2006 Annual Science Report
SETI Institute Reporting | JUL 2005 – JUN 2006
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 will conclude with a synthesis that will bring 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. We believe that we are the only NAI team whose investigations span the gamut of the roadmap’s range.
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. Subsequent weathering of the rocks released hydrogen peroxide into the environment; the previous loss of the hydrogen indicates that this is a net oxidizing mechanism. During this reporting period we succeeded in completing new physical measurements on igneous rocks, an important step toward the goal of having in hand quantitative analyses of peroxy oxygen in rocks ranging from ultramafic (peridotite, gabbro) to felsic (andesite, granite). In addition, we have shown that the hydrogen peroxide oxidizes reduced transition metal cations, foremost ferrous iron to ferric iron. This leads to the precipitation of ferric oxides in the ocean and, hence, to the deposition of Banded Iron Formations (BIF).
If the oxidation mechanisms being explored were shown to be quantitatively significant—modeling to be done later in the course of this grant—this would suggest that the oxygen transition on an Earth-like world could take place independently of the invention of any particular metabolic pathway (such as photosynthesis or methanogenesis) that have previously been proposed to drive this transition. Since Earth’s oxygen transition ultimately set the stage for the oxygen-based metabolism evidently essential for metazoa, understanding this transition is crucial to elucidating both Earth’s evolution and the evolution of complex (including intelligent) life. The team’s geological investigations are therefore tightly coupled with microbiological experiments, led by Dr. Rothschild, to understand the extent to which the proposed mechanism might have led to the evolutionary invention of oxidant protective strategies and even aerobic metabolism.
In a second investigation, oxidation driven by atmospheric hydrocarbon (and, more broadly, organic) polymerization is being investigated by Dr. Emma Bakes. Dr. Bakes’ research for the early Earth builds on analogies to processes now occurring in the atmosphere of Saturn’s moon Titan. Her mapping of the chemical sequences for anions, neutrals and cationic nitrogenated aromatic molecules in Titan’s organic haze layer is well underway, utilizing the participation of quantum chemist Alessandra Ricca. We are mapping the chemical energetics and the plausibility of each suggested reaction pathway for bicyclic nitrogenated aromatics suggestive of purine and pyrimidine bases of RNA and DNA molecules to probe the plausibility of their photochemical formation in an atmsosphere. UV penetration directly affects the survival or destruction of organic molecules and the irradiation of potential life forms and we have completed and published our investigation of how the UV radiation interacts with large molecules, tholins and the gas phase and to what degree it penetrates to the surface of Titan. Our laboratory study of hydrogen molecule synthesis on aromatics and aerosols to seek a physically plausible pathway to the accelerated oxidation of Titan and the early Earth is complete and published. She has also begun to extend her study of molecular charging to Martian regolith dust to determine how this affects the UV radiation penetration to the Martian surface. This UV penetration directly affects the survival or destruction of organic molecules and the irradiation of potential life forms.
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. 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 have completed a number of lab experiments showing that nanophase iron oxide-bearing minerals facilitate growth of photosynthetic organisms by providing protection from UV radiation. This year they have completed analyzing the data from previous years lab experiments and summarized our results in a paper that is in press in the International Journal of Astrobiology. This work showed that nanophase iron oxide-bearing minerals can facilitate growth of photosynthetic organisms by providing protection from UV radiation. Based on the spectral properties of iron oxides and the results of experiments with two photosynthetic organisms, they propose a scenario where photosynthesis, and ultimately the oxygenation of the atmosphere, depended on the protection of early microbes by nanophase ferric oxides/oxyhydroxides. They have also begun evaluating the OMEGA hyperspectral visible/near-infrared (VNIR) spectra of Mars in an effort to characterize deposits of nanophase ferric oxide-bearing minerals that could provide UV protected niches for photosynthetic microbes if they were present on Mars. This part of the project will be expanded in the coming year as the CRISM hyperspectral VNIR images become available. Concurrent with other projects, they are evaluating the spectral properties of Fe-bearing Mars analog sites on earth and analyzing spectra of Mars for Fe oxide-bearing components.
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 of these lakes 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. Bakes’ work provides insight) or nitrogen. Dr. Rocco Mancinelli, Dr. Amos Banin, Dr. David Summers, and Dr. 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. In a series of experiments in which they have searched for the nitrite reductase gene, a gene critical in denitrification, they have found none. They are currently planning a series of experiments to test the potential toxicity of the soil for nitrogen cycling microbes.
Dr. David Summers and Dr. Khare have experimentally verified that the abiotic fixation of NO to nitrite and nitrate would indeed occur, as had been postulated theoretically. It has been shown that two mechanisms exist for this fixation. One proceeds in the presence of liquid was and appears to be consistent with the proposed pathway through HNO. Another proceeds to NO2 in the absence of liquid water but, in the presence of absorbed water, the NO2 can be converted to nitric acid. Their study has expanded to include the study of the stable isotope fractionation and the effects of water layers on mineral surfaces. Specifically, analysis of the isotope fractionation in the reduction of nitrite to ammonia, which shows an average fractionation of -4 per mil. 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 Mr. Kevin Hand (a Stanford PhD student advised by 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 first-stage search of the Galileo Europa images to find overlapping images, and are continuing to work on improving our automated search method to make sure that we find all possible comparison images. We have processed a number of comparison pairs, and are currently working on automated techniques for speeding up the comparison process.
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. Mr. Kevin 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 lab measurements to enable the detection of signs of life and the discrimination between these and false biomarkers have measured IR spectra of Nitrogen Heterocycles, the class of compounds found in meteorites that include nucleobases. 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. Ultimately the behavior—and detectability—of such compounds under europan conditions will also be determined.
Dr. Peter Backus, Dr. Jill Tarter, and Dr. Mancinelli just completed the first workshop that examines the prospects that planets orbiting dwarf M stars are habitable for either microscopic or complex life. Thirty scientists from nineteen institutions in the US and UK participated. Thirteen of the participants were from six other NAI Teams. Results of the workshop are reported in a paper submitted to the journal Astrobiology. The results of the first of two workshops suggest that from the data we have 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.