The impact of asteroid shapes and topographies on their reflectance spectroscopy

TL;DR

This study compares laboratory and simulated unresolved reflectance spectroscopy of small Solar System bodies, revealing how shape and topography influence spectral observations and phase curves, with implications for remote sensing.

Contribution

It introduces a method to simulate and analyze the impact of asteroid shape and topography on their reflectance spectra using physical models and synthetic observations.

Findings

Spectral differences increase at wide phase angles, especially for bodies with high topography.

Both Hapke and MRTLS models accurately reproduce spectra, with MRTLS producing less noise.

Surface topography significantly affects the spectral parameters derived from unresolved observations.

Abstract

We report the comparison between unresolved reflectance spectroscopy of Solar System small bodies and laboratory measurements on reference surfaces. We measure the bidirectional reflectance spectroscopy of a powder of howardite and a sublimation residue composed of a Ceres analogue. The spectra are then inverted using the Hapke semi-empirical physical model and the MRTLS parametric model to be able to simulate the reflectance of the surfaces under any geometrical configuration needed. We note that both models enable an accurate rendering of the reflectance spectroscopy, but the MRTLS model adds less noise on the spectra compared to the Hapke model. Using the parameters resulting from the inversions, we simulate two spherical bodies and the small bodies (1)Ceres and (4)Vesta whose surfaces are homogeneously covered with the Ceres analogue and powder of howardite respectively. We simulate various scenarios of illumination and spectroscopic observations, spot-pointing and fly-bys, of these small bodies for phases angles between 6{\deg} and 135{\deg}. The unresolved reflectance spectroscopy of the simulated bodies is retrieved from the resulting images, and compared to the reflectance spectroscopy of the reference surface. Our results show that the photometric phase curves of the simulated bodies are different from the reference surfaces because of the variations of the local incidence and emergence angles due to the shape and topography of the surface. We observe the maximum differences at wide phase angles with the various simulated observations of (4)Vesta due to its high surface topography. Finally, we highlight the differences in the spectral parameters derived from the unresolved observations at 30{\deg} with laboratory measurements acquired under a single geometrical configuration.