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 for, anyways?

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…) 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.

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.

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 O 3 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)

O 2 production on ice worlds Europa and Ganymede have O 2 due to charged particles interacting with the icy surface. The O 3 /O 2 is not consistent with photolysis. The amount of Oxygen is small, in any case.

O 2 production on a wet Venus During a runaway greenhouse, a planet might vaporize its oceans. The photolysis of H 2 O and subsequent thermal escape of H results in atmospheric O 2. But O 3 doesn’t form as long as H sticks around, so we only expect an O 3 signature once H 2 O is completely gone. So the double detection of O 3 and H 2 O is still a robust indicator of life.

O 3 signature depends on cloud cover (des Marais et al. 2002)

O 3 depends on the host star Hotter stars produce more UV radiation, leading to more O 3 in planetary atmospheres and a hotter stratosphere. The effect on the O 3 signature is weird. CO 2 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 CH 4. Atmospheric CH 4 is thought to be inversely related to O 2 so it might be a complementary biomarker. (Des Marais et al. 2002)

Methane isn’t perfect, though There are many abiogenic ways of producing CH 4. The presence of large amounts of CH 4 in the absence of other volcanic gases would be pretty convincing, though.

What about Mars? Mars has a very tenuous atmosphere. The tiny amounts of CH 4 would never show up in a TPF-quality spectrum. If it was detected, though, the aditional presence of H 2 O vapor would be suspicious, though. (Krasnopolsky et al. 2004)

Summary Large amounts of O 2 (detected using O 3 and in the presence of H 2 O) 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 O 2. 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 CH 4, 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)