The violent collisional history of aqueously evolved (2) Pallas
Michaël Marsset, Miroslav Brož, Pierre Vernazza, Alexis Drouard, Julie Castillo-Rogez, Josef Hanuš, Matti Viikinkoski, Nicolas Rambaux, Benoît Carry, Laurent Jorda, Pavel Ševeček, Mirel Birlan, Franck Marchis, Edyta Podlewska-Gaca, Erik Asphaug, Przemyslaw Bartczak, Jérôme Berthier, Fabrice Cipriani, François Colas, Grzegorz Dudziński, Christophe Dumas, Josef Ďurech, Marin Ferrais, Romain Fétick, Thierry Fusco, Emmanuel Jehin, Mikko Kaasalainen, Agnieszka Kryszczynska, Philippe Lamy, Hervé Le Coroller, Anna Marciniak, Tadeusz Michalowski, Patrick Michel, Derek C. Richardson, Toni Santana-Ros, Paolo Tanga, Frédéric Vachier, Arthur Vigan, Olivier Witasse & Bin Yang
Nature Astronomy (2020)
Published: 10 February 2020
“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 measurement1 being compatible within error bars with the icy Ceres (2.16 ± 0.01 g cm−3)2 and the other3 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 imager5 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 (1σ uncertainty) is fully compatible with a CM chondrite-like body, as suggested by its spectral reflectance in the 3 μm wavelength region6. 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.”
