Context. Asteroid (16) Psyche is the target of the NASA Psyche mission. It is considered one of the few main-belt bodies that could be an exposed proto-planetary metallic core and that would thus be related to iron meteorites. Such an association is however challenged by both its near-and mid-infrared spectral properties and the reported estimates of its density. Aims. Here, we aim to refine the density of (16) Psyche to set further constraints on its bulk composition and determine its potential meteoritic analog. Methods. We observed (16) Psyche with ESO VLT/SPHERE/ZIMPOL as part of our large program (ID 199.C-0074). We used the high angular resolution of these observations to refine Psyche's three-dimensional (3D) shape model and subsequently its density when combined with the most recent mass estimates. In addition, we searched for potential companions around the asteroid. Results. We derived a bulk density of 3.99 ± 0.26 g·cm −3 for Psyche. While such density is incompatible at the 3sigma level with any iron meteorites (∼7.8 g·cm −3 ), it appears fully consistent with that of stony-iron meteorites such as mesosiderites (density ∼4.25 g·cm −3 ). In addition, we found no satellite in our images and set an upper limit on the diameter of any non-detected satellite of 1460 ± 200 m at 150 km from Psyche (0.2% × R Hill , the Hill radius) and 800 ± 200 m at 2,000 km (3% × R Hill ). Conclusions. Considering that the visible and near-infrared spectral properties of mesosiderites are similar to those of Psyche, there is merit to a long-published initial hypothesis that Psyche could be a plausible candidate parent body for mesosiderites.

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[1] We have studied the production of hot O and C atoms, and hot CO 2 and CO molecules in the Martian upper atmosphere and exosphere by dissociative recombination (DR) of O 2 + and CO + ions, and sputtering of the atmosphere by incident O + pick-up ions. Production and collisional thermalization of the hot particles in the upper atmosphere are described by using a unique Monte Carlo test particle approach to simulate both nonthermal processes. Velocity distributions, atmospheric loss rates, and density profiles are derived for suprathermal O, C, CO, and CO 2 at low and high solar activity. At high solar activity the hot oxygen escape rate estimated from DR of O 2 + is found to be less than two times the sputtering rate. Sputtering is found to efficiently populate the corona with molecular species such as CO and CO 2 at high solar activity and also to produce a carbon escape rate that is comparable to that derived from the major photochemical sources. Dissociation of CO 2 molecules by the impacting pick-up ions flux are found to produce about 50% of the sputtered exospheric oxygen density at high solar activity. Collisions of the background atmospheric gas with hot O atoms produced by DR of O 2 + produce densities of hot CO 2 and CO molecules larger than 10 2 cm À3 for altitudes lower than 1000 km, at both high and low solar activity. Interestingly, the hot CO 2 density scale height is observed to be process dependent. The hot oxygen energy distributions associated with sputtering and DR near the exobase are also found to follow distinct decreasing energy laws. We suggest that the effects of the solar zenithal angle (SZA), crustal magnetic fields, and atmospheric tides on the ionospheric structure may produce exospheric signatures.

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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.