The violent collisional history of aqueously evolved (2) Pallas

Abstract

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.

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Fig. 1: The two hemispheres of (2) Pallas as seen by VLT/SPHERE.
Fig. 2: Deconvolved images of (2) Pallas, compared with projections of the ADAM shape model and sketches highlighting the main geological features identified on Pallas.
Fig. 3: Mollweide projection of the 36 craters and the bright spot identified on the surface of Pallas.
Fig. 4: N-body simulations link the heavily cratered surface of Pallas to its highly inclined and eccentric orbit inducing a high average impact speed on this body.

Data availability

As soon as papers for our large programme are accepted for publication, we will make the corresponding reduced and deconvolved adaptive optics images and three-dimensional shape models publicly available at http://observations.lam.fr/astero/.

Code availability

The code used to generate the three-dimensional shape is available at https://github.com/matvii/ADAM. The modified SWIFT integrator used to model the orbital evolution of the Pallas family is available at http://sirrah.troja.mff.cuni.cz/mira/mp/.

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Acknowledgements

Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 199.C-0074 (principal investigator: P.V.). This research has made use of the Keck Observatory Archive (KOA), which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under contract with NASA. M.M. was supported by the National Aeronautics and Space Administration under grant number 80NSSC18K0849 issued through the Planetary Astronomy Program. This work was supported by the French Direction Générale de l’Armement (DGA) and Aix-Marseille Université (AMU). P.V., A.D. and B.C. were supported by CNRS/INSU/PNP. J.H., J.D. and P.S. were supported by the grant 18-09470S of the Czech Science Foundation and by the Charles University Research Programme number UNCE/SCI/023. M.Brož was supported by the grant 18-04514J of the Czech Science Foundation. E.J. is a F.R.S.-FNRS Senior Research Associate. The work of T.S.-R. was carried out through grant APOSTD/2019/046 by Generalitat Valenciana (Spain). This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 730890. This material reflects only the authors’ views and the commission is not liable for any use that may be made of the information contained herein.

Author information

P.V. is the principal investigator of the ESO large survey that acquired the images of Pallas. M.M. and P.V. designed and operated the survey in service mode. M.M. led the research on Pallas. M.M., P.V., R.F. and T.F. reduced and deconvolved the SPHERE images. A.D. performed the crater analysis. M.Brož analysed the Pallas family and ran the N-body and SPH simulations. D.C.R and E.A. provided some of the numerical codes used for the simulations. B.C. and J.H. retrieved earlier disk-resolved and disk-integrated data for Pallas from the literature. M.V. and J.H. reconstructed the three-dimensional shape of Pallas. N.R. and L.J. analysed the shape. B.C. provided the mass estimate. J.C.-R. performed the compositional analysis and thermophysical modelling of Pallas. M.M., M.Brož, P.V. and J.C.-R. worked jointly to write the manuscript. All authors discussed the results and commented on the manuscript.

Correspondence to Michaël Marsset.

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Marsset, M., Brož, M., Vernazza, P. et al. The violent collisional history of aqueously evolved (2) Pallas. Nat Astron (2020). https://doi.org/10.1038/s41550-019-1007-5

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