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Last update: October 7, 2025

Rhea and Saturn’s Rings: Orbital Dynamics

Rhea and Saturn's rings

Physical and Orbital Characteristics of Rhea

Rhea is Saturn's second-largest moon after Titan, with an average diameter of about 1,527 km and an estimated mass of 2.3 × 1021 kg. Its mean density, close to 1.24 g/cm3, indicates a mostly icy composition with a significant rocky fraction. The surface of Rhea is heavily cratered, showing features such as crater chains and linear fractures, revealing an ancient geological history.

Orbitally, Rhea revolves around Saturn at an average distance of 527,000 km and has synchronous rotation, always showing the same face to the planet. Its orbital period is about 4.5 Earth days. The orbital inclination of 0.35° and the low eccentricity contribute to stable yet significant gravitational interactions with the particles of Saturn's outer rings. Rhea orbits just beyond the E ring. It does not directly influence the main rings (A, B, C, D) but may have a marginal effect on the outer E ring and distant dust through gravitational resonances. Perturbations of the F ring are mainly caused by the nearby moons Prometheus and Pandora.

Types of Observed Perturbations

Rhea's gravitational effects on the rings appear in several forms:

Influence on Ring Stability

Numerical simulations show that Rhea's presence stabilizes some ring sections while perturbing others. These interactions create a dynamic balance between gravitational forces and interparticle collisions.

Comparative Table of Gravitational Perturbations

Gravitational effects of Rhea on Saturn's rings
Type of perturbationAffected zoneMechanismComment
Density waveOuter A ring3:2 orbital resonance with RheaCauses local periodic density variations, producing wave patterns in the ring.
Low-density zoneOuter B ringCumulative gravitational interactionParticles gradually removed, forming gaps or less dense regions.
Orbital modificationsF ringTidal forces induced by RheaSmall changes in particle velocity gradually reshape the ring's structure.

Source: NASA – Rhea

Rhea and Neighboring Moons in the Saturnian System

Orbital Position and Interactions

Rhea is part of a group of medium and outer moons of Saturn that gravitationally interact with the rings and with each other. Its immediate neighbors include Titan, Saturn's second-largest moon, as well as intermediate moons such as Iapetus, Tethys, and Dione. Each moon has a near-circular orbit with low inclination, forming a complex network of orbital resonances and mutual perturbations.

Influence on the Rings

The orbital distances between these satellites are large enough to prevent direct collisions, but cumulative gravitational effects gradually modify trajectories and orbital velocities, influencing the stability of Saturn's outer rings. Rhea, with its intermediate mass, acts as a modulator of perturbations induced by Titan and Iapetus, participating in the overall dynamics of the system.

Comparison of Physical Characteristics

Comparative analysis of physical and orbital properties of these moons also reveals significant differences in composition and structure. Rhea and Tethys are mostly icy, while Iapetus shows a marked dichotomy between a bright and a dark hemisphere.

Comparative Table of Physical Properties of Rhea and Neighboring Moons

Physical and orbital properties of Rhea and its neighboring moons
MoonDiameter (km)Mass (kg)Average orbital distance (km)Density (g/cm3)Particularities
Rhea1,5272.3 × 1021527,0001.24Heavily cratered surface, mostly ice
Tethys1,0626.2 × 1020294,0000.98Large icy plains, Odysseus crater
Dione1,1231.1 × 1021377,0001.48Combination of icy and cratered terrains, linear fractures
Iapetus1,4712.2 × 10213,561,0001.27Bright/dark dichotomy, synchronous rotation
Titan5,1511.35 × 10231,222,0001.88Dense atmosphere, cryovolcanism

Source: NASA – Solar System Exploration

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