"Sweet Moon," William
Shakespeare wrote in A Midsummer Night's Dream, "I thank thee for thy sunny
beams; I thank thee, Moon, for shining now so bright." Centuries later,
the moons of Uranus
pay homage to famous playwright.
While most of the satellites orbiting other planets take their names from
Greek mythology, Uranus' moons are unique in being named for Shakespearean
characters, along with a couple from the works of Alexander Pope.
(king and queen of the fairies) are the largest Uranian moons, and were first
to be discovered - by William Herschel in 1787. William Lassell, who had
been first to see a moon orbiting Neptune
, discovered the next two, Ariel
. Nearly a century passed before Gerard Kuiper found Miranda
in 1948. And that was it until a NASA's robot made it to distant Uranus.
The Voyager 2
spacecraft visited the Uranian system in 1986 and tripled the number of known
moons. Voyager 2 by found an additional 10, just 26-154 km (16-96 miles)
in diameter: Juliet
Since then, astronomers using the Hubble Space Telescope
and improved ground-based telescopes have raised the total to 27 known moons.
Spotting the post-Voyager moons is an impressive feat. They're tiny - as
little as 12-16 km (8-10 miles) across, and blacker than asphalt. And of
course, they're nearly 3 billion miles away.
All of Uranus's inner
moons (those observed by Voyager 2) appear to be roughly half water ice and
half rock. The composition of the moons outside the orbit of Oberon remains
unknown, but they are likely captured asteroids.
Miranda, the innermost and smallest of the five major satellites, has a
surface unlike any other moon that's been seen. It has giant fault canyons
as much as 12 times as deep as the Grand Canyon, terraced layers, surfaces
that appear very old, and others that look much younger.
Ariel has the brightest and possibly the youngest surface among all the
moons of Uranus. It has few large craters and many small ones, indicating
that fairly recent low-impact collisions wiped out the large craters that
would have been left by much earlier, bigger strikes. Intersecting valleys
pitted with craters scars its surface.
Umbriel is ancient, and the darkest of the five large moons. It has many
old, large craters and sports a mysterious bright ring on one side.
Oberon, outermost of the five major moons, is old, heavily cratered, and
shows little signs of internal activity. Unidentified dark material appears
on the floors of many craters.
Cordelia and Ophelia are shepherd moons that keep Uranus' thin, outermost "epsilon" ring well defined.
Between them and Miranda is a swarm of eight small satellites unlike any
other system of planetary moons. This region is so crowded that astronomers
don't yet understand how the little moons have managed to avoid crashing
into each other. They may be shepherds for the planet's 10 narrow rings,
and scientists think there must be still more moons, interior to any known,
to confine the edges of the inner rings.
"Well shone, Moon," wrote Shakespeare, "truly, the moon shines with a good grace."