Title: Faster Water Escape on High Obliquity Planets
Author: Wanying Kang
Status: Published in Astronomy & Astrophysics; open access
First Author’s Institution: School of Engineering and Applied Sciences Harvard University
Cambridge, MA 02138, USA
Introduction
Human civilization appears to be an infinitesimal spec when faced with the thousands of exoplanets discovered throughout recent history. Exoplanets invite newfound potential for habitability beyond Earth, with the sheer number of possibilities alone. Wanying Kang’s paper “Faster Water Escape on High Obliquity Planets” explores the effects of high obliquity on stratospheric humidity, which could lead to habitability. Earth-like planet stratospheres contain little water vapor which is why most clouds are found in the troposphere. High-obliquity affects stratospheric humidity through seasonal variation, getting moisture from high altitudes, and the nature of overall warmer surface temperature caused by high-obliquity. Kang’s experiments use a 3D general circulation model to investigate the moist stratosphere that exists in high-obliquity planets. Are these humid stratospheres a key to a habitable planet or another Venus fate by a runaway greenhouse state?
Methods
The 3D general circulation model used presents a more comprehensive understanding of the process of water escape on high-obliquity planets. The paper refers back to previous research by 1D models that neglected to consider factors such as seasonal and spatial variation. The experiment uses the increased spectral resolution to better depict radiation. H20 is considered the only greenhouse gas for simplicity and CO2 absorption is not considered. Atmosphere circulation was simulated by using a finite-volume dynamic core. The experiments used increasing isolation: one experiment set at zero obliquity, and the other at 80. These two experiments present an opportunity to better understand the cause of humid stratospheres, and if they can lead to habitable planets!
The 3D general circulation model allows for a comprehensive analysis of seasonal variation, which is present on high obliquity planets due to more intense seasons which cause warmer surface temperatures during polar days. Second, it considers moisture entering the stratosphere from higher altitudes, allowing it to escape the cold trap and make it into the upper atmosphere. Lastly, it accounts for the all-around high surface-temperatures of high obliquity planets.
Results
Upper atmospheres are significantly moister in the high-obliquity insulated experiment, and the lower obliquity experiment almost triggers runaway greenhouse affects but ultimately crashes. The high-obliquity experiment has higher water escape rates, making water vapor more visible in these models. However, both experiments prove to be inhabitable. The low obliquity experiment is inhabitable due to its potential for the runaway gas effect, whereas the high obliquity experiment also proves to be inhabitable due to water escape.
While high-obliquity planets don’t fall victim to the runaway greenhouse state, water escape speeds up and leaves planets uninhabitable. However, these experiments only account for insulation and obliquity. The results of the experiment claim that considering the rotation rate for Earth-like planets could have an effect on the cold trap and thus water escape. These experiments also don’t account for atmospheric composition. In the end, high-obliquity humid stratospheres may not prove to be habitable yet, but it does reveal that these are the exoplanets are more likely to be discovered due to surface evaporation caused by the high obliquity.
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