[en] 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[SUP]1[/SUP] being compatible within error bars with the icy Ceres (2.16 ± 0.01 g cm[SUP]-3[/SUP])[SUP]2[/SUP] and the other[SUP]3[/SUP] compatible within error bars with the rocky Vesta (3.46 ± 0.03 g cm[SUP]-3[/SUP])[SUP]4[/SUP]. Here we report high- angular-resolution observations of Pallas performed with the extreme adaptive optics-fed SPHERE imager[SUP]5[/SUP] 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[SUP]-1[/SUP] on Pallas—compared with an average of ~5.8 km s[SUP]-1[/SUP] 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[SUP]-3[/SUP] (1σ uncertainty) is fully compatible with a CM chondrite-like body, as suggested by its spectral reflectance in the 3 μm wavelength region[SUP]6[/SUP]. 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. [SUP]7,8[/SUP]), although alternative origins are conceivable.
Disciplines :
Space science, astronomy & astrophysics
Author, co-author :
Marsset, Michaël; Department of Earth, Atmospheric and Planetary Sciences, MIT, Cambridge, MA, USA ; Astrophysics Research Centre, Queen's University Belfast, Belfast, UK
Brož, Miroslav; Institute of Astronomy, Charles University, Prague, Czech Republic
Vernazza, Pierre; Aix Marseille Univ, CNRS, CNES, Laboratoire d'Astrophysique de Marseille, Marseille, France
Drouard, Alexis; Aix Marseille Univ, CNRS, CNES, Laboratoire d'Astrophysique de Marseille, Marseille, France
Castillo-Rogez, Julie; Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
Hanuš, Josef; Institute of Astronomy, Charles University, Prague, Czech Republic
Viikinkoski, Matti; Mathematics and Statistics, Tampere University, Tampere, Finland
Rambaux, Nicolas; IMCCE, Observatoire de Paris, Paris, France
Carry, Benoît; Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France
Jorda, Laurent; Aix Marseille Univ, CNRS, CNES, Laboratoire d'Astrophysique de Marseille, Marseille, France
Ševeček, Pavel; Institute of Astronomy, Charles University, Prague, Czech Republic
Birlan, Mirel; IMCCE, Observatoire de Paris, Paris, France ; Astronomical Institute of the Romanian Academy, Bucharest, Romania
Marchis, Franck; SETI Institute, Carl Sagan Center, Mountain View, CA, USA
Podlewska-Gaca, Edyta; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Poznań, Poland ; Institute of Physics, University of Szczecin, Szczecin, Poland
Asphaug, Erik; School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
Bartczak, Przemyslaw; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Poznań, Poland
Berthier, Jérôme; IMCCE, Observatoire de Paris, Paris, France
Cipriani, Fabrice; European Space Agency, ESTEC - Scientific Support Office, Noordwijk, The Netherlands
Colas, François; IMCCE, Observatoire de Paris, Paris, France
Dudziński, Grzegorz; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Poznań, Poland
Dumas, Christophe; TMT Observatory, Pasadena, CA, USA
Durech, Josef; Institute of Astronomy, Charles University, Prague, Czech Republic
Ferrais, Marin ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Origines Cosmologiques et Astrophysiques (OrCa)
Fétick, Romain; Aix Marseille Univ, CNRS, CNES, Laboratoire d'Astrophysique de Marseille, Marseille, France
Fusco, Thierry; Aix Marseille Univ, CNRS, CNES, Laboratoire d'Astrophysique de Marseille, Marseille, France ; ONERA, The French Aerospace Lab, Châtillon, France
Jehin, Emmanuel ; Université de Liège - ULiège > Département d'astrophys., géophysique et océanographie (AGO) > Origines Cosmologiques et Astrophysiques (OrCa)
Kaasalainen, Mikko; Mathematics and Statistics, Tampere University, Tampere, Finland
Kryszczynska, Agnieszka; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Poznań, Poland
Lamy, Philippe; Aix Marseille Univ, CNRS, CNES, Laboratoire d'Astrophysique de Marseille, Marseille, France
Le Coroller, Hervé; Aix Marseille Univ, CNRS, CNES, Laboratoire d'Astrophysique de Marseille, Marseille, France
Marciniak, Anna; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Poznań, Poland
Michalowski, Tadeusz; Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Poznań, Poland
Michel, Patrick; Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France
Richardson, Derek C.; Department of Astronomy, University of Maryland, College Park, MD, USA
Santana-Ros, Toni; Departamento de Física, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Alicante, Spain ; Institut de Ciències del Cosmos, Universitat de Barcelona (IEEC-UB), Barcelona, Spain
Tanga, Paolo; Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France
Vachier, Frédéric; IMCCE, Observatoire de Paris, Paris, France
Vigan, Arthur; Aix Marseille Univ, CNRS, CNES, Laboratoire d'Astrophysique de Marseille, Marseille, France
Witasse, Olivier; European Space Agency, ESTEC - Scientific Support Office, Noordwijk, The Netherlands
Yang, Bin; European Southern Observatory (ESO), Santiago, Chile)
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