left, in order of increasing distance from the planet, they
are Miranda, Ariel, Umbriel, Titania and Oberon. (Click
on the image for a larger view)
2 obtained clear, high-resolution images of each of the
five large moons of Uranus known before the encounter: Miranda,
Ariel, Umbriel, Titania and Oberon. The two largest, Titania
and Oberon, are about 1,600 kilometers (1,000 miles) in
diameter, roughly half the size of Earth's Moon. The smallest,
Miranda, is only 500 kilometers (300 miles) across, or just
one-seventh the lunar size.
10 new moons discovered by Voyager bring the total number
of known Uranian satellites to 15. The largest of the newly
detected moons, named Puck, is about 150 kilometers
(about 90 miles) in diameter, or larger than most asteroids.
analysis shows that the five large moons are ice-rock conglomerates
like the satellites of Saturn. The large Uranian moons appear,
in fact, to be about 50 percent water ice, 20 percent carbon-
and nitrogen-based materials, and 30 percent rock. Their
surfaces, almost uniformly dark gray incolor, display varying
degrees of geologic history. Very ancient, heavily cratered
surfaces are apparent on some of the moons, while others
show strong evidence of internal geologic activity.
for example, is marked by huge fault systems and canyons
that indicate some degree of geologic activity in its history.
These features may be the result of tectonic movement in
its crust. Ariel has the brightest and possibly the
geologically youngest surface in the Uranian moon system.
It is largely devoid of craters greater than about 50 kilometers
(30 miles) in diameter. This indicates that low-velocity
material within the Uranian system itself peppered the surface,
helping to obliterate larger, older craters. Ariel also
appears to have undergone a period of even more intense
activity leading to many fault valleys and what appear to
be extensive flows of icy material. Where many of the larger
valleys intersect, their surfaces are smooth; this could
indicate that the valley floors have been covered with younger
is ancient and dark, apparently having undergone little
geologic activity. Large craters pockmark its surface. The
darkness of Umbriel's surface may be due to a coating of
dust and small debris somehow created near and confined
to the vicinity of that moon's orbit.
outermost of the pre-Voyager moons, Oberon, also
has an old, heavily cratered surface with little evidence
of internal activity other than some unknown dark material
apparently covering the floors of many craters.
, innermost of the five large moons, is one of the strangest
bodies yet observed in the solar system. Voyager images,
which showed some areas of the moon at resolutions of a
kilometer or less, consists of huge fault canyons as deep
as 20 kilometers (12 miles), terraced layers and a mixture
of old and young surfaces. The younger regions may have
been produced by incomplete differentiation of the moon,
a process in which upwelling of lighter material surfaced
in limited areas. Alternatively, Miranda may be a reaggregation
of material from an earlier time when the moon was fractured
into pieces by a violent impact.
Miranda's small size and low temperature (-335 degrees Fahrenheit
or -187 Celsius), the degree and diversity of the tectonic
activity on this moon has surprised scientists. It is believed
that an additional heat source such as tidal heating caused
by the gravitational tug of Uranus must have been involved.
In addition, some means must have mobilized the flow of
icy material at low temperatures.