2009 Annual Science Report
University of Wisconsin Reporting | JUL 2008 – AUG 2009
Understanding the signatures and environments of life are the primary goals of the Wisconsin Astrobiology Research Consortium (WARC). In Year 2, WARC pursued 14 research projects and three Education and Public Outreach (EPO) projects, which involved 12 primary investigators, nine research scientists and staff, 10 post-docs, 10 graduate students, and 27 collaborators at other institutions. WARC members have collaborations with seven other NAI teams, including NASA-Ames, the two NASA-JPL teams, Carnegie, Montana State, NASA-Goddard, and Penn State. Research efforts in Year 2 may be broadly grouped into three components: 1) organic compounds, and their planetary inventories, preservation, and interactions with minerals; 2) chemical and isotopic biosignatures in modern and ancient environments; and 3) new frontiers in analytical methods, applied in situ on other planetary bodies or used for analysis of materials of the early Earth or those returned from missions. EPO activities involved a wide range of public venues, from K-12 school activities to a baseball game.
1. Organic compounds
The search for organic compounds on other planetary bodies such as Mars requires development of new methods for detecting such compounds, as well as testing and calibrating instrumentation that will eventually fly to other objects in the solar system. Part of this effort involves assembling a suite of Mars analog samples for use in instrument development. In Year 2, Pasacale Ehrenfreund’s group developed new methods of extracting Polycyclic Aromatic Hydrocarbons (PAHs) from soil samples. PAHs have figured prominently in discussions of possible life on Mars, and may occur in interstellar medium and nebulae. Ehrenfreund has characterized a number of Mars soil analogs from locations that are very dry and acidic (Figure 1). The survivability of organics in these soil analogs has been evaluated through exposure in the laboratory to cold and high-UV radiation, conditions intended to mimic those found on the surface of Mars, and the results were found to vary with soil mineralogy. In addition, Ehrenfreund’s group has subjected PAHs to space conditions on the International Space Station (EXPOSE-R project) to determine the survivability of PAH’s in space; this experiment is in progress and will last for 12-18 months.
The bridge between delivery of complex organics to a planetary body and the origin and early evolution of life depends upon understanding how cellular membranes and associated films developed. Biofilm-forming, extra-cellular polymeric substances (EPS) may have evolved as a protection from high radiation and cell lysis by minerals. Nita Sahai’s group has investigated the possible role of EPS in preventing cell lysis by comparing EPS-lacking mutant microbes with those that produce high levels of EPS, and the results suggest that EPS may have indeed evolved as a shield against toxicity of certain minerals. In addition, Sahai’s group has shown that the adsorption properties of lipids to various mineral oxides is dependent upon the surface charges of the minerals, which in turn has implications for microbe-mineral interactions and production of biominerals that may be preserved in the ancient rock record.
2. Chemical and isotopic biosignatures: laboratory and field studies
Iron is one of the most important redox-sensitive elements in the terrestrial planets of the solar system, reflecting a relatively high abundance due to nucleosynthesis reactions, and the fact that microbes may gain energy through both iron oxidation and reduction. Because iron exists largely as Fe(II) in the interiors of the terrestrial planets, but is found as Fe(III) on the surface of the Earth and Mars, in Year 2 WARC researchers investigated the biological and abiological controls on Fe(II) oxidation of basalt glass. Basaltic lavas formed the most important early crust on Earth, and is the most important lithology exposed on the surface of Mars. Eric Roden’s group demonstrated that an Fe(II)-oxidizing, nitrate-reducing culture may be sustained on basaltic glass, extending earlier work that has shown chemolithotrophic Fe(II) oxidation of clays. Additional studies of Fe(II) oxidation have focused on pyrite (FeS2), the most important combined repository of reduced Fe and S in terrestrial planets. The occurrence of Fe(III) sulfate minerals on Mars indicates that understanding biological and abiological pathways of pyrite oxidation is important for determining the origin of these deposits. Using enriched O isotope tracers, Max Coleman and collaborators have investigated the microbial oxidation of pyrite, which occurs in two steps: the first involves microbial sulfide oxidation, which is energetically most favorable for the Acidithiobacillus ferrooxidans cultures used, where the oxygen sources are largely atmospheric O2; second, sulfate is produced by inorganic oxidation with Fe(III), and the oxygen required for sulfate production comes entirely from water. It is not yet possible to extrapolate these results to conditions of low ambient O2, such as might have existed on Mars or the early Earth, but the results show that both microbial and non-biological iron and sulfide oxidation pathways are characterized by their isotopic compositions.
The large inventory of Fe(III) on the surface of the Earth, and possibly Mars, requires consideration of reductive pathways of microbial iron cycling, such as dissimilatory iron reduction (DIR). In Year 2, Eric Roden, Brian Beard, and Clark Johnson and their groups explored the effects of dissolved silica and variable pH on the Fe isotope fractionations produced by DIR, as a first step toward simulating Precambrian marine conditions, relative to the simple systems used in earlier experiments. Their results demonstrate that the Fe isotope fractionations produced by DIR may be changed under conditions of high pH and dissolved silica, which likely reflect bonding changes on the surfaces of oxides. These shifts were found in parallel abiologic experiments, confirming the hypothesis of bonding changes, rather than a “vital” effect. A test of the efficiency with which DIR may produce significant inventories of isotopically fractionated Fe in nature comes from microbiological and Fe isotope studies by this group of a modern environment that has very high Fe, but low S, fluxes; a gene sequence clone library demonstrated that DIR is responsible for Fe cycling at this site, and Fe isotope measurements of the sediment and pore fluids, as well as results obtained from incubation of the sediment, confirm the anticipated Fe isotope fractionations for DIR.
Carbonate minerals are commonly used as proxies for ancient seawater on Earth, and their importance extends beyond the Earth, as they occur in meteorites and interplanetary dust particles, and are likely present on the surface of Mars. The range of carbonate compositions ((Ca,Mg,Fe)CO3) permitted by equilibrium thermodynamics is relatively restricted, and includes calcite (CaCO3), dolomite-ankerite (CaMg(CO3)2-CaFe(CO3)2), and magnesite-siderite (MgCO3-FeCO3), but no compositions between these solid solutions (e.g., Ca-siderite). Despite their thermodynamic stability, some of these compositions have not been successfully produced in the laboratory by precipitation from super-saturated solutions, most notably dolomite and magnesite, and yet these minerals may be common in the rock record (especially dolomite). A proposed microbial role for carbonate formation comes from laboratory experiments, and it has been hypothesized that microbes may produce local pH and/or saturation conditions that overcome kinetic nucleation barriers, inducing carbonate precipitation. In addition to formation of dolomite, which is thermodynamically stable, microbes are thought to be responsible for producing carbonate compositions that are not predicted by equilibrium thermodynamics, such as Fe-bearing dolomite or Ca-bearing magnesite-siderite. Chris Romanek’s group tested inorganic “free-drift” approaches to carbonate synthesis and successfully produced Ca-siderite and Fe-dolomite, compositions not predicted by equilibrium thermodynamics and previously thought to require microbial catalysis. Huifang Xu’s group studied the effects of sulfide on formation of dolomite, and their initial experimental results indicated that sulfide may play an important role in desolvation of Mg2+ in solution, a necessary first step in dolomite formation. Using computational approaches, Nita Sahai’s group investigated the conditions and ligands necessary for Mg2+ desolvation. The experimental results obtained by these groups to date suggest that a microbial role may not be required for formation of many carbonates, and, if confirmed, establishing biosignatures for carbonates may require an isotopic approach.
Turning to the ancient rock record, a major initiative during Year 2 focused on studies of potential isotopic and mineralogical biosignatures in Neoarchean and Paleoproterozoic (~3 to 2.4 Ga) marine sedimentary rocks from the Transvaal (South Africa) and Hamersley (Australia) basins. This period of time represented a major transition in Earth’s surface environments, leading up to the first major oxygenation of the atmosphere. A puzzling feature of the C isotope record for kerogen (bulk organic matter) in shales of this age is an excursion to very low 13C/12C ratios; this has generally been ascribed to methane cycling, although the exact pathways, and potential connections to other metabolisms, remains unclear. Within this framework of large C isotope, but limited S isotope variations, the large Fe isotope excursion documented by Clark Johnson and Brian Beard and their collaborators is interpreted to reflect development of a complex microbial ecosystem that included anaerobic photosynthesis coupled to Fe(II) oxidation, oxygenic photosynthesis, aerobic methanotrophy, dissimilatory iron reduction, bacterial sulfate reduction, and anaerobic oxidation of methane coupled to sulfate and/or iron reduction. Importantly, the large decrease in 56Fe/54Fe ratios occur before an increase in atmospheric O2 contents because photosynthetic generation of Fe(III) (directly, or indirectly through O2 production) will fuel dissimilatory iron reduction before significant free O2 accumulated in the atmosphere; this model is also predicted by the inferred low seawater sulfate contents (based on S isotopes) during the major Fe isotope excursion.
WARC researchers have made a major effort in studies of banded iron formations (BIFs) of Archean and Proterozoic age (Figure 2), because these rocks record times of extensive iron cycling that is not seen today. Many workers have argued for a role for biology in BIF formation, although clear evidence for such a role has been sparse, given the relatively low organic carbon contents of BIFs and the lack of microfossils in the major oxide-facies of BIFs. Clark Johnson, Brian Beard, John Valley, and Eric Roden and their collaborators have shown, though combined C, O, and Fe isotope studies of BIFs, that a major role from dissimilatory iron reduction is required to explain the isotopic data, suggesting that virtually all of the Fe in the carbonate facies of the 2.5 Ga BIFs of the Hamersley and Transvaal basins may have been cycled by microbes. In addition, Huifang Xu and his team have modeled the banding in BIFs to record chemical self-organization processes, potentially leading to distinctive mineralogical zonations.
3. New frontiers in analytical methods
A major effort was made in Year 2 in developing methods for in situ chemical and isotopic analysis. In situ analyses are required for measurements on other planetary bodies such as Mars, and is also important for understanding the complex histories of the Earth’s oldest samples; in situ analysis will also be critical for samples that are returned from planetary missions, where sample analysis volumes must be as small as possible. WARC researchers are taking three approaches: 1) development of a miniature mass spectrometer (MMS), intended for future rover-based Mars missions; 2) large-radius secondary-ion mass spectrometry (SIMS) for Earth-based measurements, and 3) development of new laser-ablation analytical approaches for rover-based analysis, as well as Earth-based analysis using ultra-fast (femtosecond; 10-15 s) laser pulses.
Important advances were made in Year 2 in development of the MMS by Mahadeva Sinha and his collaborators. The initial plans for sample introduction via reductive gas and furnace treatment were abandoned in favor of laser ablation using neutral species, coupled to electron-impact ionization, an approach that provides far greater signals than a single-step analysis of ions directly from the ablated material. Relative to alternative designs such as inductively-coupled plasma mass spectrometry (ICP-MS), the MMS is designed to use lower power and be more shock resistant. In addition, advantages of the MMS over other mass spectrometry approaches include a solid-state CCD detector and focal-plane design, where all masses are measured simultaneously. Remaining technical challenges include compensation for the influence of minor charged species produced during ablation and optimizing the CCD array to minimize cross-channel talk.
A new area of emphasis by WARC members is applying new analytical methods to Mars geochronology, and in Year 2 this was pursued along two lines. First, Mahadeva Sinha and Brian Beard and their collaborators are developing methods for using in situ Rb-Sr geochronology on Mars, using the MMS instrument that is being developed at JPL. Their initial focus is on jarosite (KFe3(SO4)2(OH)6), given the importance of this mineral in constraining the ancient surface environments of Mars. Terrestrial jarosites vary greatly in Rb/Sr ratios, which affects their suitability for Rb-Sr geochronology. Sinha and Beard have found that low-P bearing jarosites have the highest Rb-Sr ratios, suggesting that P contents may be a good guide to determining which jarosites are suitable for geochronology. Based on ion count rates determined for the MMS, the precision of in situ Rb-Sr geochronology may be as good as ~100 m.y., which would serve as a useful screening tool for sample return missions. The second line of geochronologic studies pursued by WARC is Lu-Hf studies of Mars meteorites. Lu-Hf geochronology has been a relatively little utilized approach, in part because, until recently, analytical hurdles and uncertainty in the decay constant, has prevented widespread application of this system. Important new results obtained include relatively tight constraints on shergottite ages of 179-225 Ma, and, importantly, a new crystallization age for ALH84001 of 4086±30 Ma; this is a significantly younger age than previous estimates of the age based on complex Sm-Nd results, and demonstrates that carbonate formation on Mars may have extended to significantly younger times than previously thought.
The sampling efficiency and high sensitivity of large-radius SIMS analysis make these instruments ideally suited for in situ chemical and isotopic analysis of very small volumes in Earth-based laboratories, and John Valley and his team made major advances in developing the analytical protocols needed for Li, O, S, and Si isotope analysis. Ablated spot sizes of ~10 μm in diameter and 1 μm deep may produce precisions of a few tenths per mil in 7Li/6Li, 18O/16O, 34S/32S, and 30Si/28Si ratios; accuracy, however, may be influenced by sample surface topography, and it is now recognized that, for some minerals, crystal orientation may exert instrumental mass biases of several per mil. Fully understanding these effects, and development of strategies for correction, as well as the necessary development of standards, occupied a majority of the SIMS efforts in Year 2.
Femtosecond laser ablation (fsLA), coupled to multi-collector ICP-MS, was a new approach to in situ isotopic analysis that was explored in Year 2 by Brian Beard and Clark Johnson and their team. The advantages of ultra-fast lasers, over conventional nanosecond lasers, is elimination of differential heating effects of the sample, which produces variable elemental and isotopic mass fractionation during ablation and analysis. WARC researchers demonstrated Fe isotope analysis by fsLA to precisions of 0.15 per mil in 56Fe/54Fe ratios for iron oxides. There are no effects on accuracy of sample topography or crystal orientation by fsLA, although sample chamber design may be important. For many elements, fsLA is likely to be superior to SIMS, although the analysis volumes required for fsLA are at least 10 times larger than those used in SIMS analysis. The development work pursued in Year 2 by WARC researchers demonstrates that no single approach will cover all in situ analysis requirements.
4. Education and public outreach (EPO)
EPO activities in Year 2 targeted a broad range of public demographics. Formal training in astrobiology was led by Kay Ferrari at JPL through the Astrobiology 101 program, which involved Solar System Ambassadors (SSA), Solar System Educators (SSE), and NASA Museum Alliance members. WARC researcher Max Coleman was involved in the instructional program. Additional SSA activities included 11 events that drew 1,092 participants. In Year 2, the SSE program, which involves formal training of master teachers in astrobiology curricula, involved WARC and the JPL Titan and Icy Worlds NAI teams in joint programs. In addition, Kay Ferrari formed NASA Nationwide, a consortia of NASA volunteers, whose roster now exceeds 25,000 members. EPO efforts run from UW-Madison by Brooke Norsted included Elementary School Science Nights, where astrobiology presentations were made to over 500 people at ten Madison elementary schools; this initiative involved UW-Madison undergraduate and graduate students. Finally, in a novel approach to astrobiology EPO activities, Brooke Norsted organized Astrobiology Night at the Ballpark, where 6,252 people from the Madison area attended a Madison Mallard’s baseball game and participated in astrobiology activities.