Tuesday, September 27, 2005
Umbriel...A Shadowy World Gaurds Its Secrets
Voyager 2 flew by the Uranian moon Umbriel in 1986. Umbriel, 1,200 km in diameter, was only seen from half a million kilometers away. It reflects only 16 percent of the light hitting it, making it significantly fainter than the other major Uraniam moons. Distance and darkness conspired to limit the quality of Umbriel imaging. These images, taken during approach, show a dark world, battered with craters. A few bright markings appear. On the left-hand image, there seems to be a huge crater with a bright central peak near the bottom. Fainter, large scale albedo features can be seen a the top of this image and others, as well as in the image on the right. Paul Helfenstein, Peter C. Thomas, and Joseph Veverka proposed in a 1989 letter to the journal Nature that polygonal paterns can be seen in these features and the at they are the remnants of an ancient tectonic system similar to those seen on other moons of similar size. This remains an intriguing possibility.
The surface of Umbriel is ncient one. A collection of impact craters of various sizes was found to saturate the landscape. As Voyager drew closer, became clear that most or all of the bright features are related to craters. On the terminator, one can see two interesting craters. One, Skynd, has a bright central peak. The others, Wunda, at the top of the image, has a bright rim and no apparent central peak. This is the best color image obtained by Voyager.
This is a composite of the best Voyager images, slightly better than the original 8 km/pixel resolution. It appears to be a battered world - no features appear to be generated by anything other than impacts, unless the bright areas are produced by internal activity.
Here is a closeup systhesis from all the closeups of Wunda. It is hard to tell if the craters dug up the bright material, or if they had something to do with the impacts. Maybe it was derived from eruptions or melting caused by imacts. Or maybe they are some sort of frost left over from impacts. This issue may not be resolved until another spacecraft observes Umbriel. Here (below, left) is the best view of Wunda that can be generated with the available data.
Voyager left Umbriel behind, but the study of this world using earth-based instruments continues. The right hand image above shows a crescent view taken as Voyager receded. Perhaps compositional studies will yield clues that help us resolve some of the secrets Umbriel holds. We are left to wonder at this faint, fleeting, and fuzzy glimpse of this intriguing world.