where next in the search for life on mars? chris mckay nasa ames research center...

Where Next in the Search for Life on Mars?

Chris McKayNASA Ames Research Center

chris.mckay@nasa.gov

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?

Aliens: not on our tree of life

comparative biochemistry (life 2.0)

life is common in the universe (yeah!)

Phoenix Mission to Mars May 2008 to Nov 2008

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

5 Myr ago: 2x summer insolation

Murray et al. 1973Laskar et al. 2002

| surface melting| cold deep ice

The North Pole of Mars could have had liquid water 5 Myr ago

• Ice at the surface• Low elevationPressures >6.1 mbar

allows water to be stable• Summer Temperatures < 0oC now

but may be > 0oC at higher obliquity 5 million years ago.Liquid water on

Mars requires1. Ice2. T > 0ºC3. P> 6.1 mb

Unexpected Phoenix results

Soil pH ~8

Carbonates 3-5%

Perchlorate 0.5%

Segregated ice No organics

No nitrates } Instrument failures

bleachsalt

Perchlorates on Mars

• Perhclorates decomposeand release oxygen whenheated 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

SAM will wash the samples with a derivatization agent:N-Methyl-N-(Tert-Butyldimethylsilyl) trifluoroacetamide and this mode should be free of perchlorate.

There is a pyrolysis GCMS on the upcoming MSL mission

Sample Analysis for Mars

Laser desorption mass spectromter. Part of Luann Becker’s (JHU) MOMA instrument for the ExoMars mission.

Mass specLaser

A simple approach to searching for organics in high perchlorate soil.

Is there life in places on Earth with ice like this?

Metabolism in permafrost down to -15oCLimited by nutrient transport - not temperature(Rivkina et al. Metabolic activity of permafrost bacteria below the freezing point, Appl. Environ. Microbio., 66, 3230-3233, 2000)

Rivkina et al. 2000

The rapid growth rate phase is limited by temperature.

Microorganisms in permafrost at -10oC are inactive.

Not because theyare cold but because they are have access only to thin films ofwater

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

35 cm

+1ºC

-10ºC

dry soil

ice & soil

Mars site:dry permafrost over ground ice

-30ºC

-50ºC

dry soil

ice & soil

salts

-5ºC

-15ºC

dry soil

ice & soil

5 Myr ago

Energy for life

• Antarctica:

organic material + O2

Energy for life

• Antarctica:

organic material + O2

• Mars:

basalt rocks (Fe++) + ClO4

Nealson, K. (2003) Nature Biotechnology 21, 243-244.

Perchlorate

Perchlorate could be an electron acceptor for a redox couple that supports life on Mars.

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

nitrate formation duringAccretion shocks

nitrate decomposition by impacts

N2 + CO2 NOx

NO3 N2 + O2

3 m of nitrate in the soil

Gilichinsky et al. 2007

Limits on long term dormancy

• kT: Thermal decay: ~e-E/kT

racemization of amino acidsdegradation of organic materialnot important on Mars, -70oC

• eV: Radiation from crustal U,Th, K ~0.2rad/yrlethal dose for Deinococcus radiodurans in 100 Myron 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 smoothBiotic distributions are spiked

McKay 2004 PLoS Biol 2(9)1260-12623

Abiotic distributions are smoothBiotic distributions are spiked

McKay 2004 PLoS Biol 2(9)1260-12623

alien