Cassini sees 'snowball fight' in Saturn ring
It is like a huge snowball fight and it is taking place in the outer Solar System around Saturn.
Scientists working on the Cassini probe have witnessed small clumps of ice ploughing through one of the gas giant's main rings - its F-ring.
As they plunge through, the km-sized ice balls leave glittering trails behind them referred to as mini-jets.
Some of these collisions trace quite exotic shapes in the F-ring that look like barbs on a harpoon.
The research has been presented here at the European Geosciences Union (EGU) meeting in Vienna, Austria, by Carl Murray, a Cassini imaging team member based at Queen Mary University of London, UK.
The F-ring is the outermost of Saturn's main rings. It is located 3,000km beyond the bright A-ring and has a circumference approaching 900,000km.
The Cassini imaging team had been watching the 40km-wide Prometheus moon dance along the edge of this ring for some time.
The moon's gravitational perturbations regularly produce channels and ripples in the F-ring, and it was known some of the disturbed ice particles could clump together. But it was assumed collisions or tidal forces in their orbit around Saturn would soon break these clumps apart.
"We know that Prometheus, as well as producing regular patterns, is capable of producing concentrations of material in the ring," Prof Murray explained to BBC News.
"We just call them large snowballs, and if these things can survive - because Prometheus will come around to the same part of the F-ring again and interact with them again - they may grow, and maybe these are what form the moonlets that collide with the core of the F-ring."
Cassini's archive of pictures would certainly seem to suggest they can survive and play their own game in the F-ring.
The discovery was somewhat lucky. It was while observing Prometheus one more time that Prof Murray and colleagues noticed a jet in the ring that could not have been formed by the moon or by a quasi-moon referred to simply as S6, which is known to cross right through the ring on occasions.
And when the team examined 20,000 images stretching back over Cassini's seven years at Saturn, the researchers found 500 examples of similar rogue jets.System simulation
It is clear the ice balls collide with the F-ring at slow speed - about two meters per second. The jets they produce in their wake are about 40-180km long.
In some instances, it is the jets of lone rogues that are seen. In other cases, there is evidence that groups of ice balls have ploughed through the F-ring en masse to produce a series of jets.
Saturn's rings are composed primarily of water ice. Although the rings extend some 140,000km from the centre of the planet, their average thickness is far less than 100m.
Apart from their great beauty, scientists are fascinated by the rings because they can be used as a model to study Solar System formation.
Some of the behaviours seen in the rings are probably very similar to the ones that occurred in the disc of material that collected around the infant Sun more than four and a half billion years ago, and which eventually gave rise to the planets, Saturn included.
"The rings of Saturn are simply our closest example of an astrophysical disc," Prof Murray told the BBC.
"We're trying to understand the processes going on in Saturn's rings because they're direct analogues for processes that went on in the early history of not only our Solar System but other planetary systems as well.
"These are discs of gas and dust where larger objects form and start influencing the material around them, and then the whole system evolves."
Cassini is a cooperative mission between the US, European and Italian space agencies.
The spacecraft entered into orbit around Saturn in 2004. It is due continue its science observations until 2017, when it will then be commanded to destroy itself in the atmosphere of the gas giant.
Scientists are keen to avoid any chance that parts of Cassini will end up on Saturn's moons Enceladus or Titan (targets of interest in the search for extraterrestrial life) and contaminate them with any Earth bugs that have survived all these years on the spacecraft.