PARIS • An enigmatic ring of icy particles circling Saturn, herded into a narrow ribbon by two tiny moons, was probably born of a cosmic collision, according to a study just published.

The so-called F ring, some 140,000km beyond the sixth planet from the Sun, orbits at the border between Saturn's other rings and several moons. Scientists have long known that these so-called shepherd moons were partly responsible for keeping the F ring in tight formation. What they did not know was how this unusual configuration came into being.

Astronomers Ryuki Hyodo and Keiji Ohtsuki, from Kobe University in Japan, used computer simulations to show that Prometheus and Pandora are likely the by-product of a collision at the outer edge of Saturn's ring system. Previous speculation along these lines concluded that two icy mini-moons crashing head-on would have simply disintegrated, adding yet another ring to Saturn's collection.

But what if the objects were made of something less fragile, and hit each other at an angle? In that case, "such an impact results in only partial disruption" of the mini- moons, the authors conclude. The collision would also produce "the formation of a narrow ring of particles" which becomes a new ring.

The authors further speculate that this might not be an oddity but rather the "natural outcome" of ring formation under certain conditions for giant gas planets.

This "may explain not only Saturn's F ring, but also features of the Uranian system", Dr Aurelien Crida from France's National Centre for Scientific Research wrote in a comment in Nature Geoscience.

AGENCE FRANCE-PRESSE