Detecting Ocean Glint on Exoplanets Using Multiphase Mapping

Abstract

Rotational mapping and specular reflection (glint) are two proposed methods to directly detect liquid water on the surface of habitable exoplanets. However, false positives for both methods may prevent the unambiguous detection of exoplanet oceans. We use simulations of Earth as an exoplanet to introduce a combination of multiwavelength, multiphase, time-series direct-imaging observations and accompanying analyses that may improve the robustness of exoplanet ocean detection by spatially mapping the ocean glint signal. As the planet rotates, the glint spot appears to "blink" as Lambertian scattering continents interrupt the specular reflection from the ocean. This manifests itself as a strong source of periodic variability in crescent-phase disk-integrated reflected light curves. We invert these light curves to constrain the longitudinal slice maps and apparent albedo of multiple surfaces at both quadrature and crescent phase. At crescent phase, the retrieved apparent albedo of ocean-bearing longitudinal slices is increased by a factor of 5, compared to the albedo at quadrature phase, due to the contribution from glint. The land-bearing slices exhibit no significant change in apparent albedo with phase. The presence of forward-scattering clouds in our simulated observation increases the overall reflectivity toward crescent, but we find that clouds do not correlate with any specific surfaces, thereby allowing for the phase-dependent glint effect to be interpreted as distinct from cloud scattering. Retrieving the same longitudinal map at quadrature and crescent phases may be used to tie changes in the apparent albedo with phase back to specific geographic surfaces (or longstanding atmospheric features), although this requires ideal geometries. We estimate that crescent-phase time-dependent glint measurements are feasible for between 1 and 10 habitable zone exoplanets orbiting the nearest G, K, and M dwarfs using a space-based, high-contrast, direct-imaging telescope with a diameter between 6 and 15 m.

Oxygen False Positives on Habitable Zone Planets Around Sun‐Like Stars

“…This limit arises because silicates cannot support their own weight with greater topography. Consequently, the detection of an ocean‐continent dichotomy using time‐resolved photometric mapping (Cowan et al., 2009; Farr et al., 2018; Fujii et al., 2010; Kawahara & Fujii, 2010; Lustig‐Yaeger et al., 2018) could rule out a waterworld false positive, assuming alternative explanations for dichotomies in surface maps could be excluded. This highlights the need for large aperture direct imaging mission to ensure sufficient time‐resolution to map the surface over a planet's rotation.…”

Global Mapping of an Exo-Earth Using Sparse Modeling

“…Since we do not have any information in surface types, we assume isotropic (Lambertian) reflection for simplicity in this article. We note that one can also attempt to find signatures of nonisotropic reflection from light curves, which might indicate the presence of particular surface types, e.g., ocean (Lustig-Yaeger et al 2018). Assuming isotropic (Lambertian) reflection from the surface, G is rewritten as follows:…”

The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST

“…If an atmosphere can be confirmed, then subsequent studies will focus on determining the nature of that atmosphere, and whether there are atmospheric characteristics that could discriminate between evolutionary outcomes. Finally, a deeper dive to search for signs of habitability, including the presence of an ocean (see Robinson 2018;Lustig-Yaeger et al 2018) and biosignatures (see Schwieterman et al 2018) may be warranted for planets whose initial characterization does not preclude habitable conditions.…”