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

ALH84001 – Life on Mars?

found on Dec 27, 1984 by NSF Antarcticmeteorite program; shelved until 1993,when somebody realized it came from Mars

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

How old is ALH84001?

ALH84001

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

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

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

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

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

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?

A freshly cut chip of ALH84001,showing orange carbonatesembedded in orthopyroxene

1.5mm

Valley et al., 1997

0.1mm

A back-scattered electron imageof a carbonate globule in ALH84001

Romanek et al., 1994

More images of ALH84001 carbonates

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

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)

Terrestrial and Martianoxygen isotopes

Saxton et al., 1998

these low terrestrial values form from depleted waters at high T

marinecarbonates

lake carbonates

hydrothermaldeposits

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

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…

Sulfur isotopes in Martian meteorites

measurements 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?

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

Mass-independent atmospheric reactions on Mars: oxygen

Farquhar et al., 1998

negative correlationshelp to rule out a varietyof processes

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?