Mini moons give clue to Saturn's riddle
Astronomers reported on Wednesday that they have spotted minute moons just a hundred metres or so across the orbit of Saturn within its rings. The finding sheds light on how the planet's majestic ring system came into being, they suggest.
The prevailing theory is that the rings, so dazzlingly reflected by the distant Sun, comprise the remnants of an icy moon that long ago was smashed open by an asteroid or comet.
But a collision of this kind normally gives rise to debris in a wide range of sizes, from big lumps a kilometre wide to pebbles a few centimetres across.
Photographs and radar sensing by scout probes, though, have hitherto shown Saturn's ring particles to be remarkably small, between a few centimetres to a few metres across.
The only big exceptions were a pair of kilometre- wide moons called Pan and Daphnis that lurk within the ring system. There seemed to be nothing in the intermediate size.
Astronomers led by Matthew Tiscareno of Cornell University, New York, believe they have found some of the missing medium-sized rocks.
Their evidence comes from data sent back by the orbiting US spacecraft Cassini, which show gaps, shaped rather like a propeller blade or an elongated teardrop, in some of the rings.
Tiscareno's team deduct that the gaps are caused by "embedded moonlets" -- rocks about 100 metres across that were too small to be seen in the Cassini pictures but which must have briefly scattered the particles as they orbited, rather like a ship forms a bow wave and trails a wake that then closes behind it.