Julien Salmon

The main belt asteroid (52246) Donaldjohanson (DJ) is a likely member of the Erigone asteroid family. This implies that DJ is a fragment of a larger parent body that was destroyed in a collision about 155 million years ago. We report observations taken during a flyby of DJ by the Lucy spacecraft. We found that DJ is composed of two heavily cratered lobes, connected by a smoother neck, with overall dimensions 8.8 kilometers (km) by 4.4 km by 3.1 km. The crater density is consistent with the Erigone family’s age, except for craters <0.4 km, which have been preferentially erased. DJ rotates slowly in a tumbling state, likely owing to spin-down by radiative forces. Surface spectra show iron-bearing phyllosilicates, indicating moderate aqueous evolution on the parent body.

We present work to update current maps of the thermal properties of Enceladus using thermal observations from the Cassini Composite InfraRed Spectrometer (CIRS). In 2010, the first maps of Enceladus’ thermal inertia were published that used what CIRS data was available at the time (Howett et al., 2010). These maps were resolved into some latitude zones, and overall conveyed lower thermal inertia and albedo at higher latitudes, and confirmed that like other cold, icy moons of Saturn its surface had low (< 50 MKS) thermal inertia. Improvements to these maps using the totality of the CIRS Focal Plane 1 data (10-600 cm-1 / 16.7-1000 μm) from the mission with updated error estimates will yield better spatial resolution in addition to higher precision estimates of thermal inertia and albedo. This will be particularly useful for improving models of surface temperature or estimating endogenic heat fluxes, like those at Enceladus’ south pole, associated with dissipation of heat from beneath.Acknowledgements: Thanks are given to the NASA Cassini Data Analysis program that funded this work (80NSSC20K0477 and 80NSSC24K0373). Reference:Howett, C.J.A., Spencer, J.R., Pearl, J. and Segura, M., 2010. Thermal inertia and bolometric Bond albedo values for Mimas, Enceladus, Tethys, Dione, Rhea and Iapetus as derived from Cassini/CIRS measurements. Icarus, 206(2), pp.573-593.

This article explores the different formation scenarios of the Kronian moons system in the context of a highly dissipative Saturn, with the objective of identifying the most likely of these scenarios. First, we review the diversity of objects – moons and rings – orbiting solar system giant planets, and the diversity of their architectures, which formation scenarios must reproduce. We then identify in this broader context the specific features of the Saturn system, such as the particularly large spectrum of its moon masses, the uniqueness of Titan and the presence of both dense and tenuous rings, before discussing the applicability of the different giant planet moon formation scenarios to the Saturn case. We discuss each of the most relevant scenarios and their respective merits. Finally, we tentatively propose a “favorite” scenario and we identify the key observations to be made by future space missions and/or Earth-based telescopic observations to validate this scenario or possibly alternative ones.

This article explores the different formation scenarios of the Kronian moons system in the context of a highly dissipative Saturn, with the objective of identifying the most likely of these scenarios. First, we review the diversity of objects - moons and rings - orbiting solar system giant planets, and the diversity of their architectures, which formation scenarios must reproduce. We then identify in this broader context the specific features of the Saturn system, such as the particularly large spectrum of its moon masses, the uniqueness of Titan and the presence of both dense and tenuous rings, before discussing the applicability of the different giant planet moon formation scenarios to the Saturn case. We discuss each of the most relevant scenarios and their respective merits. Finally, we tentatively propose a "favorite" scenario and we identify the key observations to be made by future space missions and/or Earth-based telescopic observations to validate this scenario or possibly alternative ones.

We present results from an occultation of a star by the asteroid (269) Justitia on 2023 August 31 UT using a deployment of 34 stations. Of the deployed stations, 29 were successful in collecting useful data and 19 recorded a positive occultation event. All stations were regularly spaced 2.45 km apart in the cross-track direction to cover ±3σ of the ephemeris uncertainty with a planning diameter of 59 km. We find that the shape of Justitia is irregular, with large-scale facets and a circular-equivalent radius in the range of 28.5–28.9 km. The inferred albedo is p _V = 0.072 ± 0.007 using a rotation-corrected absolute magnitude of H _V = 9.72. Astrometry from the occultation is provided.

We investigate aspects of the co-accretion + giant impact scenario proposed by Morbidelli et al. (2012) for the origin of the Uranian satellites. In this model, a regular satellite system formed during gas accretion is impulsively destabilized by a Uranus-tipping impact, producing debris that ultimately re-orients to the planet’s new equatorial plane and re-accumulates into Uranus’ current large moons. We first investigate the nodal randomization of a disk of debris resulting from disruptive collisions between the hypothesized prior satellites. Consistent with Morbidelli et al., we find that an impact-generated interior c-disk with mass ≥10⁻² Uranus masses is needed to cause sufficient nodal randomization to appropriately realign the outer debris disk. We then simulate the reaccumulation of the outer debris disk into satellites and find that disks with larger initial radii are needed to produce an outer debris disk that extends to Oberon’s distance, and that Uranus’ obliquity prior to the giant impact must have been substantial, ≥40°, if its original co-accreted satellite system was broadly similar in radial scale to those at Jupiter and Saturn today. Finally, we explore the subsequent evolution of a massive, water-dominated inner c-disk as it condenses, collisionally spreads, and spawns new moons beyond the Roche limit. We find that intense tidal dissipation in Uranus (i.e., ( Q / k 2 ) U ≤ 10 2 ) is needed to prevent large icy moons spawned from the inner disk from expanding beyond the synchronous orbit, where they would be long lived and inconsistent with the lack of massive inner moons at Uranus today. We conclude that while a co-accretion + giant impact is viable it requires rather specific conditions.

We present observations of five stellar occultations for (11351) Leucus and reports from two efforts on (21900) Orus. Both objects are prime mission candidate targets for the Lucy Discovery mission. Combined results for Leucus indicate a very dark surface with p _V = 0.037 ± 0.001, which is derived from the average of the multichord occultations. Our estimate of the triaxial ellipsoidal shape is for axial diameters of 63.8 × 36.6 × 29.6 km assuming that the spin pole is normal to the line of sight. The actual shape of the object is only roughly elliptical in profile at each epoch. Significant topography is seen with horizontal scales up to 30 km and vertical scales up to 5 km. The most significant feature is a large depression on the southern end of the object as seen from a terrestrial viewpoint. For this work we developed a method to correct for differential refraction, accounting for the difference in color between the target object and the reference stars for astrometry derived from ground-based images.

We present the results from four stellar occultations by (486958) Arrokoth, the flyby target of the New Horizons extended mission. Three of the four efforts led to positive detections of the body, and all constrained the presence of rings and other debris, finding none. Twenty-five mobile stations were deployed for 2017 June 3 and augmented by fixed telescopes. There were no positive detections from this effort. The event on 2017 July 10 was observed by the Stratospheric Observatory for Infrared Astronomy with one very short chord. Twenty-four deployed stations on 2017 July 17 resulted in five chords that clearly showed a complicated shape consistent with a contact binary with rough dimensions of 20 by 30 km for the overall outline. A visible albedo of 10% was derived from these data. Twenty-two systems were deployed for the fourth event on 2018 August 4 and resulted in two chords. The combination of the occultation data and the flyby results provides a significant refinement of the rotation period, now estimated to be 15.9380 ± 0.0005 hr. The occultation data also provided high-precision astrometric constraints on the position of the object that were crucial for supporting the navigation for the New Horizons flyby. This work demonstrates an effective method for obtaining detailed size and shape information and probing for rings and dust on distant Kuiper Belt objects as well as being an important source of positional data that can aid in spacecraft navigation that is particularly useful for small and distant bodies.

The numerical modeling of coexisting circumplanetary disks/rings and satellites is particularly challenging because each part of the system requires a very different approach. Disks are generally well represented by a fluid-like dense medium, whose evolution can be calculated by a hydrocode. On the other hand, the orbital evolution of satellites is generally performed using N-body integrators. We have developed a new numerical model that combines a one-dimensional hydrocode with the N-body integrator SyMBA. The disk evolves due to its viscosity, and resonant torques from satellites. The latter is applied to the satellites as an additional “kick” to their accelerations. The integrator also includes the ability to spawn new moonlets at the disk’s outer edge if the latter expands beyond a material-dependent Roche limit, as well as the effects of tidal dissipation in the planet and/or the satellite on the satellite orbits. The resulting integrator allows one to accurately model the evolution of an inner circumplanetary disk, and the formation of satellites by accumulation of disk material, all within a single self-consistent framework. Potential applications include the formation of Earth’s Moon, the evolution of the inner Saturn system, the Martian and uranian moons, and compact exoplanetary systems.