothschild are investigating oxidation driven by diffusive loss of hydrogen formed within igneous and metamorphic rocks that incorporate water during crystallization. The major objective of this task is to study the causes for the slow but inextricable oxidation of the Earth over the first 3 Gyr of its history. Contrary to the widely held belief that planet Earth became oxidized due to the activity of early photosynthetic microorganisms, they have shown that there is an alternative, entirely abiogenic pathway toward global oxidation: the presence of oxygen anions in the minerals of common igneous rocks that have converted from a valence of 2— to a valence of 1— (peroxy). Upon stressing the rocks, the peroxy bonds break up and generate mobile electronic charge carriers, defect electrons, also known as positive holes or pholes for short. The pholes have the unusual capacity that they can flow out of the stressed rock volume, generating electric currents that can reach or exceed 100,000 amperes, if the stressed rock volume is a cubic kilometer in size. They have shown that this electric current flowing through rocks converts quantitatively into hydrogen peroxide, H₂O₂ , at the rock-water interface. This discovery opens the door to re-assess the conditions that primitive microorganisms, which lived in contact with rock surfaces, must have encountered on the early Earth.

In a second investigation, oxidation driven by atmospheric hydrocarbon (and, more broadly, organic) polymerization is being investigated by Dr. Allesandra Ricca. Dr. Ricca is investigating the chemical energetics and plausibility of reaction pathways leading to the formation of nitrogenated aromatics suggestive of purine and pyrimidine bases of RNA and DNA molecules using quantum chemistry. She has focused on the reactivity of pyridine radical cation with acetylene and shown that it is a viable reaction pathway leading to the formation of a bicyclic nitrogenated aromatic molecule. Very recent experiments performed at Virginia Commonwealth University have confirmed her results. In addition, we are investigating the reactivity of N+ with various unsaturated hydrocarbons, such as 1,3-butadiene, to form small nitrogenated heterocycles.

Understanding the oxygen balance on early Earth requires attention to sinks as well as sources of oxygen. One major sink for oxygen on early Earth would have been reduced iron. Iron could have simultaneously provided shielding against ultraviolet (UV%) light that would have been reaching Earth’s surface in the absence of the ozone shield generated by atmospheric oxygen. Nanophase ferric oxide minerals in solution could provide a sunscreen against UV while allowing the transmission of visible light, in turn making the evolution of at least some photosynthesic organisms possible. Dr. Janice Bishop and Dr. Lynn Rothschild are testing this hypothesis through coupled mineralogical and microbiological work in both the lab and the field, and examining its implications not only for Earth but for Mars as well, with an emphasis on implications for upcoming spacecraft observations. They performed experiments testing minerals as potential sunscreens. For example, nanophase-FeOx mixed with clay enabled C. reinhardtii and Euglena to survive exposure to UV radiation longer than controls not mixed with clay. In addition, absorbance spectra show a strong 674 nm absorption chlorophyll band associated with organism growth: that is, live cells exhibit a strong 674 nm absorption band whereas dead cells do not. The 674 nm band weakens with increasing UV exposure, then returns for nanophase-FeOx containing clay samples following rehabilitation in a visible light incubator. They have shown that the 674 nm band disappears and cells die in experiments without nanophase-FeOx containing clay. Current experiments involve mineralogical characterization of Fe-rich environments where photosynthetic organisms are thriving below the surface.

Environmental conditions for life in terrestrial lakes located at extreme high altitudes in Bolivia and Chile provide a good analogy to martian paleolakes dating back 3.5 Ga. Through the exploration of these lakes the survival strategies of microorganisms in very high UV environments can be elucidated. A team led by Dr. Nathalie Cabrol and Dr. Edmond Grin conducted a series of investigations examining the geology, paleobiology and extant biology of these lakes. This past year the team sampled new sites (evaporating lakes, salars, and geothermal centers), as well as laying down a new stratigraphical transect in the geological record of Laguna Verde to study the evolution of paleohabitats and life during fast changing climate conditions. The UV flux at these high altitude lakes was examined in more detail. Samples from Laguna Blanca were analyzed by Raman Spectroscopy in an attempt to develop a database for characterizing the structure and stability of biogenic carbonaceous material in such samples.

Just as global-scale changes in oxygen (or iron) were critical for the early biosphere, so too would have been global processes involving other key “biogenic” elements such as carbon (for which Dr. Allesandra Ricca’s work provides insight) or nitrogen. Dr. Rocco Mancinelli, Dr. Amos Banin, Dr. David Summers, and Dr. Bishun Khare are pursuing coupled laboratory and field research to understand the partitioning of nitrogen on early Earth and on Mars between different possible reservoirs, and (at least for Earth) the abiotic to biotic transition in this cycling.

Dr. Banin has completed the analysis of soil samples from the Atacama desert , an extreme terrestrial environment with very low biological activity. Although not completely clear what properties of the soil and environment are the limiting factors for biology, these soils are nearly devoid of organic material and contain high levels of perchlorate. They have conducted a series of field experiments, which show that at the driest sites (e.g., Yungay) there is virtually no nitrogen cycling even when the samples are wetted. They have shown that the soil is not toxic and capable of allowing a variety of microbes to flourish if organic material is added. These results are being pursued to better understand the relationship between the soil redox potential and the denitrification pathway.

Dr. Summers and Dr. Khare have completed the first experiments to demonstrate abiotic nitrogen fixation. Results show that both the theoretically predicted pathway and an alternate pathway can occur and that the observed chemistry is dependent on the amount of water and the state (liquid or gas) in which water is present. They have also discovered that NO can alternately be directly reduced to ammonia by FeS minerals in aqueous solution. Currently, they are studying pH and other variables for this reaction and are in the process of identifying a gaseous by-product. They are also conducting experiments to determine the most effective method for measuring isotope fractionation. The resulting method will be used as a benchmark for biogenic determination and for identifying the isotopic composition of samples from the Atacama, where nitrates may be of abiotic origin. These experiments begin providing a different perspective into the astrobiologically important question of the fate of N on early Mars.

The work described so far examines the evolution of planetary surface habitability. With the recognition that a subsurface ocean likely exists on Jupiter’s moon Europa, we know that habitability in possibly entirely subsurface environments must also be explored. Dr. Cynthia Phillips, Dr. Christopher Chyba, and Dr. Kevin Hand (who just received his Ph.D. from Stanford under the mentorship of Dr. Chyba) are pursuing spacecraft data analysis and modeling to examine the geology of Europa and its implications for the free energy sources that would be needed to power a europan biosphere. Dr. Phillips and Dr. Chyba are continuing a survey of images of Europa to look for any changes that occurred due to geological activity during the Galileo mission, which if present would indicate active regions of the surface. We have completed a search of the Galileo Europa images to find overlapping images, and are continuing to work on improving their automated search method to ensure that they find all possible comparison images.

These results will be coupled with the results of low-temperature laboratory experiments to make predictions about the possible abundance and survivability of any oceanic biomarkers that might reach Europa’s surface through active geology, with implications for the astrobiological exploration of Europa from either an orbiter or a surface lander. Dr. Hand, in collaboration with Dr. Robert Carlson and Dr. Chyba, is pursuing this research in Dr. Carlson’s laboratory at the Jet Propulsion Laboratory. Dr. Max Bernstein, in his laboratory at NASA Ames, is performing measurements to detect biomarkers. He is discriminating between true and false biomarkers. Dr. Bernstein has been concentrating on the kind of conditions found on icy outer Solar System bodies such as Europa and has found good agreement between theoretical prediction and experimental observation. One of his significant findings indicate that radiation can cause changes that may make it more difficult to distinguish a biomolecule from abiotic organics.

Dr. Peter Backus, Dr. Jill Tarter, and Dr. Rocco Mancinelli published results from the first workshop that examined the prospects that planets orbiting dwarf M stars are habitable for either microscopic or complex life in the Journal Astrobiology (see reference section of the report). The results suggest that there is no reson to preclude that life cannot evolve on a planet orbiting a dwarf M-star.

Education and public outreach are major and integral parts of the work of the SETI Institute’s NAI team.