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On the Role of Dissolved Gases in the Atmosphere Retention of Low-mass Low-density Planets

Chachan, Yayaati and Stevenson, David J. (2018) On the Role of Dissolved Gases in the Atmosphere Retention of Low-mass Low-density Planets. Astrophysical Journal, 854 (1). Art. No. 21. ISSN 1538-4357. https://resolver.caltech.edu/CaltechAUTHORS:20180207-084103683

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Abstract

Low-mass low-density planets discovered by Kepler in the super-Earth mass regime typically have large radii for their inferred masses, implying the presence of H_2–He atmospheres. These planets are vulnerable to atmospheric mass loss due to heating by the parent star's XUV flux. Models coupling atmospheric mass loss with thermal evolution predicted a bimodal distribution of planetary radii, which has gained observational support. However, a key component that has been ignored in previous studies is the dissolution of these gases into the molten core of rock and iron that constitute most of their mass. Such planets have high temperatures (>2000 K) and pressures (~kbars) at the core-envelope boundary, ensuring a molten surface and a subsurface reservoir of hydrogen that can be 5–10 times larger than the atmosphere. This study bridges this gap by coupling the thermal evolution of the planet and the mass loss of the atmosphere with the thermodynamic equilibrium between the dissolved H_2 and the atmospheric H_2 (Henry's law). Dissolution in the interior allows a planet to build a larger hydrogen repository during the planet formation stage. We show that the dissolved hydrogen outgasses to buffer atmospheric mass loss. The slow cooling of the planet also leads to outgassing because solubility decreases with decreasing temperature. Dissolution of hydrogen in the interior therefore increases the atmosphere retention ability of super-Earths. The study highlights the importance of including the temperature- and pressure-dependent solubility of gases in magma oceans and coupling outgassing to planetary evolution models.


Item Type:Article
Related URLs:
URLURL TypeDescription
https://doi.org/10.3847/1538-4357/aaa459DOIArticle
http://iopscience.iop.org/article/10.3847/1538-4357/aaa459PublisherArticle
https://arxiv.org/abs/1802.04296arXivDiscussion Paper
ORCID:
AuthorORCID
Chachan, Yayaati0000-0003-1728-8269
Stevenson, David J.0000-0001-9432-7159
Additional Information:© 2018 American Astronomical Society. Received 2017 August 5. Accepted 2017 December 26. Published 2018 February 7. We thank the referees for providing valuable feedback that helped us improve our manuscript. Y.C. acknowledges the support of St John's College, Cambridge, for funding this project as part of the Undergraduate Academic Research Proposal (UARP) and Learning & Research Fund (LRF) programmes.
Group:Astronomy Department
Funders:
Funding AgencyGrant Number
St. John's College, CambridgeUNSPECIFIED
Subject Keywords:planets and satellites: atmospheres; planets and satellites: composition; planets and satellites: formation; planets and satellites: interiors; planets and satellites: physical evolution
Issue or Number:1
Record Number:CaltechAUTHORS:20180207-084103683
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20180207-084103683
Official Citation:Yayaati Chachan and David J. Stevenson 2018 ApJ 854 21
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:84706
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:07 Feb 2018 16:53
Last Modified:20 Apr 2020 08:47

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