2008 Annual Science Report
SETI Institute Reporting | JUL 2007 – JUN 2008
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 most of the Roadmap Objectives that are organized under the seven broader Roadmap Goals; Goal 1: Habitable Planets; Goal 2: Life in our Solar System; Goal 3: Origins of Life; Goal 4: Earth’s Early Biosphere and its Environment ; Goal 5: Evolution, Environment, and Limits of Life ; Goal 6: Life’s Future on Earth and Beyond; Goal 7: Signatures of Life. 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. Through 2007 they continued to measure the electric currents flowing out of stressed rocks in order to gain a handle on the total amount of peroxy oxygen in a given rock. So far they have confirmed that the concentrations of peroxy oxygen in igneous rocks, in particular in gabbro and anorthosite, are higher than previously thought, but quantitative data are not yet available. In early 2008, they procured a dissolved oxygen (DO) meter and a pH meter to continuously measure the formation of hydrogen peroxide at the rock-water interface. They have set up the two meters and began operation, however they encountered a problem because the DO meter consumes dissolved oxygen during measurements. To correct for it, they are currently trying to find a way to independently determine the consumption rate.
In a second investigation, oxidation driven by atmospheric hydrocarbon (and, more broadly, organic) polymerization is being investigated by Dr. Alessandra 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. In collaboration with Prof. M. Samy El-Shall at Virginia Commonwealth University, she has demonstrated that acetylene undergoes sequential additions onto the benzene radical cation with two different mechanisms operating at low and high temperature. These findings illustrate that complex organics can form under a wide range of conditions that can be found in outer space environments.
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. Recently the bright salty soils found at Paso Robles and other sites in Gusev crater on Mars showed that this material is composed of the ferric minerals ferricopiapite, fibroferrite and/or ferristrunzite. Although these sulfates may imply the presence of brines too salty for many microbes, the UV-VIS properties of these ferric minerals could have provided solar protection for microbes able to withstand the salty conditions. In addition, analysis of MRO/CRISM hyperspectral VNIR images of Mars showed the presence of a large phyllosilicate outcrop at Mawrth Vallis, one of the potential landing sites for future missions. The most abundant clay phase found here is an Fe/Mg-smectite. The depth and breadth of this clay deposit suggests long-standing water on Mars. The iron-bearing clay also absorbs some of the UV-VIS solar radiation and could have provided solar protection to any microbes present.
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 retrieved data from the Eldonet UV station and positioned a new short UV wavelength dosimeter at the summit of the Simba (volcano). The team also explored new investigation sites, including Laguna Lejia and Laguna Aguas Calientes.
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. Alessandra 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. Denitrification tests were negative, including those from the wet test, suggesting that low water activity alone cannot account for the lack of denitrification in this system. However, when organic matter was mixed with the soil and incubated nitrite was produced. This suggests that denitrifiers are present in the soil, but the lack of organics (a potential electron donor) may account for the lack of denitrication in the system. Additionally, it is known that O2 inhibits denitrification and raises the cell’s redox level. Following from this we hypothesize that the high soil redox potential is also responsible for the lack of denitrification.
Dr. Summers and Dr. Khare have completed the first experiments to demonstrate abiotic nitrogen fixation. Work this year focused on the reduction of NO to ammonia by FeS minerals in aqueous solution. This represents a third nitrogen fixation pathway in addition to the ones identified in previous work. Product yields of ~50% for ammonia were measured. The yield of ammonia formation appears to peak at pH 7 and falls of into acidic or basic conditions. Currently, we are using isotope labeling to identify a gaseous product observed with peaks at 2235, 2100, 1300, and 1270 cm-1 in the IR. We have conducted isotope labeling experiments with 2D2O, H218O, and N18O. The gaseous compound(s) appear to be CNO, (NCO)2, or some related species (a cyanato or isocyanato species). HCNO, HNCO, HOCN, and HONC have been ruled out. The gaseous product is formed in yields that are unaffected by the pH, but which increase as the amount of FeS is increased. We are currently conducting 13C isotope labeling experiments and looking to the 14N isotope fractions of all the reactions we have been studying. 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 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. They 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 they 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. In addition, a related project to study impact gardening on Europa and its potential for surface — subsurface transport of potential biogenic materials has reached some interesting conclusions. The gardening depth on Europa is thinner than we thought, probably only about a centimeter. This means that sputtering will tend to win out over gardening, and that less material can be mixed down to Europa’s subsurface beneath the radiation processing layer to be preserved.
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. 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. They are applying the results of the workshop as they compile a list of “target” stars for SETI on the Allen Telescope Array. Under their supervision, a student in the Research Experience for Undergraduates program (sponsored by the NAF and the NAI) is sifting through a catalogue of more than a billion stars. They will continue this work and refine the derived target list based on research published subsequent to the workshop. We will publish this new catalog of roughly a million “Habstars”, stars suitable to host habitable planets, extending current work.
Education and public outreach are major and integral parts of the work of the SETI Institute’s NAI team. Among the several accomplishments are those from the “Are We Alone?” weekly science radio show. The improvement in show quality, both technical and in terms of presentation and content, continues apace. Some of the important changes in the past year include hiring a part-time assistant producer. Another recent improvement has been to broaden the scope of the show by having an expanded web presence, as well as a blog dedicated to the program. The show is also represented on Facebook, My Space, Digg and other social networking sites. Participants in Second Life can also listen to the show in a specially constructed setting.