The
Rings
All
nine previously known rings of Uranus were photographed
and measured, as were other new rings and ringlets in the
Uranian system. These observations showed that Uranus's
rings are distinctly different from those at Jupiter and
Saturn.
Radio
measurements showed the outermost ring, the epsilon,
to be composed mostly of ice boulders several feet across.
However, a very tenuous distribution of fine dust also seems
to be spread throughout the ring system.
Incomplete
rings and the varying opacity in several of the main rings
leads scientists to believe that the ring system may be
relatively young and did not form at the same time as Uranus.
The particles that make up the rings may be remnants of
a moon that was broken by a high-velocity impact or torn
up by gravitational effects.
To
date, two new rings have been positively identified. The
first, 1986 U1R, was detected between the outermost
of the previously known rings -- epsilon and delta -- at
a distance of 50,000 kilometers (31,000 miles) from Uranus's
center. It is a narrow ring like the others. The second,
designated 1986 U2R, is a broad region of material
perhaps 3,000 kilometers (1,900 miles) across and just 39,000
kilometers (24,000 miles) from the planet's center.
The
number of known rings may eventually grow as a result of
observations by the Voyager 2 photopolarimeter instrument.
The sensor revealed what may be a large number of narrow
rings -- or possibly incomplete rings or ring arcs -- as
small as 50 meters (160 feet) in width.
The
individual ring particles were found to be of low reflectivity.
At least one ring, the epsilon, was found to be gray in
color. Important clues to Uranus's ring structure may come
from the discovery that two small moons --Cordelia
and Ophelia -- straddle the epsilon ring. This finding
lends credence to theories that small moonlets may be responsible
for confining or deflecting material into rings and keeping
it from escaping into space. Eighteen such satellites were
expected to have been found, but only two were photographed.
The
sharp edge of the epsilon ring indicates that the ring is
less than 150 meters (500 feet) thick and that particles
near the outer edge are less than 30 meters (100 feet) in
diameter.
The
epsilon ring is surprisingly deficient in particles smaller
than about the size of a beachball. This may be due to atmospheric
drag from the planet's extended hydrogen atmosphere, which
probably siphons smaller particles and dust from the ring.
