Technology & Science

Uranus 'dark side' image reveals changing rings

A rare glimpse of the dark side of the rings of the planet Uranus shows they have undergone substantial changes in recent years, scientists said Thursday.

A rare glimpse of the dark side of the rings of the planet Uranus shows they have undergone substantial changes in recent years, scientists said Thursday.

This infrared image of Uranus from the W.M. Keck Observatory shows the rare ring-plane crossing in 2007 and is the first image of the unlit side of the rings of Uranus. The sun itself will cross the ring plane of Uranus on Dec. 7. ((Imke de Pater, Heidi B. Hammel and the W. M. Keck Observatory))

The first image taken from a ground-based telescope showed for the first time a view of the planet's rings edge-on to the Earth.The view, whichastronomers get only once every 42 years, provides a glare-free look at the rings and the fine dust that permeates them, said the authors of a study published in this week's edition of the journal Science.

When the rings of a planet appear "edge-on" to Earth — that is, as only a thin line seemingly cutting the planet in half — it provides a unique view because dense rings become faint and faint rings become relatively brighter.

The new view allowed astronomers using the Keck II telescope at the W.M. Keck Observatory in Hawaii and the Hubble Space Telescope to discover that the planet's rings have changed substantially since 1986, when the spaceprobe Voyager 2 last photographed the planet.

"The rings were once expected to represent a steady state between dust creation and removal processes," said lead author Imke de Pater of University of California, Berkeley, writing in Science.

"However, we now realize that these states are far from steady, and may be dominated by infrequent events, such as large impacts, that inject highly visible quantities of dust."

What has caused the changes in the rings isn't known, though the forces at work could include pressure from solar radiation, drag on the planet's magnetic field or collisions with larger bodies.

Scientists also hope the new view offers scientists a chance to find a few more moons orbiting the planet. Since the view of the rings is edge-on, it produces less glare, making it possible to see other orbiting objects, said Mark Showalter of the SETI institute.

"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," he said in a statement. "This is the unique viewing geometry that only comes along once in 42 years, when we have a chance of imaging these tiny satellites."

Uranus was discovered in 1781 but its rings weren't found until 1977 and weren't seen until Voyager's mission 21 years ago.

The planet's slow orbit around the sun — it takes 84 years to complete — means viewers from Earth had two opportunities to see the edge-on view of the planet's rings:May 3 and Aug. 16. A third crossing of the planet — on Feb. 20, 2008 — won't be visible because the sun will block the view.

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