A False Positive For Ocean Glint on Exoplanets: the Latitude-Albedo Effect

TL;DR

This paper reveals that apparent ocean glint signals on exoplanets can be false positives caused by the latitude-albedo effect, which results from the planet's obliquity and viewing geometry, complicating ocean detection.

Contribution

The study demonstrates that latitude-dependent albedo variations can mimic ocean glint signals, highlighting the need for surface mapping to accurately interpret exoplanet reflected light.

Findings

False glint signals can arise without specular reflection due to latitude-albedo effects.

The effect is prominent in zero-obliquity planets, complicating ocean detection.

Surface mapping is essential for correctly interpreting phase variations.

Abstract

Identifying liquid water on the surface of planets is a high priority, as this traditionally defines habitability. One proposed signature of oceans is specular reflection ("glint"), which increases the apparent albedo of a planet at crescent phases. We post-process a global climate model of an Earth-like planet to simulate reflected lightcurves. Significantly, we obtain glint-like phase variations even though we do not include specular reflection in our model. This false positive is the product of two generic properties: 1) for modest obliquities, a planet's poles receive less orbit-averaged stellar flux than its equator, so the poles are more likely to be covered in highly reflective snow and ice, and 2) we show that reflected light from a modest-obliquity planet at crescent phases probes higher latitudes than at gibbous phases, therefore a planet's apparent albedo will naturally increase at crescent phase. We suggest that this "latitude-albedo effect" will operate even for large obliquities: in that case the equator receives less orbit-averaged flux than the poles, and the equator is preferentially sampled at crescent phase. Using rotational and orbital color variations to map the surfaces of directly imaged planets and estimate their obliquity will therefore be a necessary pre-condition for properly interpreting their reflected phase variations. The latitude-albedo effect is a particularly convincing glint false positive for zero-obliquity planets, and such worlds are not amenable to latitudinal mapping. This effect severely limits the utility of specular reflection for detecting oceans on exoplanets.