Interior properties of the inner saturnian moons from space astrometry data

Abstract

During the thirteen years in orbit around Saturn before its final plunge, the Cassini spacecraft provided more than ten thousand astrometric measurements. Such large amounts of accurate data enable the search for extremely faint signals in the orbital motion of the saturnian moons. Among these, the detection of the dynamical feedback of the rotation of the inner moons of Saturn on their respective orbits becomes possible. Using all the currently available astrometric data associated with Atlas, Prometheus, Pandora, Janus and Epimetheus, we first provide a detailed analysis of the Cassini Imaging Science Subsystem (ISS) data, with special emphasis on their statistical behavior and sources of bias. Then, we give updated estimates of the moons' averaged densities and try to infer more details about their interior properties by estimating the physical librations for Prometheus, Pandora, Epimetheus and Janus from anomalies in their apsidal precession. Our results are compatible with a homogeneous interior for Janus and Epimetheus, within the uncertainty of the measurements. On the other hand, we found some inconsistency for Pandora and Prometheus, which might result from a dynamical mismodeling of Saturn's gravity field. Last, we show how the synergistic introduction of libration measurements directly derived from imaging should allow the moons' moments of inertia to be better constrained.

Mars moon ephemerides after 14 years of Mars Express data

“…A third but important dynamical effect was the introduction of the forced libration on the rotation of Phobos (Jacobson 2010). This perturbation introduces a secular effect on the periapsis of Phobos (Borderies & Yoder 1990;Lainey et al 2019) that can barely be masked in the fitting procedure. The secular drift, associated with the quadrupole field of Phobos on its periapsis (or any moon in a somewhat similar configuration) was reported by Borderies & Yoder (1990) and recalled by Jacobson (2010) to be…”

“…A rejection criterion beyond 3σ was applied to ground observations. Last, spacecraft data were weighted in the same way as in Lainey et al (2019). Here we present two important new data sets that were not available to the former works.…”

The NAROO digitization center

“…Using observations of natural satellites, NAROO will also be extremely useful for planetology studies. Indeed, moons can be used to test tidal mechanisms at play in giant planets (Lainey et al 2012), tidal dissipation inside very active satellites such as Io (Lainey et al 2009), characterizing the mass distribution inside moons (Lainey et al 2019), or simply determining the mass of an outer moon (Emelyanov 2005). To give just a few examples of what NAROO will be able to do, the availability of old plates of the Jovian system will allow us to test the Fuller hypothesis of a possible tidal lock mechanism currently at play within Jupiter (Fuller et al 2016).…”

Mars moon ephemerides after 14 years of Mars Express data

“…A rejection criteria beyond 3σ was applied to ground observations. Last, spacecraft data were weighted in the same way as in Lainey et al (2019). Here we present two important new data sets, not available to the former works.…”

“…A third but important dynamical effect was the introduction of the forced libration on the rotation of Phobos (Jacobson (2010)). Indeed, such perturbation introduces a secular effect on the periapsis of Phobos (Borderies & Yoder (1990), Lainey et al (2019)), barely maskable in the fitting procedure. The secular drift, associated with the quadrupole field of Phobos on its periapsis (or any moon in somewhat similar configuration) was given by Borderies & Yoder (1990) and recalled by Jacobson (2010) to be…”