atmospheric biomarkers (in extrasolar planets) nick cowan uw astronomy december 2005
Post on 21-Dec-2015
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TRANSCRIPT
Outline
• Why do we care?• How are we gonna
do it?• What are we looking
for, anyways?QuickTime™ and a
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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 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.