Monday, June 17, 2013

1306.3266 (Robin Wordsworth et al.)

Water loss from terrestrial planets with CO2-rich atmospheres    [PDF]

Robin Wordsworth, Raymond Pierrehumbert
Water photolysis and hydrogen loss from the upper atmospheres of terrestrial planets is of fundamental importance to climate evolution but remains poorly understood in general. Here we present a range of calculations we performed to study the dependence of water loss rates from terrestrial planets on atmospheric composition (CO2 and N2 levels), planetary mass, and external parameters (stellar spectrum, orbital distance and impacts). From coupled 1D climate and escape modeling, we show that CO2 can only cause significant water loss by increasing surface temperatures over a narrow range of conditions, with cooling of the middle and upper atmosphere acting as a bottleneck on escape in other circumstances. Around G-stars, efficient loss only occurs on planets with intermediate CO2 atmospheric partial pressures (0.1 to 1 bar) that receive a net flux close to the critical runaway greenhouse limit. Because G-star total luminosity increases with time but XUV/UV luminosity decreases, this places strong limits on moist stratosphere H2O photolysis for planets like Earth. In contrast, for a CO2-rich early Venus, diffusion limits on water loss are only important if clouds had a strong negative forcing (delta F<-70 W/m^2 vs. clear-sky case or albedo>0.8), implying that scenarios where the planet never had surface liquid water are indeed plausible. Around M-stars, the lack of temporal luminosity variation means water loss is primarily a function of orbital distance, with planets that absorb less flux than ~270 W/m^2 (global mean) unlikely to lose more than one Earth ocean of H2O over their lifetimes. Because of the variability of H2O delivery to planetesimals during accretion, our results suggest that many 'Earth-like' exoplanets in the habitable zone may have ocean-covered surfaces, stable CO2/H2O-rich atmospheres, and high mean surface temperatures.
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