The Role of Clouds on the Depletion of Methane and Water Dominance in the Transmission Spectra of Irradiated Exoplanets

Temperature profiles and cloud compositions in the solar system’s gaseous planets. Earth’s and Venus’s pro- files are included for reference. The horizontal dashed line marks 1 bar level where most of the visible cloud decks are located at. This pressure is therefore commonly used as a reference pressure in the solar system community. The temperature profiles are adapted from Robinson & Catling (2014).

Observations suggest an abundance of water and paucity of methane in the majority of observed exoplanetary atmospheres. We isolate the effect of atmospheric processes to investigate possible causes.

Previously, we studied the effect of effective temperature, surface gravity, metallicity, carbon-to-oxygen ratio, and stellar type assuming cloud-free thermochemical equilibrium and disequilibrium chemistry. However, under these assumptions, methane remains a persisting spectral feature in the transmission spectra of exoplanets over a certain parameter space, the Methane Valley.

In this work we investigate the role of clouds on this domain and we find that clouds change the spectral appearance of methane in two direct ways: 1) by heating-up the photosphere of colder planets, and 2) by obscuring molecular features. The presence of clouds also affects methane features indirectly: 1) cloud heating results in more evaporation of condensates and hence releases additional oxygen, causing water dominated spectra of colder carbon-poor exoplanets, and 2) HCN/CO production results in a suppression of depleted methane features by these molecules. The presence of HCN/CO and a lack of methane could be an indication of cloud formation on hot exoplanets. Cloud heating can also deplete ammonia. Therefore, a simultaneous depletion of methane and ammonia is not unique to photochemical processes.

We propose that the best targets for methane detection are likely to be massive but smaller planets with a temperature around 1450 K orbiting colder stars. We also construct Spitzer synthetic color-maps and find that clouds can explain some of the high contrast observations by IRAC's channel 1 and 2.

Karan Molaverdikhani, Thomas Henning, Paul Mollière
Comments: 24 pages, 19 figures, accepted in ApJ (relevant papers: arXiv:1809.09629 and arXiv:1908.09847)
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2007.06562 [astro-ph.EP] (or arXiv:2007.06562v1 [astro-ph.EP] for this version)
Submission history
From: Karan Molaverdikhani
[v1] Mon, 13 Jul 2020 17:59:57 UTC (9,212 KB)

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