# Water loss from terrestrial planets with $$CO_2$$-rich atmospheres

 Title: Water loss from terrestrial planets with $$CO_2$$-rich atmospheres Author: Wordsworth, Robin; Pierrehumbert, R. T. Note: Order does not necessarily reflect citation order of authors. Citation: Wordsworth, R. D., and R. T. Pierrehumbert. 2013. “Water loss from terrestrial planets with $$CO_2$$-rich atmospheres.” The Astrophysical Journal 778 (2) (November 13): 154. doi:10.1088/0004-637x/778/2/154. http://dx.doi.org/10.1088/0004-637X/778/2/154. Full Text & Related Files: 1306.3266v2.pdf (1.288Mb; PDF) Abstract: 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 a range of atmospheric and external parameters. 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 (0.1-1 bar) that receive a net flux close to the critical runaway greenhouse limit. Because G-star total luminosity increases with time but X-ray and ultraviolet/ultravoilet luminosity decreases, this places strong limits on water loss for planets like Earth. In contrast, for a CO2-rich early Venus, diffusion limits on water loss are only important if clouds caused strong cooling, implying that scenarios where the planet never had surface liquid water are indeed plausible. Around M-stars, 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 unless they lose all their atmospheric N2/CO2 early on. Because of the variability of H2O delivery 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. Published Version: doi:10.1088/0004-637X/778/2/154 Other Sources: http://arxiv.org/abs/1306.3266 Terms of Use: This article is made available under the terms and conditions applicable to Open Access Policy Articles, as set forth at http://nrs.harvard.edu/urn-3:HUL.InstRepos:dash.current.terms-of-use#OAP Citable link to this page: http://nrs.harvard.edu/urn-3:HUL.InstRepos:27846319 Downloads of this work: