The violent collisional history of aqueously evolved (2) Pallas

Asteroid (2) Pallas is the largest main-belt object not yet visited by a spacecraft, making its surface geology largely unknown and limiting our understanding of its origin and collisional evolution. Previous ground-based observational campaigns returned different estimates of its bulk density that are inconsistent with one another, one measurement 1 being compatible within error bars with the icy Ceres (2.16 ± 0.01 g cm −3 ) 2 and the other 3 compatible within error bars with the rocky Vesta (3.46 ± 0.03 g cm −3 ) 4 . Here we report high-angular-resolution observations of Pallas performed with the extreme adaptive optics-fed SPHERE imager 5 on the Very Large Telescope. Pallas records a violent collisional history, with numerous craters larger than 30 km in diameter populating its surface and two large impact basins that could be related to a family-forming impact. Monte Carlo simulations of the collisional evolution of the main belt correlate this cratering record to the high average impact velocity of ~11.5 km s −1 on Pallas—compared with an average of ~5.8 km s −1 for the asteroid belt—induced by Pallas’s high orbital inclination ( i = 34.8°) and orbital eccentricity ( e = 0.23). Compositionally, Pallas’s derived bulk density of 2.89 ± 0.08 g cm −3 (1 σ uncertainty) is fully compatible with a CM chondrite-like body, as suggested by its spectral reflectance in the 3 μm wavelength region 6 . A bright spot observed on its surface may indicate an enrichment in salts during an early phase of aqueous alteration, compatible with Pallas’s relatively high albedo of 12–17% (refs. 7 , 8 ), although alternative origins are conceivable.