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
June 7, 2024
Filed under , , , , , , , , , ,
Chemical Mapping of Temperate Sub-Neptune Atmospheres: Constraining The Deep-interior H2O/H2 Using The Atmospheric CO2/CH4
Comparisons between the theoretical transmission spectra generated by EPACRIS (solid lines) and the JWST observations for (Top) H2:H2O=50:50 and [C+N+S]=100×solar metallicity scenario of K2-18 b assuming Ab = 0.3 corresponding to the left panel of Figure 4, and (Bottom) H2:H2O=75:25 and [C+N+S]=100×solar metallicity scenario of TOI-270 d assuming Ab = 0 corresponding to the right panel of Figure 4. The grey symbols with error bars indicate JWST observations of corresponding exoplanets taken from Madhusudhan et al. (2023) for K2-18 b (Top), and Benneke et al. (2024) for TOI-270 d (Bottom). Each color represents a spectrum generated by excluding specific species: green for no CH4, red for no CO2, light blue for no OCS, blue for no H2O, teal for no CO, light brown for no NH3, and black for all species included. — astro-ph.EP

Understanding the envelope composition of sub-Neptune-type exoplanets is challenging due to the inherent degeneracy in their interior composition scenarios. Particularly, the H2O/H2 ratio, or can be expressed as the O/H ratio, in the planetary envelope provides crucial insights into the origin of these exoplanets relative to the ice line during 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 atmospheres of temperate sub-Neptunes, ranging from H2-dominated to H2O-dominated scenarios with Teq = 250-400 K, using K2-18 b (Teq = 255 K), LP 791-18 c (Teq = 324 K), and TOI-270 d (Teq = 354 K) as examples. Our models indicate that using the atmospheric CO2/CH4 ratio to infer the deep-interior H2O/H2 ratio.

Applying to recent JWST observations, our findings suggest K2-18 b likely has an interior highly enriched in water (approximately 50% H2O), exceeding the amount of water in a 100x solar metallicity scenario and suggesting a formation history that involved substantial accretion of ices. In contrast, TOI-270 d has an interior composition of approximately 25% H2O, which is comparable to the conventional metallicity framework with a metallicity higher than 100x solar metallicity. Furthermore, our models identify carbonyl sulfide (OCS) and sulfur dioxide (SO2) as strong indicators of at least a 10% water-rich envelope in temperate sub-Neptunes.

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

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. 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

Jeehyun Yang, Renyu Hu

Comments: 15 pages, 5 figures, submitted to ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2406.01955 [astro-ph.EP] (or arXiv:2406.01955v1 [astro-ph.EP] for this version)
Submission history
From: Jeehyun Yang
[v1] Tue, 4 Jun 2024 04:26:56 UTC (3,181 KB)
Astrobiology, Astrochemistry,

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) 🖖🏻