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Research Investigation 2: Development of biosignatures, paleoenvironmental proxies, and pathway tracers for aqueous-mineral systems
New observations on Mars have greatly expanded the scope of astrobiological research topics. It is clear that much greater emphasis should be placed on evaporite minerals (including sulfates and carbonates), as well as iron oxides. Moreover, the genesis of sulfate, carbonate, and iron oxide minerals likely hold a key component to understanding the evolution of life and surface environments on the early Earth. It is unlikely that there is a single biosignature that will unambiguously define whether extra-terrestrial life processes had been involved in a sample’s formation or subsequent history. Therefore a systems approach must be taken. In combination with understanding of the pathways of formation for these minerals at a mechanistic level, stable isotope compositions offer a way of understanding the paleoenvironments in which they formed, as well as pathways that may be uniquely linked to microbial catalysis.
Investigation 2 is an extensive experimental program in abiologic and biologic mineral synthesis of sulfate, carbonate, and iron oxide minerals using state-of-the-art geochemical and geomicrobiological laboratories at UW Madison, JPL, and the University of Kentucky. The solid products will be studied using the XRD, SEM, and TEM, and high-precision D/H, C, O, S, Cl, Mg, Ca, and Fe isotope and chemical analyses of the fluids and solid phases. Hypotheses will be tested using computational chemical methods, including ab initio molecular orbital calculations and kinetic reaction-transport models. The ultimate goal of Investigation 2 is to provide a rigorous interpretive context for understanding the origin of sulfate, carbonate, and iron oxide minerals that form key components to studies of the origin and evolution of life on Earth, the subject of Investigation 3, as well as the framework that will be required for studies of Mars samples through in situ measurements obtained from future robotic missions or sample return missions, which will be pursued in Investigation 4.
Our objectives in Investigation 2 are to:
- Determine the influence of microbial catalysis on the extent and rates of pyrite oxidation and attendant isotopic fractionations of O, S, and Fe, including comparison of abiotic and biologic systems, as well as the influence of O2 and Fe3+ as the oxidant;
- Study the equilibrium and kinetic O, S, Ca, Mg, and Fe isotope fractionations among the major sulfate mineral groups as possible proxies for fluid sources, formation pathways, and paleoenvironmental conditions such as humidity and temperature, as well as the degree to which isotopic and chemical compositions are retained during post-formation diagenesis;
- Investigate the mechanisms by which significant Ca-substitution occurs in Fe-Mg carbonates, which is not predicted by equilibrium thermodynamics but which appear to form during biologically-mediated carbonate precipitation;
- Study Ca, Mg, Fe, C, and O isotope fractionations in the CaCO3-MgCO3-FeCO3 system, with particular emphasis on compositions produced under biotic vs. abiotic conditions;
- Determine the O and Fe isotope fractionations that occur during abiologic oxidation of Fe2+, under equilibrium and kinetic conditions;
- Investigate the mineralogical and isotopic composition of Fe sulfate and oxide minerals formed during low-pH Fe2+ oxidation as a model for reactive groundwater systems on Mars.
The significance of these mineral groups to Astrobiology is several fold:
First, sulfate minerals tend to be widely associated with evaporative processes and hence are excellent markers of past wet conditions, including the Eh and pH of the water from which they precipitated. For example, some ferric iron sulfate minerals, such as jarosite, require formation in a low pH environment. The Mars exploration rover Opportunity has documented that sulfate minerals are a volumetrically major mineral at the Meridiani Planum landing site, where ~40 wt. % of the sedimentary rocks are sulfates. These sulfate minerals are primarily calcium and magnesium sulfates. The ferric iron sulfate jarosite has also been identified based on Mössbauer spectra. In addition, sulfate minerals are common weathering products on several of the SNC (Mars) meteorite samples, suggesting that sulfate minerals are a ubiquitous feature on Mars. Finally, generation of significant sulfate concentrations in the terrestrial Archean oceans places important constraints on the timing of oxygenation of Earth’s atmosphere.
Second, carbonate minerals are inferred from thermal emission spectra (TES) of Martian dust, suggesting ~2-3% of the Mg carbonate magnesite. Considering the very large igneous or Fe oxide surface exposures, this suggests that a significant inventory of carbonate rocks may yet be discovered on the surface of Mars. Because the past Martian atmosphere is inferred to have had higher CO2 pressures, carbonate minerals are one of the most likely repositories of the “missing” CO2 budget of Mars. In addition to TES data, some SNC meteorites contain carbonate minerals. Orthopyroxenite SNC meteorite ALH84001 contains globules and pancakes of compositionally (Ca, Fe, and Mg) variable carbonate. Finally, carbonates are ubiquitous in the Archean terrestrial record, including shales and banded iron formations.
Third, iron oxide is a ubiquitous phase on Mars, and, of course, is the source of the description “the red planet”. Large regions of the Martian surface contain significant quantities of coarse-grained gray hematite as identified by TES spectra obtained by the Mars Global Surveyor. The presence of this gray hematite has been confirmed by the Mars rover Opportunity, which has identified 5-6 mm sized concretions, so-called “blueberries”, to be almost entirely composed of hematite, 2004; Squyres et al., 2004a), strongly suggesting precipitation from aqueous solutions. In addition, iron oxides such as ferrihydrite and hematite occur in many of the SNC meteorite group meteorites. Because the major inventory of ferric Fe on Mars appears to lie in iron oxide, determining the mineralogical and isotopic signatures of various oxidation pathways is critical to understanding the origin of these deposits, or those that are found in the earliest Archean sedimentary deposits on Earth.
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