Abstract
High-resolution observations of the third largest asteroid, (2) Pallas, from SPHERE unveil a heavily cratered surface, probably due to Pallas's inclined and eccentric orbit, a density almost equal to carbonaceous chondrites and hint at surficial salt-enriched spots. 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 similar to 11.5 km s(-1) on Pallas-compared with an average of similar to 5.8 km s(-1) for the asteroid belt-induced by Pallas's high orbital inclination (i = 34.8 degrees) and orbital eccentricity (e = 0.23).
Compositionally, Pallas's derived bulk density of 2.89 +/- 0.08 g cm(-3) (1 sigma uncertainty) is fully compatible with a CM chondrite-like body, as suggested by its spectral reflectance in the 3 mu m 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.