Isotopic tools in the search for

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

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

What is normal here?
Marti et al., 1995
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How old is ALH84001?
How should we interpret this data?
Nyquist et al., 1995 2001 (review)
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What are the key events in ALH84001s geologic
history?
conventional analysis
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?
laser probe
Turner et al., 2003
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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
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How long has ALH84001 been on Earth?

• 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)
Jull et al., 1998
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How old are the carbonates globules that are
associated with the evidence of ancient Martian
life?

• carbonates represent lt1 of ALH84001, must be
  separated
• from silicates by chemical leaching

Borg et al., 1999
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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
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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
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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
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 Martian oxygen isotopes
marine carbonates
lake carbonates
hydrothermal deposits
these low terrestrial values form from depleted
waters at high T
Saxton et al., 1998
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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
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More oxygen isotope work on ALH84001
The lightest carbonates require a fluid with a
d18O lt 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
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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
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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 Mars
• surface

Farquhar et al., 2000
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Mass-independent atmospheric reactions on Mars
oxygen
negative correlations help to rule out a
variety of processes
Farquhar et al., 1998
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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
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Present and Future Mars missions
2005 Mars Reconnaissance Orbiter

• high-powered camera, subsurface
• sounding radar, infrared radiometer

2007 Phoenix Lander (lands March 2008)
2009 Mars Science Laboratory

• land on high-latitude ice fields
• (like terrestrial permafrost), look
• for volatiles, signs of life mass spec!
• http//phoenix.lpl.arizona.edu/

• long-term rover search for life
• assess suitability for habitation
• - laser zaps rock for volatile analysis
• - powered by Pu decay!