The propeller-shaped footprints of four tiny moonlets have been detected in Saturn鈥檚 rings 鈥� adding weight to a theory that the rings formed when a larger moon exploded.
The moonlets are no more than about 100 metres across, which makes them the first known objects of intermediate size to be found in the rings. They fall between ordinary ice-balls of up to 10 metres and a handful of more substantial moons a few kilometres across.
The telltale patterns appear in old images taken by the Cassini spacecraft when it entered orbit around Saturn in July 2004. They had not been spotted before because the images have very low contrast, only just above the noise level of Cassini鈥檚 camera. 鈥淭hey鈥檙e very subtle 鈥� we didn鈥檛 notice them for a while,鈥� says Matthew Tiscareno of Cornell University, lead author of the study.
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Ten million moonlets
Each propeller betrays a moonlet鈥檚 gravitational influence, which bunches other ring particles together. The team estimate that throughout the ring system there could be about ten million moonlets like these.
鈥淭he fact that objects of this size exist is intriguing,鈥� says Tiscareno. 鈥淭hey are larger than what you鈥檇 expect if the rings had accreted from dust.鈥� That challenges one theory of the rings鈥� origin, that they simply grew out of ice and dust left over from the formation of Saturn, and have girdled the planet ever since.
It adds support to a much more popular theory, that the rings were formed no more than a few hundred million years ago when a larger body was smashed to pieces. It could have been an old moon of Saturn, destroyed in a violent impact, or perhaps more likely it was an icy object from even farther out in the Solar System that got caught and crushed by Saturn鈥檚 gravity. In that case the newly discovered moonlets are just relatively large chips off the old block.
Journal reference: Nature (DOI: 10.1038/nature04581)
