Seventh planet has a blue ring

[Guest guest]

By Helen Briggs

BBC News science reporter

 

 

 

Schematic view of the outermost rings of Uranus...

Astronomers have discovered that the planet Uranus has a blue ring -

only the second found in the Solar System.

 

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 reflect blue light, giving the ring its distinctive

colour, the US team reports in Science.

 

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.

 

Bright blue

 

Astronomers have long known that the gas giant Uranus is surrounded

by rings of dark particulate matter up to ten metres in diameter.

 

But last December, two new rings - the planet's twelfth and

thirteenth - were discovered using the Hubble Space Telescope.

 

 

....and Saturn

 

Astronomers observed the ring system at infrared wavelengths with the

Keck telescope, in Hawaii.

 

The outermost ring, and its ice-bound moon Mab, could not be observed

in infrared light unlike the red inner ring.

 

A team led by Imke de Pater, professor of astronomy at the University

of California, Berkeley, found that the ring was bright blue,

something of an oddity in planetary terms.

 

" The blue colour says that this ring is predominantly submicron-sized

material, much smaller than the material in most other rings, which

appear red, " Professor de Pater said.

 

The tiny particles - less than a thousandth of the width of a human

hair - scatter and reflect predominantly blue light, much like the

very small molecules in the air that make the Earth's sky blue.

 

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.

 

Moon dance

 

" The moon orbits the planet in the ring, " Professor de Pater told the

BBC News website.

 

" 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.