Detecting Oceans on Extrasolar Planets Using the Glint Effect

TL;DR

This paper investigates the potential to detect ocean glint on extrasolar planets by modeling Earth's brightness variations, considering atmospheric effects, and assessing observability with JWST and external occulters.

Contribution

It introduces a detailed simulation approach that accounts for atmospheric scattering and validates Earth observations, enhancing methods to identify ocean glint on exoplanets.

Findings

Earth's glint can increase brightness by up to 100% at crescent phases.

Glint detectability depends on orbital inclination and wavelength.

JWST with an external occulter could observe ocean glint signals.

Abstract

Glint, the specular reflection of sunlight off Earth's oceans, may reveal the presence of oceans on an extrasolar planet. As an Earth-like planet nears crescent phases, the size of the ocean glint spot increases relative to the fraction of illuminated disk, while the reflectivity of this spot increases. Both effects change the planet's visible reflectivity as a function of phase. However, strong forward scattering of radiation by clouds can also produce increases in a planet's reflectivity as it approaches crescent phases, and surface glint can be obscured by Rayleigh scattering and atmospheric absorption. Here we explore the detectability of glint in the presence of an atmosphere and realistic phase-dependent scattering from oceans and clouds. We use the NASA Astrobiology Institute's Virtual Planetary Laboratory 3-D line-by-line, multiple-scattering spectral Earth model to simulate Earth's broadband visible brightness and reflectivity over an orbit. Our validated simulations successfully reproduce phase-dependent Earthshine observations. We find that the glinting Earth can be as much as 100% brighter at crescent phases than simulations that do not include glint, and that the effect is dependent on both orbital inclination and wavelength, where the latter dependence is caused by Rayleigh scattering limiting sensitivity to the surface. We show that this phenomenon may be observable using the James Webb Space Telescope (JWST) paired with an external occulter.