Titan's gravity field and interior structure after Cassini

Abstract

Since its arrival at Saturn in 2004, Cassini performed nine flybys devoted to the determination of Titan's gravity field and its tidal variations. Here we present an updated gravity solution based on the final data set collected during the gravity-dedicated passes, before Cassini's plunge into Saturn's atmosphere. The data set includes an additional flyby (T110, March 2015, primarily devoted to imaging Titan's north polar lakes) carried out with the low-gain antenna. This flyby was particularly valuable because the closest approach occurred at a high latitude (75°N), over an area not previously sampled. Previously published gravity results (Iess, et al., 2012) indicated that Titan is subject to large eccentricity tides in response to the time varying perturbing potential exerted by Saturn. The magnitude of the response quadrupole field, expressed in the tidal Love number k 2 , was used to infer the existence of an internal ocean. The new gravity field determination provides an improved estimate of k 2 of about 0.62, accurate to a level of a few percent. The value is higher than the simplest models of Titan suggest and the interpretation is unclear; possibilities include a high density ocean (as high as 1300 kg/m 3), a partially viscous response of the deeper region, or a dynamic contribution to the tidal response. The new solution includes higher degree and order harmonic coefficients (up to 5) and offers an improved map of gravity anomalies. The geoid is poorly correlated with the topography, implying strong compensation. In addition, the updated geoid and its associated uncertainty could be used to refine the gravity-altimetry correlation analysis and for improved interpretation of radar altimetric data.

New constraints on the location of P9 obtained with the INPOP19a planetary ephemeris

“…The new data analysis relies on the updated knowledge of the Saturnian system as acquired throughout the mission: the better accuracies achieved for the Saturn moon ephemerides, and the last gravity solutions of Saturn and its main satellites produced by the radio science team (Durante et al 2019(Durante et al , 2020Iess et al 2014Iess et al , 2019. For each arc, we solved for the spacecraft initial position and velocity, corrections for orbital trim and reaction wheel desaturation maneuvers, and RTG-induced anisotropic acceleration.…”

“…The uncertainty on the normal points is given by the estimated covariance matrix of the range biases. We also added additional normal points deduced from the radio science solutions for the gravity-dedicated flybys of Titan (Durante et al 2019) and Grand Finale Saturn pericenters (Iess et al 2019). These range normal points were obtained by considering a pass-through of range data on the spacecraft orbit reconstructions that were produced for the gravity field solutions using only Doppler data.…”

Analysis of Cassini radio tracking data for the construction of INPOP19a: A new estimate of the Kuiper belt mass

“…The reconstruction process is currently a wellestablished procedure (Roth et al 2018), thanks to the comprehensive experience of the spacecraft dynamics acquired during the mission, the enhanced precision of Saturn's satellites ephemerides (Jacobson 2016a;Boone & Bellerose 2017), and current knowledge of the gravity field of the major bodies, which are measured on dedicated flybys (Iess et al 2019;Durante et al 2019;.…”

“…Because of the limited knowledge of such forces, in some passes additional stochastic accelerations to the level of 5 × 10 −9 km/s 2 along the spacecraft Z-axis 2 and 5 × 10 −11 km/s 2 on X and Y were estimated and updated every five minutes. We extracted the gravities of Titan and Saturn, which still introduce a major effect on the spacecraft dynamics, from Durante et al (2019) and Iess et al (2019) and these values were not adjusted in our fit.…”

“…Additional points were produced based on the OD solutions of the Grand Finale passes (Iess et al 2019) and Titan flybys (Durante et al 2019) dedicated to gravity.…”