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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 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).

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. She has begun to map chemical sequences for anions, neutrals and cationic nitrogenated aromatic molecules in Titan’s organic haze layer. 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. We have also begun a laboratory study of hydrogen molecule synthesis on aromatics and aerosols to see if the suggested theoretical pathway is a plausible mechanism to the accelerated oxidation of Titan and the early Earth. This foundation enables the next stage of her research, the theoretical building of prebiotic macromolecules from the haze constituents. This work is complemented by ongoing laboratory work performed by Dr. Bishun Khare and Dr. Hiroshi Imanaka on Titan analog materials, and infrared observations of the Titan haze.

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. Based on the spectral properties of iron oxides and the results of experiments with photosynthetic organisms, Euglena and Chlamydomonas, 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. Such niches may have also existed on Mars.

They are preparing to evaluate 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. Concurrent with other projects, we are evaluating the spectral properties of Fe-bearing Mars analog sites on earth and analyzing spectra of Mars for Fe oxide-bearing components. These results are consistent with the hypothesis that early photosynthetic organisms may have existed in specific, perhaps small, niches protected by ferric oxide-bearing material. (See Figures 5, 6 & 7)

The survival of microorganisms in very high UV environments can also be tested empirically through the exploration of Earth’s highest altitude lakes and ponds, in Bolivia and Chile. Dr. Nathalie Cabrol and Dr. Edmond Grin (both of whom also this past year served on the Mars Exploration Rover team) have led a series of investigations of these lakes to examine the strategies employed by these microorganisms. This past year they have charcterized the PAR and UV flux, the geology, some microbial mat organisms and the geology and paleobiology of Laguna Blanca and Verde. They have also charcterized the PAR and UV flux of the summit lake of Licancabur as well as developing the first bathymetry map of the lake. The UV flux at those lakes is between 200 and 216% that of sea level and the level of UVB recorded is similar or exceeds that of current UVB on Mars at the equator. (See Figures 1, 2, 3 & 4)

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 nearly 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 begun a series of experiments in which they are searching for the nitrite reductase gene, a gene critical in denitrification.

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. 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. They are also completing a major project, using Galileo high-resolution imaging of Europa, to quantify the impact cratering “gardening” rate on Europa. This is important in its own right as a fundamental planetary process, but also is important in some astrobiological models because it will allow the quantification of the amount of biologically relevant material, created at Europa’s surface through radiogenic processes, that can be mixed down to the gardening depth, and thereby escape radiolysis.

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. Over the past year, Mr. Hand and Dr. Carlson have constructed the irradiation apparatus (including ice deposition chamber and diagnostics) and have preliminary experiments now underway. 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. The SETI Institute NAI team’s proposal calls for a series of workshops to bring together planetary scientists, biologists, atmospheric modelers, astrophysicists, and SETI scientists to address these issues. They are currently writing up the results of the workshop. In summary nothing about what we currently know about M-dwarfs precludes the possibility of life from originating and evolving on a terrestrial planet orbiting within the habitable zone of an M-dwarf. The results of this project will ultimately help the next generation scientific Search for Extraterrestrial Intelligence (SETI) choose the million target stars that it will survey for signs of technical civilizations using the new Allen Telescope Array (ATA), being built by the SETI Institute in partnership with the University of California, Berkeley.

Finally, during the past year Chyba and Hand completed an invited review article for Annual Reviews of Astronomy and Astrophysics titled “Astrobiology: The Study of the Living Universe.” The piece makes mention of the NAI both in the acknowledgments and in the first paragraph of the article itself; it surveys current controversial issues across the span of astrobiology and should serve to introduce many astronomers and physicists that are not (yet) astrobiologists to key ideas and controversies in the field.

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

Figure 1. Licancabur Summit Lake, 5,916 m. Colony of copepods and ostracods evolving in the lake water column between 0.5 to 4.5 m depth.

Figure 2. A. Rock Kerogen from stromatolite sample: Demineralized residue slide showing Pigmented filaments. Filaments are 5.5 µm wide on average; B-E: Pollens: B. Ambrosia artemisifolia, 20 µm; C. Chenopodium / Arnaranth, 25 µm; D. Botryococcus cf braunii, 40 µm; E: Monoporate fungai spore, 16 µm; F: Diatom: Navicula radiosa; G-H: Stromatolite thin sections. G: Cyanobacterium of the genus Oscillatoria, and H: Filamentous cyanobacteria.

Figure 3. Laguna Verde Unit 6a. Fragment of basaltic andesite rimmed by carbonate in agglomerate. Width of field: 2.3mm. Cross Nicols.

Figure 4. Summit Lake bathymetry. Typical cross section of the lake acquired by the RC boat showing the underwater topography. A 3-D map rendering is being prepared. It helps the team visualize the location of habitats and specific microbial colonies

Figure 5. This shows on image of the experiment media in a 96 well spectrometer plate. Two replicates are shown for each day of five mineral suspensions with each of twoorganisms. Initially the cultures are green due to chlorophyll pigmentation and/or red due to the iron oxide minerals. As time progressed with the experiment and the organisms died due to UV exposure, the green color faded.

Figure 6. Images of live (clear) and dead (colored) Chlamydonomonas cells following staining with Trypan blue (magnified 67X).

Figure 7. Images of live (clear) and dead (colored) Euglena cells following staining with Trypan blue (magnified 21X).