Josselin Desmars

Strong Tidal Dissipation in Saturn and Constraints on Enceladus' Thermal State from Astrometry

Tidal interactions between Saturn and its satellites play a crucial role in both the orbital migration of the satellites and the heating of their interiors. Therefore, constraining the tidal dissipation of Saturn (here the ratio k2/Q) opens the door to the past evolution of the whole system. If Saturn’s tidal ratio can be determined at different frequencies, it may also be possible to constrain the giant planet’s interior structure, which is still uncertain. Here, we try to determine Saturn’s tidal ratio through its current effect on the orbits of the main moons, using astrometric data spanning more than a century. We find an intense tidal dissipation (k2/Q = (2.3 ± 0.7) × 10 −4 ), which is about 10 times higher than the usual value estimated from theoretical arguments. As a consequence, eccentricity equilibrium for Enceladus can now account for the huge heat emitted from Enceladus’ south pole. Moreover, the measured k2/Q is found to be poorly sensitive to the tidal frequency, on the short frequency interval considered. This suggests that Saturn’s dissipation may not be controlled by turbulent friction in the fluid envelope as commonly believed. If correct, the large tidal expansion of the moon orbits due to this strong Saturnian dissipation would be inconsistent with the moon formations 4.5 Byr ago above the synchronous orbit in the Saturnian subnebulae. But it would be compatible with a new model of satellite formation in which the Saturnian satellites formed possibly over a longer timescale at the outer edge of the main rings. In an attempt to take into account possible significant torques exerted by the rings on Mimas, we fitted a constant rate da/dt on Mimas’ semi-major axis as well. We obtained an unexpected large acceleration related to a negative value of da/dt =− (15.7 ± 4.4) × 10 −15 AU day −1 . Such acceleration is about an order of magnitude larger than the tidal deceleration rates observed for the other moons. If not coming from an astrometric artifact associated with the proximity of Saturn’s halo, such orbital decay may have significant implications on the Saturn’s rings.

New constraints on Saturn's interior from Cassini astrometric data

Using astrometric observations spanning more than a century and including a large set of Cassini data, we determine Saturns tidal parameters through their current effects on the orbits of the eight main and four coorbital Moons. We have used the latter to make the first determination of Saturns Love number from observations, k2=0.390 ± 0.024, a value larger than the commonly used theoretical value of 0.341 (Gavrilov & Zharkov, 1977), but compatible with more recent models (Helled & Guillot, 2013) for which the static k2 ranges from 0.355 to 0.382. Depending on the assumed spin for Saturns interior, the new constraint can lead to a significant reduction in the number of potential models, offering great opportunities to probe the planets interior. In addition, significant tidal dissipation within Saturn is confirmed (Lainey et al., 2012) corresponding to a high present-day tidal ratio k2/Q=(1.59 ± 0.74) × 10−4 and implying fast orbital expansions of the Moons. This high dissipation, with no obvious variations for tidal frequencies corresponding to those of Enceladus and Dione, may be explained by viscous friction in a solid core, implying a core viscosity typically ranging between 1014 and 1016 Pa.s (Remus et al., 2012). However, a dissipation increase by one order of magnitude at Rheas frequency could suggest the existence of an additional, frequency-dependent, dissipation process, possibly from turbulent friction acting on tidal waves in the fluid envelope of Saturn (Ogilvie & Lin, 2004; Fuller et al. 2016).

PLUTO'S ATMOSPHERE FROM THE 2015 JUNE 29 GROUND-BASED STELLAR OCCULTATION AT THE TIME OF THE NEW HORIZONS FLYBY *

We present results from a multi-chord Pluto stellar occultation observed on 2015 June 29 from New Zealand and Australia. This occurred only two weeks before the NASA New Horizons flyby of the Pluto system and serves as a useful comparison between ground-based and space results. We find that Plutos atmosphere is still expanding, with a significant pressure increase of 5 ± 2% since 2013 and a factor of almost three since 1988. This trend rules out, as of today, an atmospheric collapse associated with Plutos recession from the Sun. A central flash, a rare occurrence, was observed from several sites in New Zealand. The flash shape and amplitude are compatible with a spherical and transparent atmospheric layer of roughly 3 km in thickness whose base lies at about 4 km above Plutos surface, and where an average thermal gradient of about 5 K km−1 prevails. We discuss the possibility that small departures between the observed and modeled flash are caused by local topographic features (mountains) along Plutos limb that block the stellar light. Finally, using two possible temperature profiles, and extrapolating our pressure profile from our deepest accessible level down to the surface, we obtain a possible range of 11.9–13.7 μbar for the surface pressure.

VESPA: A community-driven Virtual Observatory in Planetary Science

The VESPA data access system focuses on applying Virtual Observatory (VO) standards and tools to Planetary Science. Building on a previous EC-funded Europlanet program, it has reached maturity during the first year of a new Europlanet 2020 program (started in 2015 for 4 years). The infrastructure has been upgraded to handle many fields of Solar System studies, with a focus both on users and data providers. This paper describes the broad lines of the current VESPA infrastructure as seen by a potential user, and provides examples of real use cases in several thematic areas. These use cases are also intended to identify hints for future developments and adaptations of VO tools to Planetary Science.

A CONVENIENT SOLUTION TO GEOMETRIC DISTORTION AND ITS APPLICATION TO PHOEBE'S OBSERVATIONS

A simple but effective approach is proposed for measuring the geometric distortion of a CCD field of view of a ground-based telescope. For three open clusters (M35, M67, and NGC 2324), 425 CCD frames taken by a 1 m telescope at the Yunnan Observatory are used to test this approach. It is found that the geometric distortion pattern depends strongly on the corresponding filter used. The geometric distortion is then used to correct the pixel positions for Phoebe, the ninth satellite of Saturn, and its reference stars imaged in 220 CCD frames taken by the same telescope. The standard deviation of the (O - C; observed minus computed) residuals of Phoebe is significantly improved after correcting the geometric distortions.

Statistical and numerical study of asteroid orbital uncertainty

Context. The knowledge of the orbit or the ephemeris uncertainty of asteroids presents a particular interest for various purposes. These quantities are, for instance, useful for recovering asteroids, for identifying lost asteroids, or for planning stellar occultation campaigns. They are also needed for estimating the close approach of near-Earth asteroids, and the subsequent risk of collision. Ephemeris accuracy can also be used for instrument calibration or for scientific applications. Aims. Asteroid databases provide information about the uncertainty of the orbits and allow the measure of the quality of an orbit. This paper analyses these di erent uncertainty parameters and estimates the impact of the di erent measurements on the uncertainty of orbits. Methods. We dealt with two main databases, astorb and mpcorb, that provide uncertainty parameters for asteroid orbits. Statistical methods were used to estimate orbital uncertainty and to compare them with parameters from the databases. Simulations were also generated to deal with specific measurements such as the future Gaia mission or present radar measurements. Results. Relations between the uncertainty parameter and the characteristics of the asteroid (orbital arc, absolute magnitude, etc.) are highlighted. Moreover, a review of the di erent measuments are compiled and their impact on the accuracy of the orbit is also estimated.

Phoebe's orbit from ground-based and space-based observations

Context. The ephemeris of Phoebe, the ninth satellite of Saturn, is not very accurate. Previous dynamical models were usually too simplified, the astrometry is heterogeneous and, the Saturn’s ephemeris itself is an additionnal source of error. Aims. The aim is to improve Phoebe’s ephemeris by using a large set of observations, correcting some systematic errors and updating the dynamical model. Methods. The dynamical model makes use of the most recent ephemeris of planets and Saturnian satellites. The astrometry of Phoebe is improved by using a compilation of ground-based and space-based observations and by correcting the bias in stellar catalogues used for the reduction. Results. We present an accurate ephemeris of Phoebe with residuals of 0.45 arcsec and with an estimated accuracy of Phoebe’s position of less that 100 km on 1990‐2020 period.

RESULTS FROM THE 2014 NOVEMBER 15TH MULTI-CHORD STELLAR OCCULTATION BY THE TNO (229762) 2007 UK126

We present results derived from the first multi-chord stellar occultation by the trans-Neptunian object (229762) 2007 UK

Ground-based astrometry calibrated by Gaia DR1: new perspectives in asteroid orbit determination

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New orbits of irregular satellites designed for the predictions of stellar occultations up to 2020, based on thousands of new observations

, observed on 2014 November 15. The event was observed by the Research and Education Collaborative Occultation Network (RECON) project and International Occultation Timing Association (IOTA) collaborators throughout the United States. Use of two different data analysis methods obtain a satisfactory fit to seven chords, yelding an elliptical fit to the chords with an equatorial radius of