The Cassini spacecraft has revealed a previously unknown ring around Saturn. It appears to be the result of meteoroids blasting material off the surface of two of Saturn鈥檚 moons.
The new ring is very faint, and it took a unique event in Cassini鈥檚 tour around Saturn to reveal it. On Sunday, Cassini spent a record 12 hours in Saturn鈥檚 shadow, which allowed it to scrutinise the rings as they were being strongly backlit by the Sun.
Mission scientists found the new ring at the same orbital distance as Saturn鈥檚 moons Janus and Epimetheus, which measure 194 and 138 kilometres across, respectively. That location suggests they may be the source of the ring鈥檚 material.
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However, the moons are too small to have volcanic activity that could spew material into space the way Saturn鈥檚 512-kilometre-wide moon Enceladus does, says Cassini science team member Mark Showalter of the SETI Institute in Mountain View, California, US.
鈥淲hen an object is that small, it鈥檚 basically a dead ice ball,鈥� he told New Scientist. 鈥淭here really can鈥檛 be much going on except the shooting gallery out there of meteoroids and little comets.鈥�
Finer detail
The team says the moons are so small that they cannot hold on to the dust stirred up by these impacts. The dust simply escapes into space, where it spreads out into a ring, he says.
Several other rings in the solar system are thought to be generated this way. One is at the orbit of another small Saturnian moon called Atlas and others are associated with Jupiter鈥檚 moons Thebe and Amalthea and Uranus鈥檚 moon Mab.
Not all moons are associated with rings, however. There do not appear to be any rings in the orbits of Saturn鈥檚 small moons Prometheus and Pandora, for example. 鈥淲hy don鈥檛 all the little satellites have rings? That鈥檚 the more interesting question to address,鈥� Showalter says.
During its passage through Saturn鈥檚 shadow, Cassini also looked at the rings with its Composite Infrared Spectrometer. The measurements are still being analysed, but they should tell scientists how fast the particles that make up the rings rotate. This in turn provides clues to how frequently the particles collide with one another.
The team also measured the way Cassini鈥檚 radio signal varied as the spacecraft passed behind the rings as seen from Earth. This should allow the team to map the structure of the rings in finer detail than is possible from Cassini鈥檚 images.
