Atmospheric Biomarkers (in extrasolar planets)

Nick Cowan

UW Astronomy

December 2005

Outline

• Why do we care?• How are we gonna

do it?• What are we looking

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Why do we care?

• We may not find any extraterrestrial life in our solar system.

• Even if we do, it might be the result of panspermia.

• There are a lot more extrasolar planets than solar planets.

• It would be damn good impetus to build starships!

How are we gonna do it?

• Nulling Interometers• Choronographs• Infrared or Visible?• NASA: Terrestrial

Planet Finder• ESA: Darwin

(basically all that stuff E. Agol talked about yesterday…)

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TPF

What are we gonna see?

• Low resolution infrared spectroscopy.

• Integrated light from the whole planet.

• Broad absorption features tell us about the composition of the planet’s atmosphere.

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How can we tell if there’s life?

• If there’s only a little bit of life we’re out of luck.

• But, the only planet we know with any life has buckets full of it.

• On such a planet life tends to affect the planet in big ways.

• The atmosphere of a living planet is very different from the atmosphere of a dead planet.

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Atmospheric Biomarkers

• Gases which we expect to find on a living planet but not on a dead planet.

• Must understand how these gases might be produced abiotically (false positive)

• Must understand how these gases might be hidden (false negative).

Oxygen: a fine biomarker

• The 9.6 micron line of O3 is actually more sensitive (10-3 PAL).

• Oxygen likes to oxidize things.

• If you find oxygen, some photosynthetic critter must have created it, right?

• Not so fast…(Schindler & Kasting 2000)

O2 production on ice worlds

• Europa and Ganymede have O2 due to charged particles interacting with the icy surface.

• The O3/O2 is not consistent with photolysis.

• The amount of Oxygen is small, in any case.

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O2 production on a wet Venus

• During a runaway greenhouse, a planet might vaporize its oceans.

• The photolysis of H2O and subsequent thermal escape of H results in atmospheric O2.

• But O3 doesn’t form as long as H sticks around, so we only expect an O3 signature once H2O is completely gone.

• So the double detection of O3 and H2O is still a robust indicator of life.

O3 signature depends on cloud cover

(des Marais et al. 2002)

O3 depends on the host star

• Hotter stars produce more UV radiation, leading to more O3 in planetary atmospheres and a hotter stratosphere.

• The effect on the O3 signature is weird.

• CO2 can break the degeneracy.

(Segura et al. 2003)

Methane: another nice biomarker

• The Earth was toasty even when the Sun was faint.

• There must have been a stronger greenhouse gas back then, probably CH4.

• Atmospheric CH4 is thought to be inversely related to O2 so it might be a complementary biomarker.

(Des Marais et al. 2002)

Methane isn’t perfect, though

• There are many abiogenic ways of producing CH4.

• The presence of large amounts of CH4 in the absence of other volcanic gases would be pretty convincing, though.

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What about Mars?

• Mars has a very tenuous atmosphere.

• The tiny amounts of CH4 would never show up in a TPF-quality spectrum.

• If it was detected, though, the aditional presence of H2O vapor would be suspicious, though.

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(Krasnopolsky et al. 2004)

Summary

• Large amounts of O2 (detected using O3 and in the presence of H2O) in the atmosphere of an extrasolar terrestrial planet would be a smoking gun.

• It is not clear that living planets (even those with photosynthesis) will have much atmospheric O2.

• Not only would it indicate the presence of life on the planet, it would also mean that the planet is ripe for large (and possibly intelligent) animals.

• If Mars were an extrasolar planet and we had a telescope powerful enough to detect its CH4, we might think it has life.

References

• Selsis et al., A&A (2002)

• Schindler & Kasting, Icarus (2000)

• Catling & Claire, EPSL (2005)

• Segura et al., Astrobiology (2003)

• Catling et al., Astrobiology (2005)