Inferring shallow surfaces on sub-Neptune exoplanets with JWST
Abstract
Planets smaller than Neptune and larger than Earth make up the majority of the discovered exoplanets. Those with H2-rich atmospheres are prime targets for atmospheric characterization. The transition between the two main classes, super-Earths and sub-Neptunes, is not clearly understood as the rocky surface is likely not accessible to observations. Tracking several trace gases (specifically the loss of ammonia (NH3) and hydrogen cyanide (HCN)) has been proposed as a proxy for the presence of a shallow surface. In this work, we revisit the proposed mechanism of nitrogen conversion in detail and find its timescale on the order of a million years. NH3 exhibits dual paths converting to N2 or HCN, depending on the UV radiation of the star and the stage of the system. In addition, methanol (CH3OH) is identified as a robust and complementary proxy for a shallow surface. We follow the fiducial example of K2-18b with a 2D photochemical model on an equatorial plane. We find a fairly uniform composition distribution below 0.1 mbar controlled by the dayside, as a result of slow chemical evolution. NH3 and CH3OH are concluded to be the most unambiguous proxies to infer surfaces on sub-Neptunes in the era of the James Webb Space Telescope.
Citation
Tsai , S-M , Innes , H , Lichtenberg , T , Taylor , J , Malik , M , Chubb , K & Pierrehumbert , R 2021 , ' Inferring shallow surfaces on sub-Neptune exoplanets with JWST ' , Astrophysical Journal Letters , vol. 922 , no. 2 , L27 . https://doi.org/10.3847/2041-8213/ac399a
Publication
Astrophysical Journal Letters
Status
Peer reviewed
ISSN
2041-8205Type
Journal article
Rights
Copyright © 2021. The Author(s). Published by the American Astronomical Society. Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
Description
Funding: S.-M.T. acknowledges support from the European community through the ERC advanced grant EXOCONDENSE (#740963; PI: R.T. Pierrehumbert). T.L. has been supported by the Simons Foundation (SCOL award #611576).Collections
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