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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Asteroid (2) Pallas is the largest main-belt object not yet visited by a spacecraft, making its 1 surface geology largely unknown, and limiting our understanding of its origin and collisional 2 2 evolution. Previous ground-based observational campaigns returned different estimates of 3 its bulk density that are inconsistent with one another, one measurement 1 being compatible 4 within error bars with the icy Ceres (2.16±0.01 g/cm 3 ) 2 , and the other 3 compatible within 5 error bars with the rocky Vesta (3.46±0.03 g/cm 3 ) 4 . Here, we report high angular resolu-6 tion observations of Pallas performed with the extreme Adaptive-Optics (AO)-fed SPHERE 7 imager 5 on the Very Large Telescope (VLT). Pallas records a violent collisional history, with 8 numerous craters larger than 30 km in diameter populating its surface, and two large impact 9 basins that could relate to a family forming impact. Monte-Carlo simulations of the colli-10 sional evolution of the main belt correlate this cratering record to the high average impact 11 velocity of ∼11.5 km/s on Pallas -compared with an average of ∼5.8 km/s for the asteroid 12 belt, induced by Pallas' high orbital inclination (i = 34.8 • ) and orbital eccentricity (e = 0.23). 13Compositionally, Pallas' derived bulk density of (2.89 ± 0.08) g/cm 3 is fully compatible with 14 a CM chondrite-like body as suggested by its spectral reflectance in the 3-micron wavelength 15 region 6 . A bright spot observed on its surface may indicate an enrichment in salts during an 16 early phase of aqueous alteration, compatible with Pallas relatively high albedo of 12-17% 7, 8 , 17 although alternative origins are conceivable. 18 We used the sharp angular resolution (∼20 mas at 600 nm) of the SPHERE/ZIMPOL camera 5, 9 19 to characterize Pallas' bulk shape and surface properties with unprecedented details and, in turn, 20 bringing new constraints on its origin and evolution. In total, 11 series of images were acquired 21 during two apparitions as part of an ESO large program 10 . These images provide a full surface 22 coverage, resolving ∼120 to 130 pixels along Pallas' longest axis. The optimal angular resolu-23 3 tion of each image was restored with Mistral 11, 12 , a myopic deconvolution algorithm optimized for 24 images of objects with sharp boundaries, using a parametric point-spread function 13 . 25The deconvolved images unveil a strong surface topographic relief suggestive of a violent 26 collisional history ( Fig. 1). Numerous large (∼30-120-km sized) impact features, including several 27 craters with central peaks ( Supplementary Fig. 1), are ubiquitous on Pallas, forming a surface 28 reminiscent of a 'golf ball'. A total of 36 craters larger than 30 km in diameter (D c ) identified on 29 the images (Fig. 2, Fig 3 and Supplementary Table 1), implies an observed average number density 30 of N (D c ≥ 40 km) = 4.8 ± 0.7 × 10 −5 km −2 . The region with most favourable illumination in our 31 observations (Fig. 3) is more than 3 times more cratered than this average, with N (D c ≥ 40...

Infrared radiation emitted from an asteroid surface causes a torque that can significantly affect rotational state of the asteroid. The influence of small topographic features on this phenomenon, called the YORP effect, seems to be of utmost importance. In this work, we show that a lateral heat diffusion in boulders of suitable sizes leads to an emergence of a local YORP effect which magnitude is comparable to the YORP effect due to the global shape. We solve a three-dimensional heat diffusion equation in a boulder and its surroundings by the finite element method, using the FreeFem++ code. The contribution to the total torque is inferred from the computed temperature distribution. Our general approach allows us to compute the torque induced by a realistic irregular boulder. For an idealized boulder, our result is consistent with an existing one-dimensional model. We also estimated (and extrapolated) a size distribution of boulders on (25143) Itokawa from close-up images of its surface. We realized that topographic features on Itokawa can potentially induce a torque corresponding to a rotational acceleration of the order 10 −7 rad day −2 and can therefore explain the observed phase shift in light curves.

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.