Inferring Shallow Surfaces On sub-Neptune Exoplanets With JWST


The evolution of the mixing ratio profiles of NH3 (left) and HCN (right) with a 1-bar surface compared to a deep, 1000-bar atmosphere for an active (top) and a quiet (bottom) M star. The snapshot abundances at different time in the 1-bar surface model are color coded from red to blue. The abundances in the deep model do not evolve after 1 kyr because of the efficient thermochemistry in the deep atmosphere and are shown as solid grey lines. The black dotted curves indicate the initial equilibrium abundances. The upper panels are for an active M2 star at the age of 45 Myr and the lower panels for a quite M2 star at the age of 5 Gyr

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 (VULCAN) 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 (JWST).

Shang-Min Tsai, Hamish Innes, Tim Lichtenberg, Jake Taylor, Matej Malik, Katy Chubb, Raymond Pierrehumbert

Comments: Accepted for publication in ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2111.06429 [astro-ph.EP] (or arXiv:2111.06429v1 [astro-ph.EP] for this version)
Submission history
From: Shang-Min Tsai
[v1] Thu, 11 Nov 2021 19:21:06 UTC (1,649 KB)
https://arxiv.org/abs/2111.06429
Astrobiology

Please follow Astrobiology on Twitter.


  • submit to reddit