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1. The impact crater at the origin of the Julia family detected with VLT/SPHERE?

   https://doi.org/10.1051/0004-6361/201833477  

   Vernazza, P.; Brož, M.; Drouard, A.; Hanuš, J.; Viikinkoski, M.; Marsset, M.; Jorda, L.; Fetick, R.; Carry, B.; Marchis, F.; et al (October 2018, Astronomy & Astrophysics)

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   Context. The vast majority of the geophysical and geological constraints (e.g., internal structure, cratering history) for main-belt asteroids have so far been obtained via dedicated interplanetary missions (e.g., ESA Rosetta, NASA Dawn). The high angular resolution of SPHERE/ZIMPOL, the new-generation visible adaptive-optics camera at ESO VLT, implies that these science objectives can now be investigated from the ground for a large fraction of D ≥ 100 km main-belt asteroids. The sharp images acquired by this instrument can be used to accurately constrain the shape and thus volume of these bodies (hence density when combined with mass estimates) and to characterize the distribution and topography of D ≥ 30 km craters across their surfaces. Aims. Here, via several complementary approaches, we evaluated the recently proposed hypothesis that the S-type asteroid (89) Julia is the parent body of a small compact asteroid family that formed via a cratering collisional event. Methods. We observed (89) Julia with VLT/SPHERE/ZIMPOL throughout its rotation, derived its 3D shape, and performed a reconnaissance and characterization of the largest craters. We also performed numerical simulations to first confirm the existence of the Julia family and to determine its age and the size of the impact crater at its origin. Finally, we utilized the images/3D shape in an attempt to identify the origin location of the small collisional family. Results. On the one hand, our VLT/SPHERE observations reveal the presence of a large crater ( D ~ 75 km) in Julia’s southern hemisphere. On the other hand, our numerical simulations suggest that (89) Julia was impacted 30–120 Myrs ago by a D ~ 8 km asteroid, thereby creating a D ≥ 60 km impact crater at the surface of Julia. Given the small size of the impactor, the obliquity of Julia and the particular orientation of the family in the (a,i) space, the imaged impact crater is likely to be the origin of the family. Conclusions. New doors into ground-based asteroid exploration, namely, geophysics and geology, are being opened thanks to the unique capabilities of VLT/SPHERE. Also, the present work may represent the beginning of a new era of asteroid-family studies. In the fields of geophysics, geology, and asteroid family studies, the future will only get brighter with the forthcoming arrival of 30–40 m class telescopes like ELT, TMT, and GMT. 

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2. The shape of (7) Iris as evidence of an ancient large impact?

   https://doi.org/10.1051/0004-6361/201834541  

   Hanuš, J.; Marsset, M.; Vernazza, P.; Viikinkoski, M.; Drouard, A.; Brož, M.; Carry, B.; Fetick, R.; Marchis, F.; Jorda, L.; et al (April 2019, Astronomy & Astrophysics)

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   Context. Asteroid (7) Iris is an ideal target for disk-resolved imaging owing to its brightness ( V ~ 7–8) and large angular size of 0.33′′ during its apparitions. Iris is believed to belong to the category of large unfragmented asteroids that avoided internal differentiation, implying that its current shape and topography may record the first few 100 Myr of the solar system’s collisional evolution. Aims. We recovered information about the shape and surface topography of Iris from disk-resolved VLT/SPHERE/ZIMPOL images acquired in the frame of our ESO large program. Methods. We used the All-Data Asteroid Modeling ( ADAM ) shape reconstruction algorithm to model the 3D shape of Iris, using optical disk-integrated data and disk-resolved images from SPHERE and earlier AO systems as inputs. We analyzed the SPHERE images and our model to infer the asteroid’s global shape and the morphology of its main craters. Results. We present the 3D shape, volume-equivalent diameter D eq = 214 ± 5 km, and bulk density ρ = 2.7 ± 0.3 g cm −3 of Iris. Its shape appears to be consistent with that of an oblate spheroid with a large equatorial excavation. We identified eight putative surface features 20–40 km in diameter detected at several epochs, which we interpret as impact craters, and several additional crater candidates. Craters on Iris have depth-to-diameter ratios that are similar to those of analogous 10 km craters on Vesta. Conclusions. The bulk density of Iris is consistent with that of its meteoritic analog based on spectroscopic observations, namely LL ordinary chondrites. Considering the absence of a collisional family related to Iris and the number of large craters on its surface, we suggest that its equatorial depression may be the remnant of an ancient (at least 3 Gyr) impact. Iris’s shape further opens the possibility that large planetesimals formed as almost perfect oblate spheroids. Finally, we attribute the difference in crater morphology between Iris and Vesta to their different surface gravities, and the absence of a substantial impact-induced regolith on Iris. 

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

   https://doi.org/10.1051/0004-6361/202038100  

   Ferrais, M.; Vernazza, P.; Jorda, L.; Rambaux, N.; Hanuš, J.; Carry, B.; Marchis, F.; Marsset, M.; Viikinkoski, M.; Brož, M.; et al (June 2020, Astronomy & Astrophysics)

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   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 26 Al. 

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4. VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis

   https://doi.org/10.1051/0004-6361/202141781  

   Vernazza, P.; Ferrais, M.; Jorda, L.; Hanuš, J.; Carry, B.; Marsset, M.; Brož, M.; Fetick, R.; Viikinkoski, M.; Marchis, F.; et al (October 2021, Astronomy & Astrophysics)

   Context. Until recently, the 3D shape, and therefore density (when combining the volume estimate with available mass estimates), and surface topography of the vast majority of the largest ( D   ≥ 100 km) main-belt asteroids have remained poorly constrained. The improved capabilities of the SPHERE/ZIMPOL instrument have opened new doors into ground-based asteroid exploration. Aims. To constrain the formation and evolution of a representative sample of large asteroids, we conducted a high-angular-resolution imaging survey of 42 large main-belt asteroids with VLT/SPHERE/ZIMPOL. Our asteroid sample comprises 39 bodies with D   ≥ 100 km and in particular most D   ≥ 200 km main-belt asteroids (20/23). Furthermore, it nicely reflects the compositional diversity present in the main belt as the sampled bodies belong to the following taxonomic classes: A, B, C, Ch/Cgh, E/M/X, K, P/T, S, and V. Methods. The SPHERE/ZIMPOL images were first used to reconstruct the 3D shape of all targets with both the ADAM and MPCD reconstruction methods. We subsequently performed a detailed shape analysis and constrained the density of each target using available mass estimates including our own mass estimates in the case of multiple systems. Results. The analysis of the reconstructed shapes allowed us to identify two families of objects as a function of their diameters, namely “spherical” and “elongated” bodies. A difference in rotation period appears to be the main origin of this bimodality. In addition, all but one object (216 Kleopatra) are located along the Maclaurin sequence with large volatile-rich bodies being the closest to the latter. Our results further reveal that the primaries of most multiple systems possess a rotation period of shorter than 6 h and an elongated shape ( c ∕ a ≤ 0.65). Densities in our sample range from ~1.3 g cm −3 (87 Sylvia) to ~4.3 g cm −3 (22 Kalliope). Furthermore, the density distribution appears to be strongly bimodal with volatile-poor ( ρ ≥ 2.7 g cm −3 ) and volatile-rich ( ρ ≤ 2.2 g cm −3 ) bodies. Finally, our survey along with previous observations provides evidence in support of the possibility that some C-complex bodies could be intrinsically related to IDP-like P- and D-type asteroids, representing different layers of a same body (C: core; P/D: outer shell). We therefore propose that P/ D-types and some C-types may have the same origin in the primordial trans-Neptunian disk. 

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

   https://doi.org/10.1051/0004-6361/202038372  

   Yang, B.; Hanuš, J.; Carry, B.; Vernazza, P.; Brož, M.; Vachier, F.; Rambaux, N.; Marsset, M.; Chrenko, O.; Ševeček, P.; et al (September 2020, Astronomy & Astrophysics)

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   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. 

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