An Analytical Model of Wavelength-dependent Opposition Surge in Emittance and Reflectance Spectroscopy of Airless Rocky Exoplanets

TL;DR

This paper introduces a new wavelength-dependent opposition surge model for exoplanet surface spectroscopy, highlighting its importance for accurate surface property retrievals, especially at small phase angles relevant to JWST observations.

Contribution

It proposes a physically consistent opposition surge formulation that improves phase curve and spectral modeling of airless rocky exoplanets, addressing a key neglected effect in current models.

Findings

Neglecting opposition effects causes up to 30% errors in spectral retrievals.

The new model improves accuracy of surface albedo and mineralogical estimates.

Opposition effects are critical for interpreting phase curves at small angles.

Abstract

Theoretical frameworks for reflection and emission spectroscopy of exoplanet surfaces are becoming increasingly important for the characterization of rocky exoplanets, especially with the rapid growth of the detected exoplanet population and observational capabilities. The Hapke theory of reflectance and emittance spectroscopy has been widely adopted in the exoplanet community, yet a key physical effect - the opposition surge enhancement at small phase angles - remains largely neglected. This phenomenon, driven by shadow hiding and coherent backscattering, introduces a significant brightening that depends on wavelength, particle size, and surface morphology. In this paper, I propose an alternative formulation for opposition surge modeling, ensuring a smooth-to-sharp transition at small phase angles, dictated by wavelength-dependent scattering properties. I evaluate the impact of opposition surge on phase curves and surface spectra, comparing a family of models with increasing simplifications, ranging from a full wavelength-dependent opposition effect to its complete omission. My results indicate that neglecting opposition effects can introduce systematic deviations in retrieved albedos, spectral features, and phase curves, with errors reaching up to 20%-30% in certain spectral bands. Upcoming JWST observations will probe phase angles below ~10{\deg} for rocky exoplanets around M dwarfs; thus, accounting for opposition effects is crucial for accurate surface characterization. Proper treatment of this effect will lead to improved retrievals of surface albedo, mineralogical composition, and roughness properties. This study establishes a physically consistent framework for exoplanet phase-curve modeling and provides a foundation for future retrieval algorithms aimed at interpreting exoplanet surfaces.