Asteroid (16) Psyche’s primordial shape: A possible Jacobi ellipsoid

Context. Asteroid (16) Psyche is the largest M-type asteroid in the main belt and the target of the NASA Psyche mission. It is also the only asteroid of this size (D >  200 km) known to be metal rich. Although various hypotheses have been proposed to explain the rather unique physical properties of this asteroid, a perfect understanding of its formation and bulk composition is still missing. Aims. We aim to refine the shape and bulk density of (16) Psyche and to perform a thorough analysis of its shape to better constrain possible formation scenarios and the structure of its interior. Methods. We obtained disk-resolved VLT/SPHERE/ZIMPOL images acquired within our ESO large program (ID 199.C-0074), which complement similar data obtained in 2018. Both data sets offer a complete coverage of Psyche’s surface. These images were used to reconstruct the three-dimensional (3D) shape of Psyche with two independent shape modeling algorithms (MPCD and ADAM). A shape analysis was subsequently performed, including a comparison with equilibrium figures and the identification of mass deficit regions. Results. Our 3D shape along with existing mass estimates imply a density of 4.20  ±  0.60 g cm−3, which is so far the highest for a solar system object following the four telluric planets. Furthermore, the shape of Psyche presents small deviations from an ellipsoid, that is, prominently three large depressions along its equator. The flatness and density of Psyche are compatible with a formation at hydrostatic equilibrium as a Jacobi ellipsoid with a shorter rotation period of ∼3h. Later impacts may have slowed down Psyche’s rotation, which is currently ∼4.2 h, while also creating the imaged depressions. Conclusions. Our results open the possibility that Psyche acquired its primordial shape either after a giant impact while its interior was already frozen or while its interior was still molten owing to the decay of the short-lived radionuclide 26Al.Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 199.C-0074 (principal investigator: P. Vernazza). P. Vernazza, A. Drouard, M. Ferrais and B. Carry were supported by CNRS/INSU/PNP. 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. E.J. is F.R.S.-FNRS Senior Research Associate. The work of TSR was carried out through grant APOSTD/2019/046 by Generalitat Valenciana (Spain). This work was supported by the MINECO (Spanish Ministry of Economy) through grant RTI2018-095076-B-C21 (MINECO/FEDER, UE)

Binary asteroid (31) Euphrosyne: ice-rich and nearly spherical

Aims. Asteroid (31) Euphrosyne is one of the biggest objects in the asteroid main belt and it is also the largest member of its namesake family. The Euphrosyne family occupies a highly inclined region in the outer main belt and contains a remarkably large number of members, which is interpreted as an outcome of a disruptive cratering event. Methods. The goals of this adaptive-optics imaging study are threefold: to characterize the shape of Euphrosyne, to constrain its density, and to search for the large craters that may be associated with the family formation event. Results. We obtained disk-resolved images of Euphrosyne using SPHERE/ZIMPOL at the ESO 8.2 m VLT as part of our large program (ID: 199.C-0074, PI: Vernazza). We reconstructed its 3D shape via the ADAM shape modeling algorithm based on the SPHERE images and the available light curves of this asteroid. We analyzed the dynamics of the satellite with the Genoid meta-heuristic algorithm. Finally, we studied the shape of Euphrosyne using hydrostatic equilibrium models. Conclusions. Our SPHERE observations show that Euphrosyne has a nearly spherical shape with the sphericity index of 0.9888 and its surface lacks large impact craters. Euphrosyne’s diameter is 268 ± 6 km, making it one of the top ten largest main belt asteroids. We detected a satellite of Euphrosyne – S/2019 (31) 1 – that is about 4 km across, on a circular orbit. The mass determined from the orbit of the satellite together with the volume computed from the shape model imply a density of 1665 ± 242 kg m−3, suggesting that Euphrosyne probably contains a large fraction of water ice in its interior. We find that the spherical shape of Euphrosyne is a result of the reaccumulation process following the impact, as in the case of (10) Hygiea. However, our shape analysis reveals that, contrary to Hygiea, the axis ratios of Euphrosyne significantly differ from those suggested by fluid hydrostatic equilibrium following reaccumulation.This work has been supported by the Czech Science Foundation through grant 18-09470S (J. Hanuš, O. Chrenko, P. Ševeček) and by the Charles University Research program No. UNCE/SCI/023. M.B. was supported by the Czech Science Foundation grant 18-04514J. Computational resources were supplied by the Ministry of Education, Youth and Sports of the Czech Republic under the projects CESNET (LM2015042) and IT4Innovations National Supercomputing Centre (LM2015070). P. Vernazza, A. Drouard, M. Ferrais and B. Carry were supported by CNRS/INSU/PNP. M.M. was supported by the National Aeronautics and Space Administration under grant No. 80NSSC18K0849 issued through the Planetary Astronomy Program. The work of TSR was carried out through grant APOSTD/2019/046 by Generalitat Valenciana (Spain). This work was supported by the MINECO (Spanish Ministry of Economy) through grant RTI2018-095076-B-C21 (MINECO/FEDER, UE). The research leading to these results has received funding from the ARC grant for Concerted Research Actions, financed by the Wallonia-Brussels Federation. TRAPPIST 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. TRAPPIST-North is a project funded by the Université de Liège, and performed in collaboration with Cadi Ayyad University of Marrakesh. E. Jehin is a FNRS Senior Research Associate

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

Évolution des corps-parents des chondrites ordinaires : du sol à l'espace

The main asteroid belt hosts a large variety of rocky and icy bodies that range from metersto thousand of kilometers in size. The belt has been shaped by its dynamical evolution sinceits formation, in particular due to collisional processes. The numerous craters on asteroid surfacesbear witness to the violence of these impacts. The material excavated can either fall backon the surface as an ejecta blanket, or escape the asteroid gravity field. The free fragmentsundergo various non-gravitational forces and dynamical instabilities, which send them throughthe interplanetary medium to sometimes reach the Earth’s surface as meteorites. The mostabundant are the ordinary chondrites that originate from the rocky S-type asteroids.This thesis aims to bring new constraints on the evolution of the ordinary chondrite parentbodies by studying both meteorite flux and impact craters. The results encompass the differentsteps of meteorite lives, from their parent-bodies to Earth’s surface including the atmosphericentry. Specifically, the dating of ordinary chondrites recovered in the Atacama Desert showthat this ancient surface has continuously recorded the meteorite flux for several million years.Such timescales, in addition to the very high meteorite density of this region, imply that theAtacama Desert is a preferential area to constrain the collisional evolution of the ordinary chondriteparent-bodies. On another note, the analysis of high-resolution spectra acquired duringmeteorite ablation experiments changes the expectations for meteor spectroscopy’s capacity todetermine the composition of the current flux by using camera networks. Meteor spectroscopydoes not seem capable of distinguishing meteorite sub-groups but only major classes (chondrites,achondrites, irons). Finally, regarding large asteroids, the system of adaptative optics inthe visible range mounted on the VLT/SPHERE instrument allow the identification of impactcraters at their surfaces. Both crater diameter and cratering rates estimates are used to explainasteroid surface properties and as observational constraints on the origins of a few families.La ceinture principale d’astéroïdes abrite une grande variété d’objets, des corps rocheux auxcorps glacés, dont la taille varie du mètre au millier de kilomètres. Depuis sa formation, cetteceinture a été façonnée par son évolution dynamique, notamment à travers les processus decollisions. Les surfaces d’astéroïdes, recouvertes de cratères, témoignent de la violence de ces impacts.Le matériel excavé au moment du choc peut retomber en un lit d’éjectas ou bien échapperà l’attraction gravitationnelle de l’astéroïde. Sous l’effet de forces non gravitationnelles coupléesà des instabilités dynamiques, les fragments libérés voyagent ensuite dans l’espace interplanétaire,pour parfois atteindre la Terre sous la forme de météorites. Les plus abondantes d’entreelles sont les chondrites ordinaires, originaires des astéroïdes de type S à la composition rocheuse.L’objectif de cette thèse est d’apporter de nouvelles contraintes sur l’évolution des corpsparentsdes chondrites ordinaires en conjuguant l’étude du flux de météorites et celle descratères d’impact. Les résultats présentés s’articulent autour des différents stades de la viedes météorites, de leurs corps-parents jusqu’à leur chute sur Terre en passant par l’entréeatmosphérique. En particulier, la datation de chondrites ordinaires retrouvées dans le désertd’Atacama au Chili montre que cette ancienne surface a enregistré continûment le flux météoritiquesur plusieurs millions d’années. Cette échelle de temps, ainsi que la grande densité demétéorites de la région, font de ce désert une surface privilégiée pour contraindre de l’histoirecollisionnelle des corps parents des chondrites ordinaires. Par ailleurs, l’étude de spectres hauterésolutionde météorites fondues en laboratoire réévalue les apports de la spectroscopie pourla caractérisation de la composition du flux actuel par les réseaux d’observations de météores.La spectroscopie de météores ne s’avère pas en mesure de distinguer différents sous-groupesde météorites, mais seulement les grandes classes (chondrites, achondrites, métalliques). Enfin,concernant les gros astéroïdes, le système d’optique adaptative dans le visible monté surl’instrument SPHERE du Very Large Telescope (VLT) rend désormais possible l’identificationdes cratères d’impact à leur surface. La détermination des diamètres de cratères et du tauxde cratérisation sont utilisées pour expliquer les propriétés de surface d’astéroïdes et commecontrainte observationnelle sur l’origine de quelques familles

Evolution of the ordinary chondrites parent-bodies : from ground to space

L’objectif de cette thèse est d’apporter de nouvelles contraintes sur l’évolution des corps-parents des chondrites ordinaires en conjuguant l’étude du flux de météorites et celle des cratères d’impact. Les résultats présentés s’articulent autour des différents stades de la vie des météorites, de leurs corps-parents jusqu’à leur chute sur Terre en passant par l’entrée atmosphérique. En particulier, la datation de chondrites ordinaires retrouvées dans le désert d’Atacama au Chili montre que cette ancienne surface a enregistré continûment le flux météoritique sur plusieurs millions d’années. Cette échelle de temps, ainsi que la grande densité de météorite de la région, font de ce désert une surface privilégiée pour contraindre de l’histoire collisionnelle des corps parents des chondrites ordinaires. Par ailleurs, l’étude de spectres haute-résolution de météorites fondues en laboratoire réévalue les apports de la spectroscopie pour la caractérisation de la composition du flux actuel par les réseaux d’observations de météores. La spectroscopie de météores ne s’avère pas en mesure de distinguer différents sous-groupes de météorites, mais seulement les grandes classes (chondrites, achondrites, métalliques). Concernant les gros astéroïdes, l’optique adaptative dans le visible sur l’instrument SPHERE du Very Large Telescope (VLT) rend désormais possible l’identification des cratères d’impact à leur surface. La détermination des diamètres de cratères et du taux de cratérisation sont utilisées pour expliquer les propriétés de surface d’astéroïdes et comme contrainte observationnelle sur l’origine de quelques famillesThis thesis aims to bring new constraints on the evolution of the ordinary chondrite parent bodies by studying both meteorite flux and impact craters. The results encompass the different steps of meteorite lives, from their parent-bodies to Earth’s surface including the atmospheric entry. Specifically, the dating of ordinary chondrites recovered in the Atacama Desert show that this ancient surface has continuously recorded the meteorite flux for several million years. Such timescales, in addition to the very high meteorite density of this region, imply that the Atacama Desert is a preferential area to constrain the collisional evolution of the ordinary chondrite parent-bodies. On another note, the analysis of high-resolution spectra acquired during meteorite ablation experiments changes the expectations for meteor spectroscopy’s capacity to determine the composition of the current flux by using camera networks. Meteor spectro- scopy does not seem capable of distinguishing meteorite sub-groups but only major classes (chondrites, achondrites, irons). Concerning large asteroids, the adaptative optics in the visible range on the VLT/SPHERE instrument allow the identification of impact craters at their surfaces. Both crater diameter and cratering rates estimates are used to explain asteroid surface properties and as observational constraints on the origins of a few familie

The meteorite flux of the last 2 Myr recorded in Atacama

International audienc

The meteorite flux of the last 2 Myr recorded in Atacama

International audienc

The meteorite flux of the last 2 Myr recorded in Atacama

International audienc

Experimental Simulation of Meteorite Ablation during Earth Entry Using a Plasma Wind Tunnel

International audienceThree different types of rocks were tested in a high enthalpy air plasma flow. Two terrestrial rocks, basalt and argillite, and an ordinary chondrite, with a 10 mm diameter cylindrical shape were tested in order to observe decomposition, potential fragmentation, and spectral signature. The goal was to simulate meteoroid ablation to interpret meteor observation and compare these observations with ground based measurements. The test flow with a local mass‐specific enthalpy of 70 MJ kg(‐1) results in a surface heat flux at the meteorite fragment surface of approximately 16MW m(‐2). The stagnation pressure is 24 hPa, which corresponds to a flight condition in the upper atmosphere around 80 km assuming an entry velocity of 10 km s(‐1). Five different diagnostic methods were applied simultaneously to characterize the meteorite fragmentation and destruction in the ground test: short exposure photography, regular video, high‐speed imaging with 10 kHz frame rate, thermography, and Echelle emission spectroscopy. This is the first time that comprehensive testing of various meteorite fragments under the same flow condition was conducted. The data sets indeed show typical meteorite ablation behavior. The cylindrically shaped fragments melt and evaporate within about 4 s. The spectral data allow the identification of the material from the spectra which is of particular importance for future spectroscopic meteor observations. For the tested ordinary chondrite sample a comparison to an observed meteor spectra shows good agreement. The present data show that this testing methodology reproduces the ablation phenomena of meteoritic material alongside the corresponding spectral signatures

Pallas's formation and internal structure: New insights from VLT/SPHERE

International audienceLarge (D>100km) asteroids are the most direct remnants of the building blocks of planets. (2) Pallas is the third largest asteroid and the parent body of a small collisional family. Its spectral properties indicate a B-type surface, meaning Pallas is most likely linked to carbonaceous chondrite meteorites. Disc-resolved images have revealed a nearly hydrostatic shape overprinted by long-wavelength concavities (Schmidt et al. 2009, Carry et al. 2010). This was interpreted as evidence for an early phase of internal heating subsequent to Pallas's formation, followed by several large impact craters (Schmidt & Castillo-Rogez 2012). Recent estimates of Pallas's density, 2.40±0.25 g/cm3 (Schmidt et al. 2009), 3.40±0.90 g/cm3 (Carry et al. 2010) and 2.72±0.17 g/cm3 (Hanus et al. 2017), are rather inconsistent and prevent from differentiating among the various models proposed for its internal structure (Schmidt & Castillo-Rogez 2012). This currently limits our understanding of the formation and thermal evolution of Pallas. We report new high-angular resolution observations of Pallas collected in the frame of the SPHERE large survey of the asteroid belt (see Talk by P. Vernazza) with the adaptive-optics-fed SPHERE ZIMPOL camera on the VLT. 40 images acquired at 8 epochs provide a full longitudinal coverage of Pallas's southern hemisphere, with Pallas being resolved with ˜120 pixels along its longest axis. The optimal angular resolution of each image was restored with Mistral (Fusco et al. 2002), a myopic deconvolution algorithm optimised for images with sharp boundaries, which allows the identification of many craters and geological features on Pallas. A precise 3D-shape reconstruction was achieved with the ADAM software (Viikinkoski et al. 2015), providing a high precision estimate of Pallas's 3D shape, volume and hence density. Those are used to explore Pallas's early thermal evolution, its subsequent collisional evolution, and its current internal structure and composition. [1] Carry et al. 2010, Icarus, 205, 460 [2] Fusco et al. 2002, SPIE, 4839, 1065 [3] Hanus et al. 2017, A&A, 601, A114 [4] Schmidt et al. 2009, Science, 326, 275 [5] Schmidt & Castillo-Rogez, Icarus, 218, 478 [6] Viikinkoski et al. 2015, A&A, 576, A