A basin-free spherical shape as an outcome of a giant impact on asteroid Hygiea

P. Vernazza (1) , L. Jorda (1) , P Ševeček (2) , M. Brož (2) , M. Viikinkoski (3) , J. Hanuš (2) , B. Carry (4) , A. Drouard (5) , M. Ferrais (6) , M. Marsset (7) , F. Marchis (8) , M. Birlan (9) , E. Podlewska-Gaca (10) , E. Jehin (11) , P. Bartczak (10) , G Dudzinski , J. Berthier (9) , J. Castillo-Rogez (12) , F. Cipriani (13) , F Colas (9) , F Demeo , C. Dumas (14) , J Durech , R Fetick (15) , T Fusco (15) , J Grice , M Kaasalainen , A. Kryszczynska (16) , P Lamy , H Le Coroller , A. Marciniak (16) , T. Michalowski (16) , Patrick Michel (17) , N. Rambaux (18, 9) , T. Santana-Ros (16) , P Tanga , F. Vachier (9) , A Vigan , O. Witasse (13) , B Yang , M Gillon , Z. Benkhaldoun (19) , R Szakats , R. Hirsch (16) , R. Duffard (20) , A Chapman , J L Maestre

1 LAM - Laboratoire d'Astrophysique de Marseille
2 Astronomical Institute of Charles University
3 TUT - Tampere University of Technology [Tampere]
4 ESAC - European Space Astronomy Centre
5 DAPNIA - Département d'Astrophysique, de physique des Particules, de physique Nucléaire et de l'Instrumentation Associée
6 STAR - Space Sciences, Technologies and Astrophysics Research Institute
7 ARC - Astrophysics Research Centre [Belfast]
8 IMCCE - Institut de Mécanique Céleste et de Calcul des Ephémérides
9 IMCCE - Institut de Mécanique Céleste et de Calcul des Ephémérides
10 UAM - Uniwersytet im. Adama Mickiewicza w Poznaniu = Adam Mickiewicz University in Poznań
11 Institut d'Astrophysique et de Géophysique [Liège]
12 JPL - Jet Propulsion Laboratory
13 ESTEC - European Space Research and Technology Centre
14 CBS - Centre de Biochimie Structurale [Montpellier]
15 DOTA, ONERA, Université Paris Saclay (COmUE) [Châtillon]
16 Astronomical Observatory [Poznan]
17 LAGRANGE - Joseph Louis LAGRANGE
18 UPMC - Université Pierre et Marie Curie - Paris 6
19 LPHEA - LPHEA - Departement de Physique
20 IAA - Instituto de Astrofísica de Andalucía

(10) Hygiea is the fourth largest main belt asteroid and the only known asteroid whose surface composition appears similar to that of the dwarf planet (1) Ceres1,2, suggesting a similar origin for these two objects. Hygiea suffered a giant impact more than 2 Gyr ago3 that is at the origin of one of the largest asteroid families. However, Hygeia has never been observed with sufficiently high resolution to resolve the details of its surface or to constrain its size and shape. Here, we report high-angular-resolution imaging observations of Hygiea with the VLT/SPHERE instrument (~20 mas at 600 nm) that reveal a basin-free nearly spherical shape with a volume-equivalent radius of 217 ± 7 km, implying a density of 1,944 ± 250 kg m−3 to 1σ. In addition, we have determined a new rotation period for Hygiea of ~13.8 h, which is half the currently accepted value. Numerical simulations of the family-forming event show that Hygiea’s spherical shape and family can be explained by a collision with a large projectile (diameter ~75–150 km). By comparing Hygiea’s sphericity with that of other Solar System objects, it appears that Hygiea is nearly as spherical as Ceres, opening up the possibility for this object to be reclassified as a dwarf planet.

Mots clés: Dwarf planet; VLT/SPHERE instrument; High-angular-resolution imaging observation; Asteroide

Domaines: Planète et Univers [physics]; Astrophysique [astro-ph]; Planétologie et astrophysique de la terre [astro-ph.EP]

P. Vernazza, L. Jorda, P Ševeček, M. Brož, M. Viikinkoski, et al.. A basin-free spherical shape as an outcome of a giant impact on asteroid Hygiea. Nature Astronomy, 2019, 4, pp.136-141. ⟨10.1038/s41550-019-0915-8⟩. ⟨hal-02346869⟩