Discovery of a spiral ring around Saturn

A team of astrophysicists from the CEA (Commissariat à l’Energie Atomique), the Université Paris 7 Denis Diderot and the CNRS in France has discovered that one of Saturn’s rings has a spiral shape and has published its results today in the American journal Science. This unusual astronomical configuration is perhaps the result of a collision with a recently formed small moon. The discovery is a great opportunity for astrophysicists who think that this ring is the only place in our solar system where astronomical bodies are still forming. The observation of this “nursery” will undoubtedly shed light on the processes of planet and moon formation in the solar system.

The rings of Saturn, discovered in 1610 by Galileo, are composed of dust and ice orbiting around the planet. Although they seem to be continuous when viewed from the Earth, these rings are composed of vast numbers of small particles, each with its own independent orbit. They range is size from a centimetre to several metres, with some objects about a kilometre in diameter. The rings have been named, in the order of their discovery from A to G. The F ring is the furthest out of Saturn’s main rings. It is situated 140 000 km from the planet and is composed of a central brilliant ring, called the “core” and of small concentric rings, called “filaments”. Using the images from the Cassini probe, the astrophysicists have discovered that in reality the filaments have a unique structure, in the form of a spiral, that twists at least three times around itself. Although several other planets in the solar system have rings (Uranus, Neptune and Jupiter), this spiral ring around Saturn is a new category of rings, with no known equivalent.

How has the spiral been formed? By what mechanism? Astrophysicists think that a collision between the core of the F ring and small moon that is in the process of forming within the ring could be the cause. This small moon, by crossing the ring’s core, disrupts the spatial distribution of the particles of which it is composed. They then spread out in a circle around Saturn and as a result of the orbital dynamics, naturally roll up to form a spiral. This scenario has been confirmed by computer simulations. Several moons, observed by Cassini in the vicinity of the F ring, could be responsible for this collision although we can’t yet say for certain which one. Because of the proximity to the planet and the influence of gravitational forces, the moons that are currently forming are only about 1 to 2 km in diameter. They are then destroyed by the same tidal forces. The ring is therefore a place where small moons are continuously being formed and then destroyed. These observations confirm the calculations of theoretical models that for a long time have predicted the formation of small moons in such an unusual ring.

Astrophysicists think that these small moons are very young, being not more than a few years old, and are formed by the accretion of matter within the ring. As a comparison, the 9 planets in the solar system and their 160 moons were formed 4.5 billion years ago. The F ring of Saturn is probably therefore a sort of modern-day “nursery” for astronomical bodies that will provide astrophysicists with a better understanding of the mechanisms by which the celestial bodies in our solar system were originally formed. In 2009 the satellite Prometheus will collide with the F ring. It is very probable that new rings will then be formed as a result of this collision and these will be monitored directly by astrophysicists by means of the Cassini probe.