Mapping the surface of partially cloudy exoplanets is hard. Issue 1 (8th January 2022)

• Record Type: Journal Article
• Authors: Teinturier, Lucas; Vieira, Nicholas; Jacquet, Elisa; Geoffrion, Juliette; Bestavros, Youssef; Keating, Dylan; Cowan, Nicolas B
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 511:Issue 1(2022)
• Page Start: 440
• Page End: 447
• Publication Date: 2022-01-08
• Subjects: methods: numerical -- planets and satellites: atmospheres -- planets and satellites: surfaces; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac030
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: Reflected light photometry of terrestrial exoplanets could reveal the presence of oceans and continents, hence placing direct constraints on the current and long-term habitability of these worlds. Inferring the albedo map of a planet from its observed light curve is challenging because different maps may yield indistinguishable light curves. This degeneracy is aggravated by changing clouds. It has previously been suggested that disc-integrated photometry spanning multiple days could be combined to obtain a cloud-free surface map of an exoplanet. We demonstrate this technique as part of a Bayesian retrieval by simultaneously fitting for the fixed surface map of a planet and the time-variable overlying clouds. We test this approach on synthetic data and then apply it to real disc-integrated observations of the Earth. We find that 8 d of continuous synthetic observations are sufficient to reconstruct a faithful low-resolution surface albedo map, without needing to make assumptions about cloud physics. For light curves with negligible photometric uncertainties, the minimal top-of-atmosphere albedo at a location is a good estimate of its surface albedo. When applied to observations from the Earth Polychromating Imaging Camera aboard the Deep Space Climate Observatory spacecraft, our approach removes only a small fraction of clouds. We attribute this difficulty to the full-phase geometry of observations combined with the short correlation length for Earth clouds. For exoplanets with Earth-like climatology, it may be hard to do much better than a cloud-averaged map. We surmise that cloud removal will be most successful for exoplanets imaged near quarter phase that harbour large cloud systems.