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

(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.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

(704) Interamnia: a transitional object between a dwarf planet and a typical irregular-shaped minor body

Context. With an estimated diameter in the 320–350 km range, (704) Interamnia is the fifth largest main belt asteroid and one of the few bodies that fills the gap in size between the four largest bodies with D > 400 km (Ceres, Vesta, Pallas and Hygiea) and the numerous smaller bodies with diameter ≤200 km. However, despite its large size, little is known about the shape and spin state of Interamnia and, therefore, about its bulk composition and past collisional evolution. Aims. We aimed to test at what size and mass the shape of a small body departs from a nearly ellipsoidal equilibrium shape (as observed in the case of the four largest asteroids) to an irregular shape as routinely observed in the case of smaller (D ≤ 200 km) bodies. Methods. We observed Interamnia as part of our ESO VLT/SPHERE large program (ID: 199.C-0074) at thirteen different epochs. In addition, several new optical lightcurves were recorded. These data, along with stellar occultation data from the literature, were fed to the All-Data Asteroid Modeling algorithm to reconstruct the 3D-shape model of Interamnia and to determine its spin state. Results. Interamnia’s volume-equivalent diameter of 332 ± 6 km implies a bulk density of ρ = 1.98 ± 0.68 g cm−3, which suggests that Interamnia – like Ceres and Hygiea – contains a high fraction of water ice, consistent with the paucity of apparent craters. Our observations reveal a shape that can be well approximated by an ellipsoid, and that is compatible with a fluid hydrostatic equilibrium at the 2σ level. Conclusions. The rather regular shape of Interamnia implies that the size and mass limit, under which the shapes of minor bodies with a high amount of water ice in the subsurface become irregular, has to be searched among smaller (D ≤ 300 km) less massive (m ≤ 3 × 1019 kg) bodies

Impacts into rotating targets: angular momentum draining and efficient formation of synthetic families

About 10% of the observed asteroids have rotational periods lower than P = 3h and seem to be relatively close to the spin barrier. Yet, the rotation has often been neglected in simulations of asteroid collisions. To determine the effect of rotation, we performed a large number of impact simulations with rotating targets. We developed a new unified smoothed particle hydrodynamics and N-body code with self-gravity, suitable for simulations of both fragmentation phase and gravitational reaccumulation. The code has been verified against previous ones, but we also tested new features, such as rotational stability, tensile stability, etc. Using the new code, we ran simulations with Dpb = 10 and 100km monolithic targets and compared synthetic asteroid families created by these impacts with families corresponding to non-rotating targets. The rotation affects mostly cratering events at oblique impact angles. The total mass ejected by these collisions can be up to five times larger for rotating targets. We further computed the transfer of the angular momentum and determined conditions under which impacts accelerate or decelerate the target. While individual cratering collisions can cause both acceleration and deceleration,the deceleration prevails on average. Collisions thus cause asystematic spin-down of the asteroid population

Discovery of an asteroid family linked to (22) Kalliope and its moon Linus

International audience Aims: According to adaptive-optics observations, (22) Kalliope is a 150-km-wide, dense, and differentiated body. Here, we interpret (22) Kalliope in the context of the bodies in its surroundings. While there is a known moon, Linus, with a 5:1 size ratio, no family has been reported in the literature, which is in contradiction with the existence of the moon. Methods: Using the hierarchical clustering method along with physical data, we identified the Kalliope family. It had previously been associated with (7481) San Marcello. We then used various models (N-body, Monte Carlo, and SPH) of its orbital and collisional evolution, including the breakup of the parent body, to estimate the dynamical age of the family and address its link to Linus. Results: The best-fit age is (900 ± 100) Myr according to our collisional model; this is in agreement with the position of (22) Kalliope, which was modified by chaotic diffusion due to 4-1-1 three-body resonance with Jupiter and Saturn. It seems possible that Linus and the Kalliope family were created at the same time, although our SPH simulations show a variety of outcomes for both satellite size and the family size-frequency distribution. The shape of (22) Kalliope itself was most likely affected by the gravitational re-accumulation of `streams', which creates the characteristic hills observed on its surface. If the body was differentiated, its internal structure is most likely asymmetric. Movies associated to Fig. 6 are available at https://www.aanda.org</A

Understanding the edge crack phenomenon in ceramic laminates

Layered ceramic materials (also referred to as “ceramic laminates”) are becoming one of the most promising areas of materials technology aiming to improve the brittle behavior of bulk ceramics. The utilization of tailored compressive residual stresses acting as physical barriers to crack propagation has already succeeded in many ceramic systems. Relatively thick compressive layers located below the surface have proven very effective to enhance the fracture resistance and provide a minimum strength for the material. However, internal compressive stresses result in out-of plane stresses at the free surfaces, what can cause cracking of the compressive layer, forming the so-called edge cracks. Experimental observations have shown that edge cracking may be associated with the magnitude of the compressive stresses and with the thickness of the compressive layer. However, an understanding of the parameters related to the onset and extension of such edge cracks in the compressive layers is still lacking. In this work, a 2D parametric finite element model has been developed to predict the onset and propagation of an edge crack in ceramic laminates using a coupled stress-energy criterion. This approach states that a crack is originated when both stress and energy criteria are fulfilled simultaneously. Several designs with different residual stresses and a given thickness in the compressive layers have been computed. The results predict the existence of a lower bound, below no edge crack will be observed, and an upper bound, beyond which the onset of an edge crack would lead to the complete fracture of the laye