Using images acquired by NASA's Dawn mission, researchers focused on two regions of Vesta: bright avalanches in the Cornelia crater and a fresh ejecta deposit along the scarp of Matronalia Rupes.
These terrains exhibit strong brightness differences already visible in the images, but their interpretation remains delicate: a surface can appear bright because it has just been excavated, because it contains more fine particles, because it is rougher, or because it has been exposed to the space environment for a shorter time.
A study conducted at IPGP uses images from the Dawn mission and a Bayesian inversion of the Hapke model to analyze avalanches and ejecta on the asteroid Vesta. The results show that the brightest deposits correspond to the most recently mobilized surfaces, offering a new way to track regolith evolution on airless bodies. To disentangle these effects, the team used a Hapke photometric model, which relates the light reflected by a surface to the illumination and observation geometry. This model allows estimation of several effective properties of the regolith, such as its scattering power, photometric roughness, or how grains scatter light. The originality of the study is having integrated this model into a Bayesian approach, in order to obtain not a single value for each parameter, but probability distributions and thus an explicit estimate of uncertainties.
The results show that the brightest deposits are also the most recent in a geomorphological sense. In the Cornelia crater, the avalanches exhibit a higher single-scattering albedo than the crater floor and the opposite wall. At Matronalia Rupes, the fresh ejecta is brighter than the associated small crater and the scarp slope. This hierarchy remains stable even when researchers test different hypotheses about the opposition effect, a phenomenon that can amplify observed brightness when the surface is viewed almost in the direction of solar illumination.
These contrasts suggest that the bright deposits result mainly from recent mechanical processes: avalanches, impact excavation, and granular segregation during emplacement. The brighter materials may correspond to fresher, less altered regolith, or to a redistribution of fine and coarse particles during flow. Conversely, darker terrains reflect a more evolved surface, affected by space weathering, mixing with older materials, or a relative loss of fine particles.
The study thus shows that photometry can complement classical morphological analysis. Images indicate where avalanches, ejecta, or slopes are located; the light they reflect provides additional information about their surface state and relative degree of evolution. Even when certain properties remain difficult to constrain absolutely, the approach provides a robust ranking of deposit "freshness." This framework can be applied to other airless bodies, where surfaces record the combined history of impacts, mass movements, and space weathering.
Reference
D. T. Nguyen, A. Roque-Bernard, A. Lucas, S. Jacquemoud & C. Ferrari, “Bayesian inversion of the Hapke model on (4) Vesta's avalanches and ejecta: photometric constraints on regolith evolution”, Astronomy & Astrophysics, to be published. DOI: 10.1051/0004-6361/202557890.