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The Planetary Society Blog
By Emily Lakdawalla
Europlanet : Swings and Roundabouts with Rings
Aug. 24, 2007 | 00:48 PDT | 07:48 UTC
by Doug Ellison in Potsdam
Unfortunately, the session including my presentation overlapped with the session in which Imke de Pater presented the latest results of observations of the ring crossing of Uranus. Thus I'm stuck working from the press release, but fortunately it's fairly verbose. For a ring-crossing 101 Emily ‘previewed' this event not too long ago here and here.
What Imke's team do is image the brightness of the rings seen edge-on to understand their density. Of course, when you see the ‘middle' of the rings, you're also looking through all the other rings as well, so the rings should always look cumulatively brightest toward the centre. However, if you do the maths, starting at one edge you can peel the rings apart like an onion and calculate the discrete ring ‘brightness' across their entire width. Infrared Keck images of Uranus ring crossingThe sun-it side of the rings of Uranus (two left images) captured in the infrared by the Keck II telescope in 2004 and 2006 as Uranus approached the point where its rings appear edge-on from Earth. On May 28, 2007, less than a month after the first ring-plane crossing on May 3, Keck II captured the unlit side of the rings for the first time ever (right). The dotted lines show the position of the epsilon (upper line) and zeta (lower line) rings, with other rings indicated.
Credit: Imke de Pater (UC Berkeley), Heidi B. Hammel (SSI, Boulder) and the W. M. Keck Observatory |
By comparing the rings as seen 21 years ago by Voyager 2 with observations taken at the Keck telescope during the first crossing in 2007, they have seen that the rings have changed significantly. You have to be careful when looking at the images taken over the past few years because the subtle changes in geometry have a significant impact on how bright the rings appear. By observing with Keck (with its awesome near infrared adaptive optics), the Very Large Telescope, and Hubble close to this ring crossing, the brighter outer rings grow faint as the larger metrer-sized rocks hide behind one another, and the thinner rings of finer material get illuminated as they merge into a thin band. VLT image of Ring Crossing Chile's Very Large Telescope, operated by the European Southern Observatory, caught the rings of Uranus a mere two hours after Earth had crossed to the lit side of the ring plane. This false-color image, a composite of infrared images, was obtained by the NAOS-CONICA infrared camera at 9:00 UT on August 16, 2007. With a ring opening of only 0.003°, the thin rings nearly disappear. At right, the region around the planet has been enhanced to show a thin line, which is sunlight glinting off the ring edges and reflected by dust clouds embedded within the system. The picture at left shows the planet and four of its largest moons. On the planet, banding in the atmosphere and a bright cloud feature near the planet's south polar collar are clearly visible.
Credit: Daphne Stam (TU Delft), Markus Hartung (ESO, Chile), Mark Showalter (SETI) and Imke de Pater (UC Berkeley and TU Delft) |
They are still trying to understand the mechanisms involved in the changes - perhaps the rings are actually being moved - the broadest inner ‘zeta' ring seems to have migrated outwards by several thousand kilometers - but it is equally possible that we're in fact seeing a new ring and the zeta ring has gone, or even impacts that could also cause changes. The authors of a paper going to press about this are not at a point of a fully understanding the mechanisms involved. For those of you who think of ring systems as a flat, stable, pretty disks that punctuates the outer planets - think again!
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