Detecting Lava Oceans on Hot Exoplanets Using the Glint Effect

Theory and models predict that extremely hot rocky exoplanets (T>850 K) could be covered with lava oceans. However, direct observational evidence of lava oceans remains elusive. Here we show that phase curves can be used to distinguish between planets with smooth, molten surfaces (lava-ocean) versus rough, solid surfaces (Moon- or Mercury-like). To do so, we argue that lava oceans should be smooth enough to exhbit specular reflection, which gives rise to an ocean "glint". We develop both numerical and analytical models which solve for the reflected and emitted light of a surface with specular versus Lambertian reflection. We show that the phase curve of a specular surface is much flatter than the well-known sinusoidal shape of a Lambert surface, and causes the phase curve amplitude to be noticeably smaller than the secondary eclipse depth. Incorporating Fresnel’s law, we predict that two peaks will appear near transit for low-albedo surfaces. Our results suggest that phase curve variations caused by the glint effect can be used to detect smooth, molten surfaces such as lava oceans. This detection method holds promise for characterization of hot rocky exoplanets with thin atmospheres using JWST.