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Where Next in the Search for
Life on Mars?
Chris McKay
NASA Ames Research Center
[email protected]
JHU, 9 April 2010
Ans: Ice
1. On a dry cold world, ice can be a
source of liquid water.
(Go to the North Polar Region of Mars)
Follow Phoenix!
2. Ancient ice can preserve evidence of
past life.
(Go to the South Polar Region of Mars)
The Astrobiology Strategy
(NRC report 2007)
 follow the water
- search for organics
- characterize to detect signs of life
- return to Earth for analysis
Why do we search for a second
genesis of life on other worlds?
 comparative biochemistry (life 2.0)
 life is common in the universe (yeah!)
Aliens:
not on our tree of life
Phoenix Mission to Mars May 2008 to Nov 2008
5 Myr ago: 2x summer insolation
| cold deep ice
| surface melting
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Murray et al. 1973
Laskar et al. 2002
The North Pole of Mars could have
had liquid water 5 Myr ago
Liquid water on
Mars requires
1. Ice
2. T > 0ºC
3. P> 6.1 mb
• Ice at the surface
• Low elevationPressures >6.1 mbar
allows water to be stable
• Summer Temperatures < 0oC now
but may be > 0oC at higher obliquity
5 million years ago.
Unexpected Phoenix results
Soil pH ~8
Carbonates 3-5%
Perchlorate 0.5%
Segregated ice
No organics
No nitrates
}

Instrument
failures
bleach
salt
Perchlorates on Mars
• Perhclorates decompose
and release oxygen when
heated to ~350ºC
- used in rockets
- destroys organics
• Strong anti-freeze (-70ºC)
- allow liquid on Mars today
• Consumed by microorganisms
- terminal electron acceptor ≈ nitrate
Organics on Mars
•Everything we thought we knew about
organics on Mars from the Viking pyrolysis
GCMS is wrong
•The upper limit is not ppb, but 0.1%
(a factor of a million correction)
Good news for future missions:
organics might be high.
Bad news for future missions:
need non-thermal methods
There is a pyrolysis GCMS on the upcoming MSL mission
Sample Analysis for Mars
SAM will wash the samples with a derivatization agent:
N-Methyl-N-(Tert-Butyldimethylsilyl) trifluoroacetamide
and this mode should be free of perchlorate.
A simple approach to searching for
organics in high perchlorate soil.
Laser
Mass spec
Laser desorption mass spectromter.
Part of Luann Becker’s (JHU) MOMA instrument for the
ExoMars mission.
Is there life in places on Earth with ice like this?
Metabolism in permafrost down to -15oC
Limited by nutrient transport - not temperature
(Rivkina et al. Metabolic activity of permafrost bacteria below the
freezing point, Appl. Environ. Microbio., 66, 3230-3233, 2000)
The rapid growth rate phase is limited by temperature.
Rivkina et al. 2000
Microorganisms in permafrost at
-10oC are inactive.
Not because they
are cold but
because they are
have access only
to thin films of
water
Dry permafrost
• Ice cemented ground beneath dry permafrost that is always
below -10ºC is habitable
(consistent with Rivkina et al. 2000)
• In the dry permafrost of the dry valleys of Antarctica we
find aerobic heterotrophs consuming organic material
ultimately produced by cryptoendoliths in the sandstone
Antarctic site:
dry permafrost over
deep ground ice
Mars site:
dry permafrost over
ground ice
salts
+1ºC
-10ºC
dry soil
-30ºC
dry soil
-50ºC
ice & soil
35 cm
ice & soil
5 Myr ago
-5ºC
dry soil
-15ºC
ice & soil
Energy for life
• Antarctica:
organic material + O2
Energy for life
• Antarctica:
organic material + O2
• Mars:
basalt rocks (Fe++) + ClO4
Perchlorate could be
an electron acceptor
for a redox couple that
supports life on Mars.
Perchlorate
Nealson, K. (2003)
Nature Biotechnology 21, 243-244.
Habitability of the Phoenix site 5
Myr ago
• Carbon as CO2 
• Liquid water 
• Energy 
• Nitrogen ? (nitrates in the soil??)
Nitrates on Mars
Manning et al. Icarus, 2008
N2 + CO2  NOx
NO3  N2 + O2
nitrate formation during
Accretion shocks
nitrate decomposition
by impacts
3 m of nitrate in the soil
Limits on long term dormancy
• kT: Thermal decay: ~e-E/kT
racemization of amino acids
degradation of organic material
not important on Mars, -70oC
• eV: Radiation from crustal U,Th, K ~0.2rad/yr
lethal dose for Deinococcus radiodurans in 100 Myr
on Mars hundreds of lethal dose over 3.5 Gyr
• Its dead, Jim
If we find organic material in a
frozen sample on Mars.
Can we tell if it was ever alive?
If its like us (DNA)
then is easy, but less interesting
If its alien then hard, but interesting
Abiotic distributions are smooth
Biotic distributions are spiked
McKay 2004 PLoS Biol 2(9)1260-12623
Abiotic distributions are smooth
Biotic distributions are spiked
alien
McKay 2004 PLoS Biol 2(9)1260-12623