Ring-a-Rheases

Saturn's moon Rhea

Everyone knows about Saturn’s rings, even if they have never observed these marvels with their own eyes through a telescope. And Saturn is not the only planet in our solar system with these bands of dust, gas, rock and chunks of ice around their equators. Jupiter, Uranus and Neptune also have ring systems.

But it appears that not only do the gas giant planets have rings; it looks as if at least one of Saturn’s moons has them too. Planetary scientists have discovered that the satellite Rhea is encircled by a disk of material that may be some kind of ring system.

Rhea – the largest satellite of Saturn without an atmosphere – was first observed in 1672 by the astronomer Giovanni Domenico Cassini. The moon is an icy body with a diameter of 1,530 km, or a little over a tenth that of Earth. Our own Moon’s diameter is around a quarter that of the planet.

Quite how Rhea could have rings form around its equator is a mystery. Such features are the result of mechanical forces, and they tend to be associated with particularly massive bodies. In the case of Saturn, the thinking is that loose material left over following the formation of the planet was shepherded by the moons into the ring system through gravitational resonances. If that is so, how can a moonless moon possibly have a ring system?