Abstract
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.
Highlights
Sub-Neptune-sized planets (Rp ∼ 1.6–3.5 R⊕ ) constitute the main population of exoplanets we have discovered (Hsu et al 2019)
(0.25 ρ⊕ – 0.75 ρ⊕ ) planets can either consist of a dense core enclosed by a massive H2 /He envelope or a lighter core with a shallow H2 -rich atmosphere
We find that the trends of NH3 and HCN destruction remain robust
Summary
Sub-Neptune-sized planets (Rp ∼ 1.6–3.5 R⊕ ) constitute the main population of exoplanets we have discovered (Hsu et al 2019). Their formation (Bean et al.2021), atmospheric composition (Moses et al 2013), and interior structure (Madhusudhan et al 2020; Aguichine et al 2021) are not well understood. The low-density (0.25 ρ⊕ – 0.75 ρ⊕ ) planets can either consist of a dense core enclosed by a massive H2 /He envelope or a lighter core (dominated by rocky material and/or water) with a shallow H2 -rich atmosphere
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