Low Albedo Surfaces of Lava Worlds

Abstract

Hot super Earths are exoplanets with short orbital periods ($<$ 10 days), heated by their host stars to temperatures high enough for their rocky surfaces to become molten. A few hot super Earths exhibit high geometric albedos ($>$ 0.4) in the Kepler band (420-900 nm). We are motivated to determine whether reflection from molten lava and quenched glasses (a product of rapidly cooled lava) on the surfaces of hot super Earths contributes to the observationally inferred high geometric albedos. We experimentally measure reflection from rough and smooth textured quenched glasses of both basalt and feldspar melts. For lava reflectance values, we use specular reflectance values of molten silicates from non-crystalline solids literature. Integrating the empirical glass reflectance function and non-crystalline solids reflectance values over the dayside surface of the exoplanet at secondary eclipse yields an upper limit for the albedo of a lava-quenched glass planet surface of $\sim$0.1. We conclude that lava planets with solid (quenched glass) or liquid (lava) surfaces have low albedos. The high albedos of some hot super Earths are most likely explained by atmospheres with reflective clouds (or, for a narrow range of parameter space, possibly Ca/Al oxide melt surfaces). Lava planet candidates in TESS data can be identified for follow-up observations and future characterization.

18 pages, 14 figures, 4 tables, published in ApJ

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• Hot Rocks Survey II: The thermal emission of TOI-1468 b reveals a hot bare rock
• 55 Cancri e's occultation captured with CHEOPS
• Convective shutdown in the atmospheres of lava worlds
• Population-level Hypothesis Testing with Rocky Planet Emission Data: A Tentative Trend in the Brightness Temperatures of M-Earths
• CHEOPS and TESS view of the ultra-short period super-Earth TOI-561 b
• A general machine learning model of aluminosilicate melt viscosity and its application to the surface properties of dry lava planets
• Low 4.5 μm Dayside Emission Disfavors a Dark Bare-Rock scenario for the Hot Super-Earth TOI-431 b
• Clouds in partial atmospheres of lava planets and where to find them
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• The impact of ultraviolet heating and cooling on the dynamics and observability of lava planet atmospheres
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• An internal heating mechanism operating in ultra-short-period planets orbiting magnetically active stars