Theoretical Spectra of Terrestrial Exoplanet Surfaces

TL;DR

This paper presents a theoretical framework for detecting and characterizing rocky exoplanet surfaces through spectral features in thermal emission and reflectance, enabling identification of surface composition and presence of water or ice.

Contribution

It introduces a self-consistent model for exoplanet spectra that distinguishes surface features from atmospheric signatures, and proposes specific spectral indicators for surface composition.

Findings

Silicate surfaces produce detectable Si-O features in thermal emission bands.

Brightness temperature variations can reach up to 20 K due to silicate features.

Reflectance spectra can differentiate between rocky and icy surfaces based on albedo differences.

Abstract

We investigate spectra of airless rocky exoplanets with a theoretical framework that self-consistently treats reflection and thermal emission. We find that a silicate surface on an exoplanet is spectroscopically detectable via prominent Si-O features in the thermal emission bands of 7 - 13 \mu m and 15 - 25 \mu m. The variation of brightness temperature due to the silicate features can be up to 20 K for an airless Earth analog, and the silicate features are wide enough to be distinguished from atmospheric features with relatively high-resolution spectra. The surface characterization thus provides a method to unambiguously identify a rocky exoplanet. Furthermore, identification of specific rocky surface types is possible with the planet's reflectance spectrum in near-infrared broad bands. A key parameter to observe is the difference between K band and J band geometric albedos (A_g (K)-A_g (J)): A_g (K)-A_g (J) > 0.2 indicates that more than half of the planet's surface has abundant mafic minerals, such as olivine and pyroxene, in other words primary crust from a magma ocean or high-temperature lavas; A_g (K)-A_g (J) < -0.09 indicates that more than half of the planet's surface is covered or partially covered by water ice or hydrated silicates, implying extant or past water on its surface. Also, surface water ice can be specifically distinguished by an H-band geometric albedo lower than the J-band geometric albedo. The surface features can be distinguished from possible atmospheric features with molecule identification of atmospheric species by transmission spectroscopy. We therefore propose that mid-infrared spectroscopy of exoplanets may detect rocky surfaces, and near-infrared spectrophotometry may identify ultramafic surfaces, hydrated surfaces and water ice.