M-type (22) Kalliope: A tiny Mercury^★,★★

¹ Aix Marseille Université, CNRS, CNES, Laboratoire d’Astrophysique de Marseille, Marseille, France
e-mail: marin.ferrais@lam.fr
² Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France
³ Charles University, Faculty of Mathematics and Physics, Institute of Astronomy, V Holešovičkách 2, 18000 Prague 8, Czech Republic
⁴ IMCCE, CNRS, Observatoire de Paris, PSL Université, Sorbonne Université, Paris, France
⁵ Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, Słoneczna 36, 60–286 Poznań, Poland
⁶ Institut de Planetologie et d’Astrophysique de Grenoble, UGA-CNRS, France; Institut Universitaire de France, Paris, France
⁷ SETI Institute, Carl Sagan Center, 189 Bernado Avenue, Mountain View, CA 94043, USA
⁸ Department of Earth, Atmospheric and Planetary Sciences, MIT, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
⁹ Mathematics and Statistics, Tampere University, 33720 Tampere, Finland
¹⁰ ONERA The French Aerospace Laboratory, Châtillon, France
¹¹ Astronomical Institute of the Romanian Academy, 5-Cuţitul de Argint, 040557 Bucharest, Romania
¹² Institute of Physics, University of Szczecin, Wielkopolska 15, 70–453 Szczecin, Poland
¹³ Department of Astronomy, Mount Holyoke College, South Hadley, MA 01075 USA
¹⁴ Oukaimeden Observatory, High Energy Physics and Astrophysics Laboratory, Cadi Ayyad University, Marrakech, Morocco
¹⁵ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
¹⁶ European Space Agency, ESTEC – Scientific Support Office, Keplerlaan 1, Noordwijk 2200 AG, The Netherlands
¹⁷ TMT Observatory, 100 W. Walnut Street, Suite 300, Pasadena, CA 91124, USA
¹⁸ Observatoire du Bois de Bardon, 16110 Taponnat, France
¹⁹ Open University, School of Physical Sciences, The Open University, MK7 6AA UK
²⁰ Space sciences, Technologies and Astrophysics Research Institute, Université de Liège, allée du 6 Août 17, 4000 Liège, Belgium
²¹ Laboratoire Atmosphères, Milieux et Observations Spatiales, CNRS & Université de Versailles Saint-Quentin-en-Yvelines, Guyancourt, France
²² Université de Toulouse – UPS-OMP – IRAP, Toulouse, France
²³ Lunar and Planetary Laboratory, University of Arizona, 1629 E University Boulevard, Tucson, AZ 85721-0092, USA
²⁴ Departamento de Fisica, Ingeniería de Sistemas y Teoría de la Señal, Universidad de Alicante, Alicante, Spain
²⁵ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (IEEC-UB), Marti Franquès 1, 08028 Barcelona, Spain
²⁶ INAF – Osservatorio Astronomico di Brera, Merate, Italy
²⁷ European Southern Observatory (ESO), Alonso de Cordova 3107, 1900 Casilla Vitacura, Santiago, Chile
²⁸ Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Av. Ejercito 441, Santiago, Chile

Received: 26 January 2022
Accepted: 14 March 2022

Abstract

Context. Asteroid (22) Kalliope is the second largest M-type asteroid in the main belt and is orbited by a satellite, Linus. Whereas the mass of Kalliope is already well constrained thanks to the presence of a moon, its volume is still poorly known, leading to uncertainties on its bulk density and internal structure.

Aims. We aim to refine the shape of (22) Kalliope and thus its diameter and bulk density, as well as the orbit of its moon to better constrain its mass, hence density and internal structure.

Methods. We acquired disk-resolved observations of (22) Kalliope using the VLT/SPHERE/ZIMPOL instrument to reconstruct its three-dimensional (3D) shape using three different modeling techniques. These images were also used together with new speckle observations at the C2PU/PISCO instrument as well as archival images from other large ground-based telescopes to refine the orbit of Linus.

Results. The volume of (22) Kalliope given by the shape models, corresponding to D = 150 ± 5 km, and the mass constrained by its satellite’s orbit yield a density of ρ = 4.40 ± 0.46 g cm⁻³. This high density potentially makes (22) Kalliope the densest known small body in the Solar System. A macroporosity in the 10–25% range (as expected for this mass and size), implies a grain density in the 4.8–5.9 g cm⁻³ range. Kalliope’s high bulk density, along with its silicate-rich surface implied by its low radar albedo, implies a differentiated interior with metal contributing to most of the mass of the body.

Conclusions. Kalliope’s high metal content (40–60%) along with its metal-poor mantle makes it the smallest known Mercury-like body. A large impact at the origin of the formation of the moon Linus is likely the cause of its high metal content and density.