THE SOLAR SYSTEM
Miranda’s orbit. The only newcomer to exceed 100 kilo-
metres (60 miles) in diameter is Puck, which was imaged
from a range of 500,000 kilometres (312,000 miles) and
found to be dark and roughly spherical; three craters
were seen, and given the rather bizarre names of Bogle,
Lob and Butz.
Incidentally, it may be asked why the names of the
Uranian moons come from literature, not mythology. The
names Titania and Oberon were suggested by Sir John
Herschel, and the later satellites were also given names
coming either from Shakespeare or from Pope’s poem The
Rape of the Lock. This is certainly a departure from the
norm, and arguably an undesirable one, but the names are
now well established, and all have been ratified by the
International Astronomical Union.
The nine innermost satellites are presumably icy, but
nothing is known about their physical make-up. Cordelia
and Ophelia act as shepherds to the Epsilon ring. A careful
search was made for similar shepherds inside the main
part of the ring system, but without success; if any such
shepherds exist, they must be very small indeed.
The four largest members of the family are not alike.
In general they are denser than the icy satellites of Saturn,
and so must contain more rock and less ice; the proportion
of rocky material is probably between 50 and 55 per cent.
All have icy surfaces, but there are marked differences
between them. Umbriel is the darkest of the four, with a
rather subdued surface and one bright feature, called
Wunda, which lies almost on the equator – so that with the
pole-on view it appears near the edge of the disk; it may
be a crater, but its nature is uncertain. Umbriel is fainter
than the other major satellites, and in pre-Voyager days
was assumed to be the smallest, though in fact it is mar-
ginally larger than Ariel. Oberon is heavily cratered, and
some of the craters such as Hamlet, Othello and Falstaff
have dark floors, due perhaps to a mixture of ice and car-
bonaceous material erupted from the interior; on the limb,
near the crater Macbeth, there is a high mountain. Titania
is distinguished by high ice-cliffs, and there are broad,
branching and interconnected valleys, so that there seems
to have been more past internal activity than on Oberon.
Ariel also has very wide, branching valleys which look as
though they have been cut by liquid – though, needless to
say, all the satellites are far too lightweight to retain any
trace of atmosphere.
Miranda has an amazingly varied surface. There are
regions of totally different types – some cratered, some
relatively smooth; there are ice-cliffs up to 20 kilometres
(over 12 miles) high, and large trapezoid-shaped areas or
‘coronae’ which were initially nicknamed ‘race-tracks’.
The three main coronae (Arden, Elsinore and Inverness)
cover much of the hemisphere which was imaged by
Voyager 2. It has been suggested that during its evolution
Miranda has been broken up by collision, perhaps several
times, and that the fragments have subsequently re-
formed. This may or may not be true, but certainly it
would go some way to explaining the jumble of surface
features now seen.
Five small outer satellites, with retrograde motion,
have been found: they are named Caliban, Stephano,
Sycorax, Prospero and Setebos. All are reddish, and pre-
sumably asteroidal.cratered, with fault-scarps
and graben, suggesting
considerable past tectonic
activity, and there is
evidence of erosion.OberonPlanetary orbitPlanetary orbitTitaniaUmbrielArielMiranda▲ Titania. Voyager 2 took
this picture 24 January 1986
from 483,000 km (302,000
miles). It shows details
down to 9 km (5.6 miles).
The surface is generally
cratered, with ice-cliffs and
trench-like features; there
is considerable evidence
of past tectonic activity. Puck.Discovered on
30 December 1985 and
imaged on 24 January 1986
from a range of 500,000 km
(300,000 miles). Resolution
is 10 km (6 miles). Three
craters were recorded:
Bogle, Lob and Butz. Puck
is roughly spherical, with
a darkish surface. Ariel. This Voyager 2
image was taken from
169,000 km (106,000 miles);
the resolution is 3.2 km
(2 miles). The surface is▼ Satellite orbits.Orbits
of the five larger satellites.
Voyager discovered ten
more small moons, within
the orbit of Miranda.D108- 151 UNIVERSE UK 2003CB 7/4/03 5:16 pm Page 127