Bayesian inversion of the Hapke model on (4) Vesta’s avalanches and ejecta: Photometric constraints on regolith evolution

Université Paris Cité, Institut de physique du globe de Paris, CNRS, 75005 Paris, France

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Received: 29 October 2025
Accepted: 11 May 2026

Abstract

Context. The Dawn mission revealed significant photometric variability on (4) Vesta, particularly in areas with craters, avalanches, cliffs, and ejecta. Understanding how surface processes control these variations is essential to understanding regolith evolution on airless bodies.

Aims. We test the hypothesis that the photometric behavior of bright units can be explained by granular segregation during mass wasting and impact emplacement rather than by subsequent optical maturation.

Methods. Using a Bayesian approach, we inverted a four-parameter Hapke model at two sites: bright avalanches in the Cornelia crater and a fresh ejecta deposit on the Matronalia Rupes scarp. For each geomorphological unit, we adjusted median reflectance factors for phase angles ranging from 10° to 70°. Because the available data do not identify the opposition effect robustly, the main inversion was complemented by sensitivity tests that use literature-based fixed shadow-hiding opposition effect (SHOE) prescriptions, which are reported explicitly for Cornelia. We retrieved the Hapke parameters {ω, θ^-, b, c} with full posteriors.

Results. Fresh, bright deposits at both sites exhibit higher single-scattering albedo (ω) than adjacent older or fine-depleted surfaces. The photometric roughness (θ^-) is likely higher for the Cornelia avalanches than the crater floor and opposite wall. At Cornelia, sensitivity tests show that ω, θ^-, and the relative ordering of terrains remain stable, whereas the absolute values of the phase-function parameters (b, c) depend more strongly on how SHOE is represented.

Conclusions. Granular segregation during emplacement explains the brightest units, while optical maturation and surface stabilization explain the darker ones. Even without direct opposition coverage, the baseline inversion and Cornelia SHOE sensitivity tests yield a consistent relative “freshness ranking” that complements morphological superposition. This ranking provides a transferable framework for interpreting regolith processes on airless bodies.