麻豆传媒

Two of Saturn鈥檚 small moons look eerily like flying saucers, new observations by the Cassini spacecraft reveal. The moons, which lie within the giant planet鈥檚 rings, may have come by their strange shape by gradually accumulating ring particles in a ridge around their equators.

The Voyager spacecraft discovered the moons, called Pan and Atlas, in the early 1980s. Pan, which is 33 kilometres wide, orbits Saturn within a gap in the planet鈥檚 A ring called the Encke Division (scroll down for image), while the 39-km-wide Atlas orbits just outside the A ring.

Both moons have a flattened shape, being wider than they are tall. But their uncanny resemblance to UFOs only became clear recently, when Cassini viewed them with its powerful cameras.

Carolyn Porco, Cassini imaging team leader at the Space Science Institute in Boulder, Colorado, US, says she was looking forward to getting high-resolution images of the moons. 鈥淚 very much suspected that their shapes might tell us something about their origins,鈥� she told 麻豆传媒. 鈥淏ut I never anticipated something that might look like a flying saucer.鈥�

The images revealed that the smooth ridges girdling the moons鈥� equators lie in the same plane as Saturn鈥檚 rings. They are also as thick as the vertical distance that the moons appear to travel as they move through the rings, with Atlas鈥檚 ridge between 3 and 5 kilometres thick and Pan鈥檚 between 1.5 and 4 km thick.

Now, scientists led by S茅bastien Charnoz of the University of Paris in France have run computer simulations suggesting that these ridges are made of material swept up from Saturn鈥檚 rings.

Walnut moon

The origin of the planet鈥檚 famous rings is still a mystery. But one theory suggests that early in the solar system, one or more large, icy bodies broke up near the planet, creating detritus that then settled into flat rings.

If that is so, Pan and Atlas鈥檚 cores may have been fragments of this breakup. After the rings flattened into a plane, ring particles may have fallen onto the moons, building up equatorial ridges. The ridges 鈥渃ould be considered as 鈥榝ossilised鈥� accretion discs that once may have surrounded Pan and Atlas鈥�, the researchers write in the journal Science.

The process probably stopped long ago, since the moons鈥� current orbits are thought to prevent the tenuous material still remaining around them to settle onto their surfaces.

It is not yet clear whether the same process has occurred elsewhere in the solar system. Saturn鈥檚 鈥榳alnut鈥� moon Iapetus, which is much larger than Pan and Atlas, has a pronounced ridge around its equator, but that moon lies Saturn鈥檚 main ring system.

鈥淪hortly after the ridge was seen on Iapetus, some people did make a quick speculation on rings, but the dynamics are very different for that distance from Saturn, and for that mass of satellite,鈥� says team member Peter Thomas of Cornell University in Ithaca, New York, US. 鈥淭he morphology of the ridge also suggests internal, rather than external, origins.鈥�

And Neptune has several relatively large moons that lie in its ring system, but they have not yet been imaged at high resolution. 鈥淥ne thing we must do when we get back to Neptune is get a very close look at them to see if their shapes also tell us something of their formation,鈥� Porco says.

Cassini: Mission to Saturn 鈥� Learn more in our continually updated .

Journal reference: (vol 318, p 1622)