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

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(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 m3 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.

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Fig. 1: VLT/SPHERE deconvolved images of the four largest main belt objects.
Fig. 2: Comparison between the deconvolved images of Hygiea and the corresponding shape model projections.
Fig. 3: SPH simulations reveal a nearly spherical shape for Hygiea following post-impact reaccumulation.
Fig. 4: Asphericity of Solar System objects as a function of their mean radius.

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 3D shape models publicly available at http://observations.lam.fr/astero/.

Code availability

The code used to generate the 3D shape is freely available at https://github.com/matvii/ADAM. The code used to perform the SPH simulations is freely available at https://gitlab.com/sevecekp/sph.


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P.V., A.D. and B.C. were supported by CNRS/INSU/PNP. M.Brož was supported by grant 18-04514J of the Czech Science Foundation. J.H. and J.D. were supported by grant 18-09470S of the Czech Science Foundation and by the Charles University Research Programme no. UNCE/SCI/023. This project has received funding from the European Union’s Horizon 2020 research and innovation programmes under grant agreement nos 730890 and 687378. This material reflects only the authors’ views, and the European Commission is not liable for any use that may be made of the information contained herein. TRAPPIST-North is a project funded by the University of Liège, in collaboration with Cadi Ayyad University of Marrakech (Morocco). TRAPPIST-South is a project funded by the Belgian Fonds (National) de la Recherche Scientifique (F.R.S.-FNRS) under grant FRFC 2.5.594.09.F. E.J. and M.G. are F.R.S.-FNRS Senior Research Associates.

Author information

P.V. designed the research. P.V., M.M., R.F. and T.F. reduced and deconvolved the SPHERE images. M.V. and J.H. reconstructed the 3D shape of Hygiea. L.J. and P.V. performed the analysis of Hygiea’s shape. P.Š. and M.Brož ran the SPH simulations. M.F. and E.J. acquired and reduced the TRAPPIST data. M.M. and L.J. produced the albedo map. P.V. and F.D. served as principal investigators to acquire the near-infrared spectral data. B.C. provided the mass estimate. P.V., L.J., P.Š. and M.Brož worked jointly to write the manuscript. All authors discussed the results and commented on the manuscript.

Correspondence to P. Vernazza.

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Vernazza, P., Jorda, L., Ševeček, P. et al. A basin-free spherical shape as an outcome of a giant impact on asteroid Hygiea. Nat Astron (2019) doi:10.1038/s41550-019-0915-8

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