Seven moons discovered recently around Saturn and Neptune have complex and violent pasts, suggest two teams of astronomers.
Five confirmed moons 鈥� and one candidate moon 鈥� around Neptune appear to have been captured by the distant planet after they were born from collisions in the outer Solar System, according to a new study.
And the strange orbits of two tiny moons around Saturn 鈥� discovered by the European-US Cassini spacecraft on Monday 鈥� hint at brutal beginnings.
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Researchers now report detailed orbital analyses of Neptune鈥檚 moons, whose discovery was announced in 2003. Their orbits, though stable, are stretched out, tilted, and in three cases, backwards 鈥� with the satellites moving in the opposite direction to Neptune鈥檚 rotation. That suggests the moons could not have formed by simply condensing from disks of material around their host planet.
Two other moons, Triton and Nereid, also have unusual orbits, so now seven of Neptune鈥檚 13 moons are classified as 鈥渋rregular鈥�.
鈥淭here鈥檚 been a real debate in the literature about what happened to Neptune鈥檚 satellites,鈥� says JJ Kavelaars, a planetary scientist at the National Research Council of Canada in Victoria and an author of the new study.
Parent body
He and his colleagues believe one or more collisions 鈥� possibly between a comet and Nereid itself 鈥� spawned the five moons, which are all between 30 and 55 kilometres wide. Then the system was disturbed when Neptune captured the 2700-km-wide Triton from the Kuiper Belt, a collection of primordial space rocks beyond Neptune.
To test their theory, the researchers will try to study the colours of the faint objects. 鈥淚f we find they all have very similar surface compositions, it would make it more reasonable to say they were collisional fragments of a parent body,鈥� Kavelaars told New 杏吧原创. Such features have linked irregular moons around Jupiter and Saturn into families.
Kavelaars says 40 or so smaller moons 鈥� a few kilometres wide 鈥� may orbit Neptune. But these are undetectable by even the best ground-based telescopes, which were pushed to their limits to find the five new moons.
But such tiny moons have just been discovered around Saturn, thanks to the Cassini spacecraft now in orbit around the ringed planet.
The moons are the planet鈥檚 smallest known satellites. They span just three and four kilometres across, sizes approaching those of the largest ring particles, say astronomers.
Debris disks
The moons are located about 200,000 km from Saturn鈥檚 centre, between the orbits of two of Saturn鈥檚 33 larger moons, Mimas and Enceladus.
鈥淚t is surprising to find satellites in this region,鈥� says discoverer Sebastien Charnoz, a planetary scientist at the University of Paris, France. Gravitational tugs from the larger satellites make their positions precarious, so the mini-moons will likely be forced into different orbits in the future.
This destabilising force also has implications for the moons鈥� past. 鈥淥ne thing is almost sure 鈥� they did not form where they are seen today,鈥� Charnoz told New 杏吧原创.
Collisions may have also created these satellites, says Kavelaars, when larger moons crashed into each other in their orbits around Saturn.
鈥淭he actual rings of Saturn are probably debris disks from the breakup of satellites,鈥� he says. 鈥淪o it鈥檚 pretty obvious there鈥檚 a continuous process churning up the area around Saturn and creating new small moons.鈥�
Journal reference: Nature (vol 430, p 865)