A New Spectral Library

for Modeling the Surfaces of Hot, Rocky Exoplanets

Kimberly Paragas,¹ Heather A. Knutson,¹ Renyu Hu ,^2,1 Bethany Ehlmann,¹ Giulia Alemanno,³
Jörn Helbert,³ Alessandro Maturilli,³ Michael Zhang,⁴ George Rossman,¹

¹Division of Geological and Planetary Sciences, California Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA
²Jet Propulsion Laboratory, 4800 Oak Grove Dr, Pasadena, CA 91101, USA
³Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstrasse 2, 12489, Berlin-Adlershof, Germany
⁴Department of Astronomy & Astrophysics, University of Chicago, 5640 S Ellis Ave, Chicago, IL 60637, USA

Abstract

JWST ’s Mid InfraRed Instrument (MIRI) provides a unique opportunity to spectroscopically characterize the surface compositions of terrestrial exoplanets for the first time. Close-in, rocky planets orbiting M dwarfs are ideal targets for these studies, and recent Spitzer and JWST measurements have shown that many have little to no atmosphere. There are currently six hot rocky exoplanets for which thermal emission spectra have been or will soon be measured with MIRI. Current models for the bare-rock spectra of these planets often utilize a spectral library from Hu et al. (2012), which consists of models calculated from room temperature measurements of powdered mineral and rock materials, meant to broadly represent archetypes of rocky planet surfaces. Here we present an expanded spectral library that includes hemispherical reflectance measurements for a greater variety of compositions, varying textures (solid slab, coarsely crushed, and fine powder), as well as high temperature (up to 830 K) emissivity measurements for select samples. We incorporate this new spectral library into the open-source retrieval package PLATON and use it to show that the predicted dayside temperatures for individual materials can vary significantly within each broad surface compositional type. Additionally, we demonstrate that changing the texture of a material, and therefore surface, can drastically alter its hemispherical reflectance and corresponding predicted dayside temperature, while temperature-dependent changes in spectral features are likely undetectable at the precision of current exoplanet observations. This new library illustrates the importance of spectroscopically resolved thermal emission measurements, as distinct from surface albedo constraints, for characterizing the surface compositions of hot, rocky exoplanets.