2011 Annual Science Report
University of Wisconsin Reporting | SEP 2010 – AUG 2011
The focus of the WARC Team is on the signatures and environments of life, and in Year 4, the team significantly expanded its research and Education and Public Outreach (EPO) efforts into new directions within this framework. Twenty-six research projects were pursued in Year 4, including 15 continuing projects and 11 new initiatives. EPO efforts in Year 4 also expanded, and ten major projects and programs were run through NASA-JPL and the University of Wisconsin.
Research Topic 1: Early surface conditions on Mars and Earth – Implications for Life
Four entirely new research projects for WARC were pursued in Year 4 that were aimed at determining the environments that existed on Mars and Earth between ~4.0 and 3.5 Ga, as well as the role that core formation has in providing conditions favorable to supporting life. WARC research in the previous year provided the first rigorous igneous crystallization age for Mars meteorite ALH84001 by Lu-Hf geochronology, and in Year 4, Brian Beard’s group applied the Rb-Sr isotope system to studying the carbonates contained in ALH84001. Beard found that the meteorite experienced a shock event at 3951±22 Ma, significantly younger than the igneous crystallization age of 4091±30 Ma determined by Lu-Hf, and, based on petrographic relations, the shock age is interpreted to be the carbonate formation age. Importantly, the initial 87Sr/86Sr ratio for the carbonates and associated minerals is very high, requiring a surface environment high in Rb/Sr, most likely reflecting clays on the surface of Mars at ~4 Ga. Turning to early Archean terrestrial rocks, Clark Johnson led two studies in Year 4 that combined U-Th-Pb geochronology with Fe isotope measurements on the 3.4 Ga Apex Basalt and Marble Bar Chert from the Pilbara craton, Australia. It has been previously hypothesized that ferric iron minerals in these units reflect oxidation by free oxygen at ~3.4 Ga, which, if true, should have been accompanied by an ancient enrichment in U, given the high solubility of U(VI). By coupling the 238U-206Pb and 235U-207Pb isotope systems, and referencing to the immobile and non-redox-sensitive 232Th-208Pb isotope systems, Johnson and colleagues showed that the 3.4 Ga rocks from the Pilbara craton were not associated with ancient U enrichment and hence are unlikely to record oxygen-rich early Archean conditions. Moreover, Fe isotope analyses of the Marble Bar jaspers showed the highest δ56Fe values yet measured on Earth, indicating oxidation under conditions of very limited oxidant, supporting the interpretation that no free oxygen existed on Earth at this time; the data are instead interpreted to reflect oxidation by anoxygenic phototrophs, a metabolism that is deeply rooted and therefore likely to have occurred early in Earth’s history.
Finally, Max Coleman led a study in Year 4 that investigated the effect of core evolution on Earth’s magnetic field, which bears on the extent of magnetic shielding of cosmic radiation, which in turn has implications for the evolution of life. Coleman’s group hypothesized that extensive magnetic shielding likely occurred only after turbulent flow in the liquid outer core subsided, which would have been after 1 or 2 b.y. of inner core solidification that allowed differential rotation of the inner and outer core. These results suggest that life which evolved in the first 1 to 2 b.y. of Earth history would have had to develop strategies to cope with high cosmic radiation.
Research Topic 2: Organic compounds in the terrestrial planets – Inventories, evolution, and survivability
Organic compounds are, of course, the building blocks of life, and WARC research in Year 4 on organic material covered a broad range of topics. Five projects were pursued, including one new project aimed at in situ C isotope analysis of microfossils. Nita Sahai led a continuing investigation of the delivery of pre-biotic amino acids to the early Earth based on studies of carbonaceous chondrites, with a focus on factors that may determine chirality, based on observations that amino acids in some meteorites exhibit significant asymmetry in chirality. Observations from meteorites were compared with mineral absorption studies, and no chiral preference was shown during sorption, indicating that the chiral asymmetry observed in some meteorites cannot reflect condensation or sorption processes. In a second continuing project led by Sahai, the role of mineral surfaces in stabilizing lipid membranes as possible “proto cells” was investigated. Sahai’s group found that lipid membrane stability in the presence of minerals is highly variable, dependent upon surface charge, solution chemistry, and mineral particle size. This on-going work hopes to constrain the conditions required to promote development of “proto cells”, which in turn can inform models for the early evolution of life. A third project led by Sahai continued studies of the role of extra-cellular polymeric substances (EPS) as a possible strategy for early life in dealing with toxic mineral surfaces. The results to date provide evidence that EPS, and biofilms in general, may have evolved as a means for protecting microbial communities from toxic effects of mineral substrates, in addition to the traditional interpretation that such substances developed to provide physical stabilization of microbial communities on rocky substrates. In Year 4, John Valley’s team began a new project on carbon isotope analysis by SIMS (Secondary Ion Mass Spectrometry), initially concentrating on methods and standard development, a critical aspect given the large ranges in carbon abundances in rocks. Valley’s group demonstrated that high-precision C isotope data can be obtained on sub-components of cells, and showed that fossil cell walls and interiors may have distinct isotopic compositions in samples of 2.7 to 3.4 Ga age. The fifth component of research related to organic compounds in Year 4 was a continuation of the EXPOSE-R studies led by Pascale Ehrenfreund, where samples were exposed to space radiation on the International Space Station (ISS). Samples were returned to Earth in Year 4 of the grant, providing a total exposure to space radiation that greatly exceeded that previously obtained in laboratory experiments. Full analysis of these experiments will occur in Year 5 of the grant and will provide a rigorous evaluation of the effects of space exposure of organic compounds.
Research Topic 3: The transition to an oxygenated Earth
Three research projects pursued in Year 4 targeted the biogeochemical changes in the rock record that mark the transition to an oxygenated Earth. Valley’s group continued two studies that involved SIMS stable isotope analysis, one focussed on O and Si isotopes in 2.5 Ga (and older) Banded Iron Formations (BIFs), and a second aimed at multiple S isotopes in 2.4 Ga rocks that immediately preceded the Great Oxidation Event (GOE). SIMS analysis demonstrated extremely large ranges in O isotope compositions, particularly for oxide minerals, that likely reflect unique formation pathways and hydrothermal histories; such information would not be attainable by conventional bulk isotopic analyses, and demonstrate that near-primary isotopic compositions may be recovered by careful SIMS analysis. Silicon isotopes were shown to be relatively insensitive to the effects of metamorphism, demonstrating promise for distinguishing continental, hydrothermal, and biological pathways involved in Si cycling in BIFs. Turning to multiple S isotopes in rocks deposited immediately before the GOE, Valley’s team documented an extremely large range in mass-dependent S isotope variations on the microscale, indicating that dissimilatory sulfate reduction was active at this time. These results contrast with the wide range in mass-independent S isotope variations found in the same analyses, which suggests that overall atmospheric oxygen contents remained low. These rocks, therefore, probably capture a condition at 2.4 Ga where significant seawater sulfate existed while atmospheric oxygen contents remained low. Finally, Clark Johnson’s group pursued an integrated isotopic study of the Neoarchean Campbellrand carbonate platform, South Africa, continuing their work from the previous year, where new datasets for C, O, Fe, and Sr isotopes were integrated with existing data for S and Mo isotopes. These data provide a detailed view of the paleoenvironmental and paleoecological conditions of the Neoarchean, including which types of samples record ambient marine conditions, and which are dominated by microbial cycling after deposition. Collectively, the dataset provides strong evidence for free oxygen in the photic zone of the oceans between 2.7 and 2.5 Ga, generated by oxygenic photosynthesis, but little free oxygen in the atmosphere. Moreover, the results demonstrate that, in general, BIFs are dominated by post-depositional microbial Fe cycling, and few of the isotopic compositions of BIFs can be traced to ambient marine conditions. In contrast, shallow-water Ca-Mg carbonates, unless altered, faithfully record ambient marine conditions in the photic zone.
Research Topic 4: Life detection in the rock record I – Iron oxides
Eric Roden’s team led three investigations aimed at understanding the biosignatures that may be left in the rock record by microbial Fe cycling, including one project that is new for Year 4. The first project investigated the ability of Fe(II)-oxidizing, chemolithotrophic microorganisms to live on Fe(II)-bearing igneous minerals, as well as clays. Roden’s group found that a wide variety of Proteobacteria are capable of catalyzing solid-phase Fe(II), including many species not previously known to have this capability. These results bear on the possibility that early life on Mars, when Fe(II)-clays may have been abundant, could have used this metabolic pathway. A second project investigated stable iron isotope fractionations in abiological and biological systems involving Si-bearing ferrihydrite and aqueous Fe(II). This system was chosen because the Precambrian oceans were likely saturated in silica, and the Archean oceans likely had significant quantities of Fe(II). These experiments showed that Fe isotope exchange is enhanced in biological systems, likely reflecting the close proximity of aqueous Fe(II) that is produced by in situ reduction of Fe(III), in contrast to simple juxtaposition of Fe(II) and Fe(III) hydroxides in abiologic systems. A new project for Year 4 involved a field study where biogenic magnetite forms via dissimilatory iron reduction. At the field site, iron reduction runs essentially to completion, producing little Fe isotope variability in the samples. This contrasts with the large Fe isotope variations measured at other field sites where iron reduction is incomplete, providing an interpretive framework for studies of the ancient rock record in terms of fine-scale Fe isotope variability.
Research Topic 5: Life detection in the rock record II – carbonates
A major focus of Year 4 research on biosignatures involved carbonate minerals, and six projects were pursued in Year 4, three of which were new. Chris Romanek’s group continued their extensive series of abiologic synthesis experiments using free-drift and chemostat approaches, where calcite seed crystals were used to nucleate growth of Mg-bearing carbonates. Romanek’s team was able to produce a wide range of carbonate compositions through changes in experimental conditions, and they demonstrated that the use of seed crystals, which reduce nucleation energy barriers to carbonate precipitation, are an effective means for growing carbonate under near-equilibrium conditions. The experiments done in Romanek’s lab were used for stable Mg isotope analysis, a second project, new for Year 4, led by Brian Beard. Beard’s team found that the stable Mg isotope compositions of Mg-calcite were independent of carbonate composition, and that the fractionation between Mg in solution and calcite was mildly dependent on temperature. The isotopic fractionations appear to follow an equilibrium relation, providing the first rigorously determined stable Mg isotope fractionation factors for carbonates. These results demonstrate the promise of stable Mg isotope geochemistry for determining the origin of carbonates, as well as the Mg isotope composition of seawater, which may eventually find use as a proxy for continental weathering. Huifang Xu led a new experimental study that tested the hypothesis that sulfide catalyzes dewatering of Mg in solution, therefore promoting dolomite formation. Xu’s team found that even small amounts of sulfide promoted dolomite formation. In contrast, Nita Sahai’s group found, via computational approaches, that aqueous sulfide is an unlikely mechanism for Mg dehydration, in apparent conflict with the experimental results. A possible reconciliation of the new computational results and experiments may lie in the effect of sulfide at the surface of an amorphous carbonate precursor, and this will be pursued in future work. Xu additionally pursued two studies aimed at understanding the possible role of biological ligands in promoting dolomite precipitation. The first study was a continuation of work in the previous year that investigated production of disordered dolomite in experiments that used agar gel, as well as halophilic bacteria, which, upon annealing at elevated temperature, converted to true dolomite. The second study, new for Year 4, produced protodolomite using extracellular polysaccharides, which are excreted by many microorganisms. Collectively, the two studies suggest that biological ligands can indeed catalyze dolomite formation, suggesting that the presence of dolomite in the rock record may, by itself, implicate life.
Research Topic 6: Mars analog environments
Research on Mars analog environments continued to be an important mission-related component of work in Year 4, and two projects were pursued. Pascale Ehrenfreund continued research on Mars analog field sites, including the Mars Desert Research Station (MDRS) in Utah. A major finding of this work was that “life detection” approaches may yield dramatically different results depending upon the instrumentation and methodology used. For example, on-site Polymerase Chain Reaction (PCR) identified several microbial domains at MDRS, but amino acid and polycyclic aromatic hydrocarbon (PAH) measurements were highly variable. Soil samples had a great diversity of mineralogy, including highly reactive clays and oxides, suggesting that sorption of PAH and amino acids may inhibit traditional approaches to life detection, masking the presence of microbial populations that PCR indicates are clearly present. Max Coleman led a second project, a continuation of work on Rio Tinto, Spain, a locality where oxidative weathering of pyrite produces highly acidic waters and a wide variety of ferric oxides and sulfates, analogous to ancient Mars environments inferred by the Mars Exploration Rovers. A key issue in understanding the biogeochemical pathways involved in pyrite oxidation is the source of oxygen in sulfate, be it from water or the atmosphere, and Coleman’s team showed that measurement of three stable isotopes of oxygen (16O, 17O, and 18O) have the potential for discriminating between biological and abiological pathways, as well as the source of oxygen in sulfate.
Research Topic 7: New technologies for astrobiology
Three projects were pursued in Year 4 that were focussed on developing new technologies for astrobiology, ranging from new ways to explore the surface of Mars to establishing the micro-analytical methods that will be required for sample return missions from Mars. Max Coleman’s group led a new initiative that explored the use of Tumbleweed rovers for exploring the surface of Mars. These vehicles are designed to migrate randomly over the Mars surface, propelled by transient wind, therefore using minimal power. In addition, Tumbleweed rovers may be built for significantly less cost than traditional rovers, allowing large areas of the landscape to be covered. Mahadeva Sinha continued work on developing a Miniature Mass Spectrometer (MMS) for deployment on Mars, which would be capable of chemical and isotopic analysis via laser ablation. The unique capabilities of the MMS include a 100 % duty cycle, and high sensitivity due to the use of neutral species for analysis via electron-impact ionization. A new focus for the development of the MMS in Year 4 was K-Ar geochronology, which should have widespread applications on Mars as a means for screening samples for sample return missions. A third project was directed by John Valley, which continued development work for high-precision stable isotope measurements using Secondary Ion Mass Spectrometry (SIMS). Work continued on O and S isotope analysis, where Valley’s team has developed strategies for dealing with crystal orientation effects that are now known to affect SIMS analysis for certain minerals. In addition, in Year 4 Valley’s group began work on the protocols required for SIMS analysis of carbon isotopes in organic material, including individual microfossils, which presents very significant challenges.
Education and Public Outreach (EPO) activities
The EPO portfolio was significantly expanded in Year 4 with ten projects and programs, divided between NASA-JPL and the University of Wisconsin – Madison. Kay Ferrari (JPL) continued her work with the Solar System Educator (SSE) and Solar System Ambassador (SSA) events, which included an online SSE astrobiology training program through a nationwide website, SSE workshops that reached over 1,500 teachers, and SSA events that reached over 300,000 people. In addition, a SSE workshop was held at the Astronomical Society of the Pacific Conference in Year 4. Additional EPO activities centered at JPL included Space Day for more than 250 K-8 students in the Los Angeles area, and participation in an undergraduate summer internship program at JPL, in collaboration with Max Coleman. Brooke Norsted (UW-Madison) continued work on the Imagine Mars program, expanding into new schools in Year 4. In addition, a new initiative, “Astrobiology in Your Backyard” was started in Year 4, which was brought to three summer music festivals. Other UW-Madison EPO activities included publication of the NASA-sponsored “Life in the Extreme Trading Cards”, and completion of the first phase of a permanent astrobiology exhibit in the Geology Museum.