PART 4 | THE SOLAR SYSTEM
The eccentricity of the ε ring is
apparently caused by the eccentric
orbits of Ophelia and Cordelia.The narrowness of the rings suggests they are shepherded by small moons.
Voyager 2 found Ophelia and Cordelia shepherding the ε ring. Other small
moons must be shepherding the other narrow rings. Such moons must
be structurally strong to hold themselves together inside the
planet’s Roche limit.The albedo of the ring particles is only about 0.015, darker than lumps of coal.
If the ring particles are made of methane-rich ices, radiation from the planet’s
radiation belts could break the methane down to release carbon and darken the ices.
The same process may darken the icy surface of Uranian moons.UranusWhen the
Voyager 2
spacecraft looked
back at the rings
illuminated from behind by
the sun, the rings were not
bright. That is, the rings are not
bright in forward-scattered light.
That means they must not contain
small dust particles. The nine main
rings contain particles no smaller than
meter-sized boulders.When the
Voyager 2
spacecraft looked
back at the rings
illuminated from behind by
the sun, the rings were not
bright. That is, the rings are not
bright in forward-scattered light.
That means they must not contain
small dust particles. The nine main
rings contain particles no smaller than
meter-sized boulders.Uranus
4
1986
U2R(^65)
51 50 49 48 47 46 45 44 43 42 43 45 46
Minutes from midoccultation
42 44
45,000 47,000 49,000
Distance from center of Uranus (km)
51,000 49,000 47,000 45,000 51,000
47 48 49 50 51
Intensity
ε ε
δ δ
γ β β γ
α α
α
β
γδ
η
λ
ε
Ophelia
Cordelia
The rings of Uranus were discovered in 1977, when Uranus crossed in front
of a star. During this occultation, astronomers saw the star dim a number of
times before and again after the planet crossed over the star. The dips in brightness
were caused by rings circling Uranus.
More rings were discovered by Voyager 2. The
rings are identified in different ways depending
on when and how they were discovered.
Notice the eccentricity of
theε ring. It lies at different
distances on opposite sides
of the planet.
1
2
Ring particles don’t last forever as
they collide with each other and are
exposed to radiation. The rings of Uranus
may be resupplied with fresh particles
occasionally as impacts on icy moons
scatter icy debris.
3
2a
Collisions among the large particles in the
ring produce small dust grains. Friction with
Uranus’s tenuous upper atmosphere plus
sunlight pressure act to slow the dust grains
and make them fall into the planet. The
Uranian rings actually contain very little dust.