Seventh planet has a blue ring

Astronomers have discovered that the planet Uranus has a blue ring. Like the blue ring of Saturn, it probably owes its existence to an accompanying small moon. Scientists suspect subtle forces acting on dust in the rings allow smaller particles to persist while larger ones are recaptured by the moon. Smaller particles- less than a thousandth of the width of a human hair- reflect blue light, giving the ring its distinctive colour, much like the very small molecules in the air that make the Earth’s sky blue. All other rings – those around Jupiter, Saturn, Uranus and Neptune – are made up of both large and small particles, making the rings reddish in appearance.

Astronomers have long known that the gas giant Uranus is surrounded by rings, but last December, two new rings – the planet’s twelfth and thirteenth – were discovered using the Hubble Space Telescope. The more common rings are reddish because they also contain many larger particles, which gives the reflected light its colour, and may be made up of reddish material, perhaps from iron. It appears that the outer blue rings of Saturn and Uranus are strikingly similar, not least because they are both associated with small moons.

“The moon orbits the planet in the ring,” Professor de Pater told. “It is continuously impacted by very tiny particles [micrometeorites]. On a moon that doesn’t have any atmosphere these tiny particles impact the moon at high velocity, and throw stuff up into space. “Because the moon is so small, it escapes the moon and goes into orbit around the planet. “The smaller particles stay in orbit around the planet but the larger particles smash back into the moon.” The work was carried out in collaboration with Mark Showalter, of the Search for Extra-Terrestrial Intelligence (Seti) Institute in California; Heidi Hammel, of the Space Science Institute, Colorado; and Seran Gibbard, of Lawrence Livermore National Laboratory in California. The scientists plan to carry out further observations next year, when the faint rings of Uranus will be more visible.

Questions

1. Is the Uranus the only one planet in our Solar System that has a blue ring?
2. What causes the reddish colour of the more common rings of the planets?
3. Why is it most likely that the existence of a blue ring is closely associated with the presence of small moons round the planets?

Key

1. No, Saturn also has one.
2. It is caused by the reflection of light on larger particles and probably some kind of reddish material perhaps iron.
3. It is suspected that the forces acting on dust in the rings allow smaller particles to stay in orbit while larger ones are recaptured by the moon.