Planet Uranus rings the changes

From: http://news.bbc.co.uk/2/hi/science/nature/6961087.stm

Aug 23, 2007

Astronomers have captured remarkable new images of the rings of Uranus.

The rings are currently edge-on to Earth, in an event that only happens every 42 years.

A team, led by Imke de Pater from University of California, Berkeley, US, has analysed the rings' structure, with some surprising results.

The group tells the journal Science their images show that the rings are changing much more quickly than researchers had previously believed.

The study compares pictures of the rings obtained in May with data gathered over the last seven years.

The edge-on perspective is considered favourable for seeing particular features in the rings.

It makes the outer rings that contain centimetre- to metre-sized rocks seem dimmer as they obscure each other; but those that are normally almost transparent layers of dust become more visible as the material merges into a thin band along the line of sight.

Astronomers can also look for other properties not measurable from other angles, like warps or waves in the ring structure, the thickness of different rings and their inclination and orientation.

Clearer view

The research team has been using an infrared camera on Hawaii's Keck II telescope to make its observations.

The group also employed the telescope's adaptive optics system which corrects the distortions introduced into images by the Earth's turbulent atmosphere.

The pictures show the nearly edge-on rings appearing as a bright line bisecting a dim Uranus, which appears dark in the infrared.

"The improvements to the adaptive optics systems allowed us to capture unbelievably crisp images of Uranus," said Marcos van Dam from the W. M. Keck Observatory.

The images revealed that the inner rings of micron-sized dust have changed significantly since the Voyager 2 spacecraft photographed the Uranus system 21 years ago. Today the inner rings are much more prominent than expected.

"People tend to think of the rings as unchanging, but our observations show that not to be the case," said Dr de Pater. "There are a lot of forces acting on small dust grains, so it is not that crazy to find that the arrangement of rings has changed."

From orbit

The Hubble Space Telescope also imaged Uranus earlier this month.

Scientists hope this data could expose some of Uranus' small moons. There is even a chance that some previously unknown moons will be discovered.

"Two little satellites called Cordelia and Ophelia straddle the brightest ring, the epsilon ring, and keep it in place, but people have always assumed there must be a bunch more of these satellites that are confining the nine other narrow rings," said Mark Showalter from the Seti Institute in California.

"This is the unique viewing geometry that only comes along once in 42 years, when we have a chance of imaging these tiny satellites, because normally they are lost in the glare of the rings. Now, the rings are essentially invisible."

The research team will be continuing to gather data over the next few months.

The group is waiting for the Uranus equinox, which occurs in December. At this time, the planet's rings are edge-on to the Sun, and the position of the Earth will allow astronomers to see the rings at a steep angle with no glare from our star.