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Exoplanet Exploration

Application of new paleomagnetic and rock magnetism techniques to test the origin of magnetites in ALH84001 carbonates

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Application of new paleomagnetic and rock magnetism techniques to test the origin of magnetites in ALH84001 carbonates
Previous analyses of the ~4 Ga Martian meteorite ALH84001 have revealed magnetite in carbonate 'blebs' strikingly similar to those made by magnetotactic bacteria on Earth (McKay et al., 1996). Additionally, clumped-isotope studies have shown the carbonates precipitated at temperatures < 20°C (Halevy et al., 2011), consistent with paleomagnetic analyses indicating that the interior of the meteorite was not heated to temperatures exceeding 40°C (Weiss et al., 2000). Great debate has raged concerning the possible preserved biological magnetites trapped in the carbonate blebs along fracture surfaces in this meteorite. Two leading hypotheses exist to explain the magnetite crystals associated with the carbonate blebs: formation through alteration of the carbonate and pyroxene in the bulk rock (Treiman and Essene, 2011) and deposition of mature magnetite in an aqueous solution (Thomas-Keprta et al., 2009). The latter hypothesis is consistent with a biogenic origin for some of the magnetite crystals. The two distinct scenarios would lead to dramatically different magnetization properties, which can be measured and characterized in the laboratory to discriminate between the hypotheses. The former leads to intense relative magnetizations, while the latter leads to much weaker remanence. A robust analysis based on this difference, which distinguishes between chemical and depositional magnetization, is the Fuller et al. (1988) test of natural remanent magnetization. Similarly, the susceptibility of anhysteretic remanence (ARM) directly measures the microscopic clumping (nearest-neighbor) arrangement of single-domain magnetites, via the Cisowski (1981) ARM acquisition test. Detrital magnetites should clump together magnetostatically during transport (e.g., Kobayashi et al., 2006), whereas exsolved magnetites should remain spatially separate. Thus far, the small size and associated magnetization of the blebs have precluded any paleomagnetic or rock-magnetic studies on the carbonates themselves; however, the new ultra-high resolution scanning superconducting magnetic microscopes have increased our measurement sensitivity 4 orders of magnitude over conventional superconducting magnetometers. With this instrument, we have demonstrated recently the ability to measure quantitatively the magnetic moments associated with 50-µm sized ALH84001carbonate fragments. Using these new measurements in conjunction with well established paleomagnetic laboratory demagnetization techniques it is now possible to conduct the critical tests to resolve the origin of magnetization in the carbonates. Through demagnetization and careful determination of the magnetic structures we may also be able to estimate the strength of the magnetic field present during carbonate precipitation (or magnetite exsolution event). The strength of the paleofield is an indication of the ability of Mars to maintain a dense atmosphere enabling habitable conditions. Two main goals exist for the proposed work: determine if the magnetization in the carbonate blebs is chemical or detrital, and determine the field strength on the Martian surface at the time of carbonate formation, as distinct from the cooling of the orthopyroxene. These goals are directly relevant to two of the research emphases in the exobiology solicitation: biosignatures and life elsewhere and early evolution of life and the biosphere. Terrestrial magnetites formed by magnetotactic bacteria have revealed much information about past environments and the direction and strength of the Earth's magnetic field. The presence of biogenic magnetites would also give insight to the redox stratification in the environment in which they were formed. Should the magnetites in the ALH84001 meteorite be biogenic, they would have existed long before the earliest known magnetotactic bacteria on Earth, from 1.8 Ga (Chang et al., 1989), and would predate by several hundred million years the first hints of life on Earth. More »

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