Evidence for differentiation of the most primitive small bodies

Benoit Carry (1) , P. Vernazza (2) , F. Vachier (3) , M. Neveu (4, 5) , J. Berthier (3) , J. Hanuš (6) , M. Ferrais (2, 7) , Laurent Jorda (2) , M. Marsset (8) , M. Viikinkoski (9) , P. Bartczak (10) , R. Behrend (11) , Z. Benkhaldoun (12) , M Birlan (3, 13) , J. Castillo-Rogez (14) , F. Cipriani (15) , F Colas (3) , A. Drouard (2) , G. P. Dudziński (10) , J. Desmars (16, 3) , C. Dumas (17) , J. Ďurech (6) , R. Fetick (2) , T. Fusco (2) , J. Grice (1, 18) , E. Jehin (7) , M. Kaasalainen (19) , A. Kryszczynska (10) , Philippe Lamy (20) , F. Marchis (21) , A. Marciniak (10) , T. Michalowski (10) , P. Michel (1) , M. Pajuelo (3, 22) , E. Podlewska-Gaca (10, 23) , N. Rambaux (3) , T. Santana-Ros (24, 25) , A. Storrs (26) , P. Tanga (1) , A. Vigan (2) , B. Warner (27) , M. Wieczorek (1) , Olivier Witasse (15) , Bin Yang (28)

1 LAGRANGE - Joseph Louis LAGRANGE
2 LAM - Laboratoire d'Astrophysique de Marseille
3 IMCCE - Institut de Mécanique Céleste et de Calcul des Ephémérides
4 University of Maryland [College Park]
5 GSFC - NASA Goddard Space Flight Center
6 Institute of Astronomy [Prague]
7 STAR - Space Sciences, Technologies and Astrophysics Research Institute
8 EAPS - Department of Earth, Atmospheric and Planetary Sciences [MIT, Cambridge]
9 Department of Mathematics [Tampere]
10 Astronomical Observatory [Poznan]
11 Geneva Observatory
12 Oukaimeden Observatory
13 Astronomical Institute of Romanian Academy
14 JPL - Jet Propulsion Laboratory
15 ESTEC - European Space Research and Technology Centre
16 IPSA - Institut Polytechnique des Sciences Avancées
17 Thirty Meter Telescope Observatory
18 OU - The Open University [Milton Keynes]
19 University of Tampere [Finland]
20 HELIOS - LATMOS
21 SETI Institute
22 PUCP - Pontificia Universidad Católica del Perú = Pontifical Catholic University of Peru
23 Institute of Physics [Szczecin]
24 Universidad de Alicante
25 ICCUB - Institut de Ciencies del Cosmos
26 Towson University [Towson, MD, United States]
27 CS3 - Center for Solar System Studies
28 ESO - European Southern Observatory

Dynamical models of Solar System evolution have suggested that the so-called P-and D-type volatile-rich asteroids formed in the outer Solar System beyond Neptune's orbit and may be genetically related to the Jupiter Trojans, the comets and small Kuiper-belt objects (KBOs). Indeed, the spectral properties of P/D-type asteroids resemble that of anhydrous cometary dust. We aim at gaining insights into the above classes of bodies by characterizing the internal structure of a large P/D-type asteroid. Methods. We report high-angular-resolution imaging observations of P-type asteroid (87) Sylvia with VLT/SPHERE. These images were used to reconstruct the 3D shape of Sylvia. Our images together with those obtained in the past with large ground-based telescopes were used to study the dynamics of its two satellites. We also model Sylvia's thermal evolution. The shape of Sylvia appears flattened and elongated (a/b∼1.45 ; a/c∼1.84). We derive a volume-equivalent diameter of 271 ± 5 km, and a low density of 1378 ± 45 kg•m −3. The two satellites orbit Sylvia on circular, equatorial orbits. The oblateness of Sylvia should imply a detectable nodal precession which contrasts with the fully-Keplerian dynamics of its two satellites. This reveals an inhomogeneous internal structure, suggesting that Sylvia is differentiated. Sylvia's low density and differentiated interior can be explained by partial melting and mass redistribution through water percolation. The outer shell would be composed of material similar to interplanetary dust particles (IDPs) and the core similar to aqueously altered IDPs or carbonaceous chondrite meteorites such as the Tagish Lake meteorite. Numerical simulations of the thermal evolution of Sylvia show that for a body of such size, partial melting was unavoidable due to the decay of long-lived radionuclides. In addition, we show that bodies as small as 130-150 km in diameter should have followed a similar thermal evolution, while smaller objects, such as comets and the KBO Arrokoth, must have remained pristine, in agreement with in situ observations of these bodies. NASA Lucy mission target (617) Patroclus (diameter ≈140 km) may, however, be differentiated.

Keywords: Minor planets; asteroids; Sylvia; Kuiper belt

Domains: Sciences of the Universe [physics]

Benoit Carry, P. Vernazza, F. Vachier, M. Neveu, J. Berthier, et al.. Evidence for differentiation of the most primitive small bodies. Astronomy and Astrophysics - A&A, 2021, 650, A129 (15pp.). ⟨10.1051/0004-6361/202140342⟩. ⟨insu-03184663v2⟩