Water loss from terrestrial planets with CO2-rich atmospheres (1306.3266v2)

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 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 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.

Analysis of Water Loss from Terrestrial Planets with CO$_2$-Rich Atmospheres

The paper authored by Wordsworth et al. addresses a critical question in planetary science: the factors influencing water retention and loss on terrestrial planets, particularly those possessing a CO$_2$-rich atmosphere. This paper undertakes a systematic evaluation of water loss mechanisms driven by water photolysis and hydrogen escape in the upper atmospheres of such planets. By examining the effects of different atmospheric compositions and stellar inputs, the research contributes to the understanding of climate evolution and potential habitability conditions on rocky planets.

The authors explore the hypothesis that CO$_2$ influences water loss by significantly affecting surface temperatures, largely due to the greenhouse effect. Their analyses suggest that CO$_2$ may only facilitate water loss under specific and narrow conditions where increased surface temperatures do not concurrently cool the middle and upper atmosphere sufficiently to impede atmospheric escape. Their focus is on planets orbiting G-stars, specifically examining the implications for Earth-like conditions. Efficient water loss is limited to planets with an intermediate range of CO$_2$ partial pressures (0.1 to 1 bar) when receiving stellar radiations near the critical runaway greenhouse threshold.

A pivotal finding of the paper is an intricate coupling between photolysis, atmospheric CO$_2$ partial pressure, and stellar radiation in establishing upper atmospheric conditions that permit significant water escape. The evolution of a star's luminosity, particularly the disparity in the growth rates of total and XUV/UV luminosity, acts as a determinant for water loss potential. For G-star systems, the increase in total luminosity over time contrasts with a decrease in XUV/UV luminosity, creating a window that restricts water loss under certain atmospheric conditions. This aspect of stellar evolution implies Earth has remained within the bounds for conserving substantial water bodies despite early solar variability.

Conversely, examination of early Venus reveals scenarios where initial water abundance could have led to complete loss due to extreme insolation and resultant atmospheric conditions. Here the authors note the potential for a lack of significant surface water if early atmospheric conditions favored a scenario without significant cloud-driven cooling.

The paper extends its implications to planets orbiting lower-mass M-stars. In these systems, the primary determinant of water loss is proximity to the star due to their lower luminosity and prolonged elevated XUV/UV emissions. Planets beyond a critical distance, absorbing less than approximately 270 W/m$^2$, demonstrate a reduced likelihood of cumulative water loss equivalent to one Earth ocean over geologic timescales, assuming the initial presence of N$_2$/CO$_2$.

Through rigorous modeling, the authors highlight scenarios where planets attaining an Earth-like position within a habitable zone may yet differ fundamentally in surface and atmospheric composition due to variable water loss. Some 'Earth-like' exoplanets may remain ocean-covered, retaining thick, stable CO$_2$/H$_2$O-rich atmospheres with elevated surface temperatures potentially conducive to persistent life, albeit potentially restraining complex life development due to limited exposed land nutrient supply.

Future work in this vein could improve constraints on CO$_2$ and H$_2$O atmospheric behavior with enhanced cloud simulations or through 3D climate modeling, addressing current approximations regarding infrared cooling and photolysis impacts. This research advances the conceptual framework for interpreting exoplanetary atmospheres and emphasizes the crucial interplay between atmospheric composition, stellar radiation, and geological conditions in determining long-term planetary water inventories.