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 contribute to the observationally inferred high geometric albedos. We experimentally measure reflection from rough-and smoothtextured 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 ∼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.

Exploring Super-Earth Surfaces: Albedo of Near-Airless Magma Ocean Planets and Topography

“…The inferred high spherical albedo values of several hot super-Earths (Demory 2014;Malavolta 2018) are well above what the current literature predicts for magmatic surfaces (Essack et al 2020;Rouan et al 2011;Edgett and Rice 1997). Two main explanations have been put forward: reflective exotic magmas (Rouan et al 2011) and reflective volcanic/mineral atmospheres (Hamano et al 2015;Pluriel et al 2019).…”

“…We also lack experimental data on how the refractive indices evolve when there is a phase change which could be problematic as we use the refractive indices for the solid phases whilst magmas are fluids. However, as shown by Essack et al (2020), the albedo of 100% molten and 100% solidified basaltic glass remains relatively constant which implies that the refractive indices might not vary too strongly. In any case, the dependence of the refractive indices on the temperature and phase of the materials are poorly constrained and warrant more research.…”

ProbingKepler’s hottest small planets via homogeneous search and analysis of optical secondary eclipses and phase variations

“…Alternatively, for near-airless planets, high albedos could be the result of specular reflections from the moderately wavy lava surfaces made of metallic species such as iron oxides (Modirrousta-Galian et al 2021). However, low albedos (A g 0.1) have also been predicted for lava-ocean planets under a different theoretical framework by Essack et al (2020). In this case, the occultation signal would be mostly due to high thermal emission (e.g.…”

Detectability of Rocky-Vapour atmospheres on super-Earths with Ariel

“…In agreement with the previous study [25], we assume a volatile-free Bulk Silicate Earth (BSE) composition as the magma composition and zero as the albedo value. As the albedo of molten silicates is as low as ≤0.1 [14,41], we neglect this effect as it would not significantly affect the surface temperature. If one considers optically thin atmospheres and the albedo value, A, of 0.1, the temperature change is only a few percent as…”