Exoplanetology: Exoplanets & Exomoons

Chemical Mapping Of Temperate sub-Neptune Atmospheres: Constraining the Deep-interior H2O/H2 Using the Atmospheric CO2/CH4

By Keith Cowing
Status Report
astro-ph.EP
August 5, 2024
Filed under , , ,
Chemical Mapping Of Temperate sub-Neptune Atmospheres: Constraining the Deep-interior H2O/H2 Using the Atmospheric CO2/CH4
The figure shows a schematic diagram of the feeding zone of planets in the protoplanetary disk with respect to the ice-line on the top and the upper atmospheric (P=0.1–2 mbar) CO2+CH4 mixing ratios plotted against the deep interior (P=20 bar) H2O-to-H2 ratios on the bottom. It should be noted that the position of the ice-line is depicted arbitrarily to better illustrate the concept of the current study. All notations follow Figure 1. The only difference is that the constraints on the H2O/H2 ratio for each planet are now based on the combination of the CO2 /CH4 ratio (Section 3.1.1) and individual abundances (Section 3.1.2). — astro-ph.EP

Understanding the planetary envelope composition of sub-Neptune-type exoplanets is challenging due to the inherent degeneracy in their interior composition scenarios. Particularly, the planetary envelope’s H2O/H2 ratio, which can also be expressed as the O/H ratio, provides crucial insights into its original location relative to the ice line during planetary formation.

Using self-consistent radiative transfer modeling and a rate-based automatic chemical network generator combined with 1D photochemical kinetic-transport atmospheric modeling, we investigate various atmospheric scenarios of temperate sub-Neptunes, ranging from H2-dominated to H2O-dominated atmospheres with equilibrium temperatures (Teq) of 250-400 K.

This study includes examples such as K2-18 b (Teq = 255 K), LP 791-18 c (Teq = 324 K), and TOI-270 d (Teq = 354 K). Our models indicate that the atmospheric CO2/CH4 ratio can be used to infer the deep-interior H2O/H2 ratio. Applying this method to recent JWST observations, our findings suggest K2-18 b likely has an interior that is 50% highly enriched in water, exceeding the water content in a 100xZ scenario and suggesting a planetary formation mechanism involving substantial accretion of ices.

In contrast, our model suggests that approximately 25% of TOI-270 d’s interior is composed of H2O, which aligns with the conventional metallicity framework with a metallicity higher than 100xZ. Furthermore, our models identify carbonyl sulfide (OCS) and sulfur dioxide (SO2) as strong indicators for temperate sub-Neptunes with at least 10% of their interior composed of water.

These results provide a method to delineate the internal composition and formation mechanisms of temperate sub-Neptunes (Teq < ~500 K) via atmospheric characterization through transmission spectroscopy.

Jeehyun Yang, Renyu Hu

Comments: 12 pages, 5 figures, accepted to be published in ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2406.01955 [astro-ph.EP] (or arXiv:2406.01955v3 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2406.01955
Focus to learn more
Submission history
From: Jeehyun Yang
[v1] Tue, 4 Jun 2024 04:26:56 UTC (3,181 KB)
[v2] Sun, 28 Jul 2024 19:49:18 UTC (7,307 KB)
[v3] Fri, 2 Aug 2024 15:49:18 UTC (3,949 KB)
https://arxiv.org/abs/2406.01955
Astrobiology

Keith Cowing

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻

Follow on Twitter
MORE FROM Exoplanetology: Exoplanets & Exomoons