Last update: November 4, 2025

Saturn: The Harmony of Orbital Resonances

Saturn and its rings as seen by the Cassini probe

Saturn, the lightest planet

Saturn is the sixth planet in the solar system and the second in mass after Jupiter. Mostly composed of hydrogen and helium, it beautifully illustrates how gravity can create structures of stunning beauty.

Saturn's equatorial diameter is about 120,536 km (≈9.5 Earth diameters - 12,742 km). With an average density of only 0.69 g/cm³, Saturn is the least dense planet in the entire solar system. This means that if it could be placed in a giant ocean of water, it would float. This lightness is due to its composition, dominated by hydrogen and helium, which together make up more than 96% of its mass. Its internal structure consists of a small rocky core, a metallic hydrogen mantle, and a thick gaseous envelope.

N.B.:
This low density explains the planet's flattened shape: the rapid rotation speed (period of about 10 hours 33 minutes) causes strong oblation, reducing the polar radius (54,364 km) compared to the equatorial radius (60,268 km).

A planet where gravity becomes geometry

The beauty of the rings lies in their apparent visual simplicity, hiding extraordinary mathematical complexity. The observed divisions, such as the famous Cassini Division, are not cosmic accidents but regions where orbital resonance effects with Saturn's moons prevent any accumulation of matter.

Its rings, extremely thin, are not solid disks but a myriad of ice, dust, and rock particles orbiting at precise speeds, according to Kepler's law \((T^2 \propto r^3)\). Each particle, orbiting around the planetary center of mass, follows a trajectory dictated by the balance between centripetal force \((F_c = m v^2 / r)\) and gravity \((F_g = G M m / r^2)\). This subtle balance defines the stability of the rings and explains their division into distinct zones. Thus, the famous Cassini Division is about 4,800 km wide, between rings A and B, resulting from an orbital resonance with the moon Mimas.

N.B.:
The Cassini Division was first observed in 1675 by Giovanni Domenico Cassini (1625-1712), a Franco-Italian astronomer. It represents the first empirical understanding of a natural orbital resonance.

The dynamics of the rings: a ballet of resonances

The concept of orbital resonance is central to Saturn's structure. When ring particles complete, for example, two revolutions while Mimas completes one, they undergo periodic perturbations. These resonant oscillations expel matter from certain areas, creating the observed gaps. The beauty of Saturn thus arises from this mathematical interaction between gravity and motion, a dynamic order whose regularity the human eye perceives as aesthetic.

Aesthetic and mechanical characteristics of Saturn's rings
Ring	Diameter (km)	Aesthetic characteristic	Mechanical cause
D Ring	66,900 - 74,510	Very faint and diffuse	Resonance with electromagnetic forces
C Ring	74,658 - 91,975	Transparent and subtle	Dispersion by micro-satellites
B Ring	91,975 - 117,507	Brightest and densest	Intense gravitational confinement
Cassini Division	117,507 - 122,340	Well-defined dark band	2:1 resonance with Mimas
A Ring	122,340 - 136,775	Bright with radial structures	Density waves created by moons

Source: NASA Solar System Exploration - Saturn and Cassini-Huygens Mission to Saturn.

A model of fragile stability

Saturn's rings are not eternal. Measurements from the Cassini mission have shown that ring material literally "rains" onto the planet due to the magnetic field. It is estimated that these structures could disappear in less than 100 million years, a negligible time on the cosmic scale. Thus, Saturn offers us, for a limited time, the contemplation of a natural organization where dynamic balance becomes art.

The Mysterious Worlds in Orbit

Saturn is not only surrounded by spectacular rings; it also reigns over a true miniature solar system, composed of more than 145 confirmed moons. Each of these moons has its own identity, from icy giants to small irregular worlds, forming a cosmic family of remarkable diversity.

Physical Characteristics of Saturn's Main Moons
Moon	Discoverer / Year	Diameter (km)	Density (g·cm⁻³)	Main Composition	Physical Peculiarity	Mythological Character
Titan	Christiaan Huygens (1655)	5,150	1.88	Water ice, nitrogen, hydrocarbons	Dense atmosphere, liquid methane seas. Titan alone accounts for over 96% of the total mass of Saturn's moons.	The Titans, giant gods defeated by Zeus during the Titanomachy
Rhea	Gian Domenico Cassini (1672)	1,528	1.23	Water ice and silicates	May have a tenuous ring of debris	Rhea, Titaness mother of the Olympian gods
Iapetus	Gian Domenico Cassini (1671)	1,471	1.09	Water ice and dark materials	Two-tone surface, 20 km equatorial ridge	Iapetus, one of the Titans, father of Prometheus, Atlas, and Epimetheus
Dione	Gian Domenico Cassini (1684)	1,123	1.48	Water ice and silicate rocks	Numerous ancient tectonic fractures	Dione, Titaness associated with fertility and mother of Aphrodite in some traditions
Tethys	Gian Domenico Cassini (1684)	1,062	0.98	Almost pure water ice	Huge Ithaca Chasma valley (2,000 km)	Tethys, Titaness of the seas and wife of Oceanus
Enceladus	William Herschel (1789)	504	1.61	Water ice, salts, organic compounds	Active cryovolcanic plumes, internal ocean	Enceladus, giant buried under Mount Etna, symbol of subterranean forces
Mimas	William Herschel (1789)	396	1.15	Water ice	Giant Herschel crater (130 km), "Death Star" shape	Mimas, giant killed by Ares during the Gigantomachy
Hyperion	William Bond and William Lassell (1848)	270	0.54	Porous water ice	Chaotic rotation, spongy surface	Hyperion, Titan of light and father of the Sun, Moon, and Dawn
Phoebe	William Pickering (1899)	213	1.63	Water ice, carbon, silicates	Retrograde orbit, captured object from the Kuiper Belt	Phoebe, Titaness of brightness and grandmother of Apollo and Artemis
Janus	Audouin Dollfus (1966)	179	0.63	Water ice and silicates	Shares the same orbit as Epimetheus; orbital swap every 4 years	Janus, Roman god with two faces, guardian of beginnings and transitions