lecture14

Slide1: 

Isotopic tools in the search for life on Mars I 11/16/05 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

Slide2: 

ALH84001 – Life on Mars? found on Dec 27, 1984 by NSF Antarctic meteorite program; shelved until 1993, when somebody realized it came from Mars

Slide3: 

How do we know ALH84001 originated 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 observed during Viking expedition Marti et al., 1995

Slide4: 

How old is ALH84001? Nyquist et al., 1995; 2001 (review) How should we interpret this data?

Slide5: 

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 impact event…. 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? conventional analysis laser probe

Slide6: 

When was ALH84001 ejected from Mars? Eugster et al., 1997b 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

Slide7: 

How long has ALH84001 been on Earth? Jull et al., 1998 step-wise combustion of meteorite organics come off at low T (low d13C, modern 14C!) -carbonates combusted at high T (high d13C, low 14C) bottom line: All organics in ALH84001 likely represent recent contamination (5ka to 20ka)

Slide8: 

How old are the carbonates globules that are associated with the evidence of ancient Martian life? Borg et al., 1999 carbonates represent <1% of ALH84001, must be separated from silicates by chemical leaching

Slide9: 

Pb-Pb isochrons for ALH84001 carbonates Borg et al., 1999 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?

Slide10: 

A freshly cut chip of ALH84001, showing orange carbonates embedded in orthopyroxene 1.5mm Valley et al., 1997 0.1mm A back-scattered electron image of a carbonate globule in ALH84001 Romanek et al., 1994 More images of ALH84001 carbonates

Slide11: 

Early stable isotope work on ALH84001 Romanek et al., 1994 - 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. Taken together, results suggest a warm, wet planet at 4.5Ga BUT could also be explained by high-T metasomatism (Harvey et al., 1996)

Slide12: 

Terrestrial and Martian oxygen isotopes Saxton et al., 1998 these low terrestrial values form from depleted waters at high T marine carbonates lake carbonates hydrothermal deposits

Slide13: 

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

Slide14: 

More oxygen isotope work on ALH84001 Saxton et al., 1998 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…

Slide15: 

Sulfur isotopes in Martian meteorites measurements of ALH84001 d34S D33S of Martian meteorites Farquhar et al., 2000 Greenwood et al., 1997 What d34S and D33S values would we expect for sulfur-reducing bacteria?

Slide16: 

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 experiment conceptual model of Martian sulfur cycle

Slide17: 

Mass-independent atmospheric reactions on Mars: oxygen Farquhar et al., 1998 negative correlations help to rule out a variety of processes

Slide18: 

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

Slide19: 

Present and Future Mars missions 2005 Mars Reconnaissance Orbiter high-powered camera, subsurface sounding radar, infrared radiometer 2007 Phoenix Lander (lands March 2008) land on high-latitude ice fields (like terrestrial permafrost), look for volatiles, signs of life; mass spec! http://phoenix.lpl.arizona.edu/ 2009 Mars Science Laboratory long-term rover; search for life; assess suitability for habitation - laser zaps rock for volatile analysis - powered by Pu decay!