THE propeller-shaped footprints of four moonlets have been detected in Saturn’s rings. Their existence adds weight to the theory that the rings formed when a larger moon broke up.
The moons are no more than about 100 metres across. That makes them the first known ring-dwellers of intermediate size, being larger than small ice balls up to 10 metres across and smaller than the more substantial moons a few kilometres across.
The telltale patterns have been found in images taken by the Cassini spacecraft when it entered Saturn orbit in July 2004. “They’re very subtle – we didn’t notice them for a while,” says Matthew Tiscareno of Cornell University in Ithaca, New York, lead author of the study (Nature, DOI: 10.1038/nature04581).
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Each propeller betrays the gravitational disturbance caused by a moonlet, which bunches other ring particles together. Saturn’s ring could be harbouring as many as 10 million moonlets like these, the team estimates.
The existence of the moonlets hints at a violent origin for the rings. “They are larger than what you’d expect if the rings had accreted from dust,” Tiscareno says. He thinks the moonlets and the rings formed when a larger moon broke up, perhaps in an impact with another body.
