Hydrogen escape from waterworlds and H2 greenhouse habitable Super-Earths

Details Hydrogen escape from waterworlds and H2 greenhouse habitable Super-Earths Hydrogen escape from waterworlds and H2 greenhouse habitable Super-Earths

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p23f..01p&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P23F-01

Biology

[0325] Atmospheric Composition And Structure / Evolution Of The Atmosphere, [0328] Atmospheric Composition And Structure / Exosphere, [5200] Planetary Sciences: Astrobiology, [5405] Planetary Sciences: Solid Surface Planets / Atmospheres

Scientific paper

We re-examine the problem of hydrogen escape from Super-Earth class planets, with an eye to resolving issues raised by recent observational and theoretical work in this area. One of the problems treated is motivated by the low-density hot Super-Earth GJ1214b, which has a density suggesting it is water-rich, but does not appear to have a hydrogen envelope. This poses the question of the lifetime of such planets, and the rate at which water would be lost by photolysis and subsequent hydrogen escape. The second class of questions is posed by recent theoretical work by Pierrehumbert and Gaidos (ApJL 2011) which extends the boundary of habitable zones for Super-Earths by invoking the greenhouse effect due to the collisional continuum of H2. Here the question is how close the planet can be to its host star before it loses its atmosphere in an unacceptably short time. To address the first question, we examine the effect of atmospheric chemistry on hydrogen escape, using a highly idealized atmosphere consisting of only water vapor and its photolysis products. The essential issue here is that the buildup of oxidants as hydrogen escapes can lead to back-reaction with H and H2, choking off escape, if there is no sink of oxidant at the surface of the planet. The problem crucially involves vertical mixing in the upper atmosphere, since gravitational segregation of H and H2 can separate reactants and retard the back-reaction. The problem is closely related to the problem of "false positives" in O2 as a biosignature, treated by Selsis and co-workers. We explore this problem in a range of simplified two-box models of the upper atmosphere. The crux of the second question amounts to a refinement of the rate of EUV driven H and H2 escape from hydrogen-dominated Super-Earths in distant orbits. In Pierrehumbert and Gaidos (2011), an upper bound based on energy-limited escape was employed, but this does not fully incorporate additional constraints governing hydrodynamic escape. We re-examine the problem using a one-dimensional time-dependent compressible hydrocode. The implications of transition to collisionless flow before the critical (Mach 1) point is reached are also discussed, using the approximate Jeans boundary condition introduced by Tian and Kasting.

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