# Interior properties of the inner Saturnian moons from space astrometry data

## TL;DR

This study analyzes over ten thousand astrometric measurements from Cassini to investigate the interior properties and orbital dynamics of Saturn's inner moons, providing new insights into their densities and internal structures.

## Contribution

It offers the first detailed analysis of Cassini ISS data on Saturn's inner moons, estimating their densities, interior properties, and moments of inertia using orbital and libration data.

## Key findings

- Janus and Epimetheus likely have homogeneous interiors.
- Inconsistencies found in Pandora and Prometheus may be due to modeling issues.
- Libration measurements can improve constraints on moons' moments of inertia.

## 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.