Atmospheric Biomarkers (in extrasolar planets)

1. Atmospheric Biomarkers (in extrasolar planets) Nick Cowan UW Astronomy December 2005

2. Outline • Why do we care? • How are we gonna do it? • What are we looking for, anyways?

3. 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!

4. 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

5. 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.

6. 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.

7. 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).

8. 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)

9. 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.

10. 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.

11. O3 signature depends on cloud cover (des Marais et al. 2002)

12. 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)

13. 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)

14. 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.

15. 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. (Krasnopolsky et al. 2004)

16. 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.

17. 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)