isotopic tools in the search for life on mars 11/29/12
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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 - PowerPoint PPT PresentationTRANSCRIPT
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Isotopic tools in the search for life on Mars 11/29/12
Evidence for ice on Mars from ESA Mars Express Camera (top) and the Viking lander
Pack ice in the Wedell Sea, Antarctica
25km
25km
Murray et al., 2005
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ALH84001 – Life on Mars?
found on Dec 27, 1984 by NSF Antarcticmeteorite program; shelved until 1993,when somebody realized it came from Mars
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How do we know ALH84001originated from Mars?
step-heating SNC meteorites
What is “normal” here?
- measure gas isotopes in tiny glass vesicles
- N, Ar, and Xe isotope ratios match Mars atmosphere values, as observedduring Vikingexpedition
Marti et al., 1995
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How old is ALH84001?
ALH84001
Nyquist et al., 1995; 2001 (review) How should we interpret this data?
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Turner et al., 2003
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 impactevent….
Plus, during ejection ALH84001experienced extremelyhigh pressures and temperatures(300-400ºC; 35-40 GPa)
Why are these events so crucial to the interpretation of ALH84001 as evidenceof life on Mars?
conventional analysis
laser probe
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When was ALH84001 ejected from Mars?
Eugster et al., 1997b
Cosmogegnic exposure dating: need to model ingrowth of cosmogenic isotopesthrough time using bulk chemistry
3He, 21Ne, and 38Ar measurements all give similar age range: 12 to 16Ma
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How long has ALH84001 been on Earth?
Jull et al., 1998
step-wise combustion of meteorite
-organics come off at low T(low 13C, high 14C!)
-carbonates combusted at high T(high 13C, low 14C)
bottom line: Most organics in ALH84001 likelyrepresent recent contamination (5ka to 8ka);after a fall to Earth at ~13ka
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How old are the carbonates globules that are associatedwith the evidence of ancient Martian life?
Borg et al., 1999
•carbonates represent <1% of ALH84001, must be separated from silicates by chemical leaching
circles = leachates from Borg et al., 1999triangles = whole rock analyses (plagioclase, pyroxene, etc)dashed line = 4.5Ga crystallization agesolid line = best fit to carbonate leachate data
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Pb-Pb isochrons for ALH84001 carbonates
Borg et al., 1999
So Rb-Sr and Pb-Pb datingare near-concordant
but
what kinds of things do we have to worry about wheninterpreting this data?
Could this data be describedas a mixing line between terrestrial Pb and an enrichedPb source in ALH84001?
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A freshly cut chip of ALH84001,showing orange carbonatesembedded in orthopyroxene
1.5mm
Valley et al., 1997
0.1mmA back-scattered electron imageof a carbonate globule in ALH84001
Romanek et al., 1994
More images of ALH84001 carbonates
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MARS carbonate globules
EARTH carbonate globulesfrom Spitsbergen Island, Norway
precipitated from hydrothermal fluids derived from volcanics
but earliest terrestrial life probablyformed around hydrothermal vents….
Thiemans et al., 2002
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Early stable isotope work on ALH84001
Romanek et al., 1994
* 13C relative to PDB, 18O relative to SMOW
- 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 equilibratedoxygen isotopes between the different mineral phases.
- carbon isotopes agree with 13C of Martian atmosphere, as measured in gas trapped in a Martian meteorite.
Taken together, results suggest a warm, wet planet at 4.5Ga
BUT could also be explained by high-T metasomatism (Harvey et al., 1996)
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Terrestrial and Martianoxygen isotopes
Saxton et al., 1998
these low terrestrial values form from depleted waters at high T
marinecarbonates
lake carbonates
hydrothermaldeposits
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More stable isotope work on ALH84001
Valley et al., 1997
-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
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More oxygen isotope work on ALH84001
Saxton et al., 1998
The lightest carbonates require a fluid with a 18O < 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…
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Sulfur isotopes in Martian meteoritesmeasurements of ALH84001 34S
33S of Martian meteorites
Farquhar et al., 2000
Greenwood et al., 1997
What 34S and 33S values would we expect for sulfur-reducing bacteria?
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Farquhar et al., 2000
Mass-independent atmospheric reactions on Mars: sulfur
-explains sulfur isotopes w/o invoking biotic activity
-implies presence of oxidized sulfur species on Mar’s surface
laboratory experimentconceptual model of Martian sulfur cycle
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Mass-independent atmospheric reactions on Mars: oxygen
Farquhar et al., 1998
negative correlationshelp to rule out a varietyof processes
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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)
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Burns Cliff sedimentary layers?
hematite water?