The
southern hemisphere of Umbriel displays heavy cratering in this Voyager
2 image, taken 24 January 1986, from a distance of 557,000 kilometers. This
frame, taken through the clear- filter of Voyager's narrow-angle camera,
is the most detailed image of Umbriel, with a resolution of about 10 km.
Umbriel is the darkest of Uranus' larger moons and the one that appears
to have experienced the lowest level of geological activity. It has a diameter
of about 1,200 km and reflects only 16 percent of the light striking its
surface; in the latter respect, Umbriel is similar to lunar highland areas.
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Umbriel
is heavily cratered but lacks the numerous bright- ray craters seen on the
other large Uranian satellites; this results in a relatively uniform surface
albedo (reflectivity). The prominent crater on the terminator (upper right)
is about 110 km across and has a bright central peak. The strangest feature
in this image (at top) is a curious bright ring, the most reflective area
seen on Umbriel. The ring is about 140 km in diameter and lies near the
satellite's equator. The nature of the ring is not known, although it might
be a frost deposit, perhaps associated with an impact crater. Spots against
the black background are due to 'noise' in the data. |
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