Mapping the surface of partially cloudy exoplanets is hard

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 disk-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 then apply it to real disk-integrated observations of the Earth. We find that eight days of continuous synthetic observations are sufficient to reconstruct a faithful low resolution surface albedo map, without needing to make assumptions about cloud physics. For lightcurves 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 DSCOVR 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.

Synergies Between Venus & Exoplanetary Observations

“…Unfortunately in the near term it could be that a modern Venus-like cloud and haze layer will prevent JWST from resolving atmospheric species that could give clues to exoplanetary atmospheric evolution histories. Clouds in general make observing even major species very challenging with JWST (Fauchez et al 2019;Teinturier et al 2022), although there may be some opportunities when observing more arid planets with fewer clouds (Ding and Wordsworth 2022). Thirdly, can we discern the longevity of any postulated climate state in Venus' history?…”

Measuring the variability of directly imaged exoplanets using vector Apodizing Phase Plates combined with ground-based differential spectrophotometry

“…Further in the future, multi-wavelength variability measurements obtained in reflected light may even enable exocartography of directly imaged Earth-like exoplanets (e.g. Luger et al 2019Luger et al , 2022Kawahara 2020;Kuwata et al 2022;Teinturier et al 2022).…”

Blue Marble, Stagnant Lid: Could Dynamic Topography Avert a Waterworld?

“…Proposed next-generation direct imaging missions might be capable of enough precision for the exo-cartography of small planets-solving the inverse problem of 2D albedo distributions from time-resolved light curves-which might discriminate between land and ocean surfaces (Cowan & Fujii 2018;Farr et al 2018;Lustig-Yaeger et al 2018;Aizawa et al 2020;Kawahara 2020). Interpretations of the data may remain highly model dependent and burdened by cloud removal, however (Paradise et al 2021;Teinturier et al 2022). Ocean fractions might also be discerned from near-infrared polarimetric observations (Takahashi et al 2021).…”