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Isotopic tools in the search for life on Mars 11/29/12 25km 25km Pack ice in the Wedell Sea, Antarctica Evidence for ice on Mars from ESA Mars Express Camera (top) and the Viking lander Murray et al., 2005 ALH84001 – Life on Mars? found on Dec 27, 1984 by NSF Antarctic meteorite program; shelved until 1993, when somebody realized it came from Mars How do we know ALH84001 originated from Mars? step-heating SNC meteorites - measure gas isotopes in tiny glass vesicles - N, Ar, and Xe isotope ratios match Mars atmosphere values, as observed during Viking expedition Marti et al., 1995 What is “normal” here? How old is ALH84001? ALH84001 Nyquist et al., 1995; 2001 (review) How should we interpret this data? What are the key events in ALH84001’s geologic history? Ar-Ar step-wise heating gives an age of ~3.90 ± 0.9Ga for a major impact event…. conventional analysis Plus, during ejection ALH84001 experienced extremely high pressures and temperatures (300-400ºC; 35-40 GPa) Why are these events so crucial to the interpretation of ALH84001 as evidence of life on Mars? laser probe Turner et al., 2003 When was ALH84001 ejected from Mars? Cosmogegnic exposure dating: need to model ingrowth of cosmogenic isotopes through time using bulk chemistry 3He, 21Ne, and 38Ar measurements all give similar age range: 12 to 16Ma Eugster et al., 1997b How long has ALH84001 been on Earth? step-wise combustion of meteorite -organics come off at low T (low d13C, high 14C!) -carbonates combusted at high T (high d13C, low 14C) bottom line: Most organics in ALH84001 likely represent recent contamination (5ka to 8ka); after a fall to Earth at ~13ka Jull et al., 1998 How old are the carbonates globules that are associated with the evidence of ancient Martian life? •carbonates represent <1% of ALH84001, must be separated from silicates by chemical leaching circles = leachates from Borg et al., 1999 triangles = whole rock analyses (plagioclase, pyroxene, etc) dashed line = 4.5Ga crystallization age solid line = best fit to carbonate leachate data Borg et al., 1999 Pb-Pb isochrons for ALH84001 carbonates So Rb-Sr and Pb-Pb dating are near-concordant but what kinds of things do we have to worry about when interpreting this data? Could this data be described as a mixing line between terrestrial Pb and an enriched Pb source in ALH84001? Borg et al., 1999 More images of ALH84001 carbonates 1.5mm A freshly cut chip of ALH84001, showing orange carbonates embedded in orthopyroxene 0.1mm A back-scattered electron image of a carbonate globule in ALH84001 Valley et al., 1997 Romanek et al., 1994 MARS carbonate globules EARTH carbonate globules from Spitsbergen Island, Norway precipitated from hydrothermal fluids derived from volcanics but earliest terrestrial life probably formed around hydrothermal vents…. Thiemans et al., 2002 Early stable isotope work on ALH84001 - oxygen isotopic compositions of ALH84001 carbonates indicate precipitation temperatures of 0 to 80ºC, using open system model and making a variety of assumptions - host rock d18O = 4.6‰, so rock not subjected to high T, which would have equilibrated oxygen isotopes between the different mineral phases. - carbon isotopes agree with d13C of Martian atmosphere, as measured in gas trapped in a Martian meteorite. Romanek et al., 1994 * d13C relative to PDB, d18O relative to SMOW Taken together, results suggest a warm, wet planet at 4.5Ga BUT could also be explained by high-T metasomatism (Harvey et al., 1996) Terrestrial and Martian oxygen isotopes marine carbonates lake carbonates these low terrestrial values form from depleted waters at high T Saxton et al., 1998 hydrothermal deposits More stable isotope work on ALH84001 -isotopic data inconsistent with high-T precipitation of carbonates -favor T between 150ºC and 300ºC -also point out that many terrestrial processes produce isotopic values out of equilibrium at low T Valley et al., 1997 More oxygen isotope work on ALH84001 The lightest carbonates require a fluid with a d18O < 0‰ SMOW Using this water to precipitate all carbonates yields a range of T from 70 to 300ºC … probably different fluid compositions at different temperatures…. … not precipitated at the same time… Saxton et al., 1998 Sulfur isotopes in Martian meteorites measurements of ALH84001 d34S Greenwood et al., 1997 What d34S and D33S values would we expect for sulfur-reducing bacteria? D33S of Martian meteorites Farquhar et al., 2000 Mass-independent atmospheric reactions on Mars: sulfur conceptual model of Martian sulfur cycle laboratory experiment -explains sulfur isotopes w/o invoking biotic activity -implies presence of oxidized sulfur species on Mar’s surface Farquhar et al., 2000 Mass-independent atmospheric reactions on Mars: oxygen negative correlations help to rule out a variety of processes Farquhar et al., 1998 Curiosity Rover, August 6, 2012-? Analytic capabilities for studies of life on Mars: - mini-Thermal Emission Spectrometer (bulk chemistry of rocks; remote measurement) - Mossbauer spectrometer (Fe-bearing minerals; contact) - alpha-particle X-ray spectrometer (elemental chemistries; contact) Burns Cliff sedimentary layers? hematite water?