The Moons and Rings of Uranus
By F. E. Harris
July 27, 2014
Can the Uranus system give us some clues
about what to expect, when New Horizons reaches Pluto? Uranus has many
moons, and thin rings similar to Pluto. The moons of Uranus are almost as diverse
as the moons of Saturn or Jupiter. One of them might be a good analog for
Pluto, another for Charon, and so on down the line of Pluto's moons.
Varied terrain on Miranda indicates complex geologic
history. January 24, 1986. Range, 21,000 miles.
When the Voyagers launched in 1977, it was decided they would not visit
Pluto, for several good reasons. Neptune's moon Triton, of roughly similar
size, served as a sort of stand-in for Pluto, since it was thought the two
objects would be fairly similar. As the Voyager mission progressed, we
learned that every rocky moon in the outer Solar system is unique. No two
could be assumed to be alike.
As ground based telescopes, and the Hubble space telescope studied Pluto
in the years that followed Voyager's outer planets flybys, It was found
that Pluto may not be very much like Triton. Pluto has its own moons, a
part-time atmosphere, and large variations in color and brightness on its
surface. Unfortunately, neither ground or space based telescopes could
resolve much detail. Mostly, they could only raise questions. The New
Horizons space probe would eventually be launched, to resolve the
questions. It will reach Pluto next year.
Section 1: The Moons:
Figure 3. Best image of Oberon shows cratering and large peak on
moon's lower limb. January 24, 1986. Range 410,000 miles.
The major moons of Uranus, discovered in 1787, were named after the fairy
characters in Shakespeare's A Midsummer Night's Dream. Oberon
is named after the mythical king of the fairies in the play.
Oberon, also designated Uranus IV, is the outermost major moon of the planet Uranus.
It is the second-largest and second most massive of the Uranian moons, and the ninth
most massive moon in the Solar System. With large variations in brightness
on its surface, it might be the best analog for Pluto. It is about 2/3 the
diameter of Pluto. From Wikipedia:
It is likely that Oberon formed from the accretion disk that
surrounded Uranus just after the planet's formation. The moon consists of
approximately equal amounts of ice and rock, and is probably differentiated
into a rocky core and an icy mantle. A layer of liquid water may be present
at the boundary between the mantle and the core. The surface of Oberon,
which is dark and slightly red in color, appears to have been primarily
shaped by asteroid and comet impacts. It is covered by numerous impact
craters reaching 210 km in diameter. Oberon possesses a system of chasmata
(graben or scarps) formed during crustal extension as a result of the
expansion of its interior during its early evolution.
Figure 4. Color composite shows evidence of
impact scars and past geologic
activity on Titania. January 24, 1986. Range, 300,000 miles.
Titania is the largest of the moons of Uranus and the eighth largest
moon in the Solar System, with a diameter of 1,578 kilometres (981 mi).
Titania is named after the queen
of the fairies in Shakespeare's A Midsummer Night's Dream. With a diameter
that is almost 90% of Pluto, Titania is also a good analog for it. From
Scientists have recognized three classes of geological feature on Titania:
craters, chasmata (canyons) and rupes (scarps). The surface of Titania
is less heavily cratered than the surfaces of either Oberon or Umbriel,
which means that it is much younger. The crater diameters range from a
few kilometers at the low end to 326 kilometers for the largest known crater,
Gertrude. Some craters (for instance, Ursula and Jessica) are surrounded by
bright impact ejecta (rays) consisting of relatively fresh ice. All large
craters on Titania have flat floors and central peaks. The only exception
is Ursula, which has a pit in the center. To the west of Gertrude there
is an area with irregular topography, the so-called "unnamed basin", which
may be another highly degraded impact basin with the diameter of about 330
kilometres (210 mi).
Titania's surface is intersected by a system of enormous faults, or scarps.
In some places, two parallel scarps mark depressions in the satellite's crust,
forming grabens, which are sometimes called canyons.
The most prominent among Titania's canyons is Messina Chasma, which runs for
about 1,500 kilometres (930 mi) from the equator almost to the south pole.
The grabens on Titania are 20-50 kilometres (12-31 mi) wide and have a
relief of about 2-5 km.
The scarps that are not related to canyons are called rupes, such as
Rousillon Rupes near Ursula crater.
Figure 5. Highest-resolution picture of Titania displays prominent
fault valleys nearly 1,000 miles long. January 24, 1986.
Range, 229,000 miles.
Figure 6. Mosaic of Ariel, most detailed
view from Voyager 2 shows numerous faults and valleys.
January 24, 1986. Range, 80,000 miles.
Ariel is the fourth-largest of the moons of Uranus. It is almost exactly
half the diameter of Pluto. When I look at Ariel and its deep canyons, it reminds me a good
deal of Triton.
When one looks at a globe of Earth, with the oceans romoved, so that the
mid-ocean ridges and other signs of plate tectonics are visible, it
reminds me a lot of Ariel and Triton. There are clear signs that the ice
on each moon was moved around by subsurface convection. The sparse data
that we have on Ariel suggests that the surface may be divided into
highlands and lowlands, much like Earth's Moon's highlands and marias, or
the Earth's continents and oceans. From Wikipedia:
The observed surface of Ariel can be divided into three terrain types:
cratered terrain, ridged terrain and plains. The main surface features are
impact craters, canyons, fault scarps, ridges and troughs.
The cratered terrain, a rolling surface covered by numerous impact craters
and centered on Ariel's south pole, is the moon's oldest and most
geographically extensive geological unit. It is intersected by a
network of scarps, canyons (graben) and narrow ridges mainly occurring in
Ariel's mid-southern latitudes. The canyons, known as chasmata, probably
represent graben formed by extensional faulting, which resulted from global
tensional stresses caused by the freezing of water (or aqueous ammonia) in
the moon's interior ...
The second main terrain type-ridged terrain-comprises bands of ridges and
troughs hundreds of kilometers in extent. It bounds the cratered terrain
and cuts it into polygons. ...
The youngest terrain observed on Ariel are the plains: relatively
low-lying smooth areas that must have formed over a long period of time,
judging by their varying levels of cratering. The plains are found on
the floors of canyons and in a few irregular depressions in the middle of
the cratered terrain. In the latter case they are separated from the
cratered terrain by sharp boundaries, which in some cases have a lobate
pattern. The most likely origin for the plains is through volcanic processes...
The kinds of terrain seen on Ariel almost demand a nearby planet to
provide tidal heating. I think Ariel might be a good analog for Pluto's
largest moon, Charon. New Horizons will prove or disprove this theory,
Figure 7. Heavy cratering is
seen in this, the most detailed view of
Umbriel. January 24, 1986. Range, 346,000 miles.
Umbriel is very dark and cratered, indicating that its surface is very
old. Because Pluto is relatively light, I do not consider Umbriel a very
good candidate as an analog, but I could be wrong. From Wikipedia:
Umbriel's surface is the darkest of the Uranian moons, and reflects less
than half as much light as Ariel, a sister satellite of similar size.
Umbriel has a very low Bond albedo of only about 10% as compared to 23% for Ariel.
The reflectivity of the moon's surface decreases from 26% at a phase angle
(geometric albedo) to 19% at an angle of about 1°. This phenomenon is called
opposition surge. The surface of Umbriel is slightly blue in color,
while fresh bright impact deposits (in Wunda crater, for instance)
are even bluer. ...
Scientists have so far recognized only one class of geological feature
The surface of Umbriel has far more and larger craters than do
Ariel and Titania and shows the least geological activity. ...
Figure 8. Computer mosaic
of Miranda images shows varied geologic regions at high
resolution. January 24, 1986.
Miranda, the fifth largest moon of Uranus, is so strange that I've
included five pictures of it. I think Miranda is probably the best analog
for Charon and the smaller moons of Pluto, but the chance of surprise
is very high.
Unusual "Chevron" figure seen on approach to Miranda.
January 24, 1986. Range, 26,000 miles.
Miranda is the smallest and innermost of Uranus's five major moons.
It was discovered by Gerard Kuiper on February 16, 1948.
Miranda shows more evidence of past geologic activity than
any of the other Uranian satellites.
Voyager 2 image of Miranda taken shortly before closest approach.
January 24, 1986. Range 19,000 miles.
Miranda's surface has patchwork regions of broken terrain indicating intense
geological activity in the moon's past, and is criss-crossed by huge canyons.
Large 'racetrack'-like grooved structures, called coronae, may have formed via
extensional processes at the tops of diapirs, or upwellings of warm ice.
The ridges probably represent extensional tilt blocks. The canyons probably
represent graben formed by extensional faulting. Other features may be due to
cryovolcanic eruptions of icy magma. The diapirs may have changed the density
distribution within the moon, which could have caused Miranda to reorient itself,
similar to a process believed to have occurred at Saturn's geologically
active moon Enceladus.
Miranda displays rugged, high-elevation terrain (right),
lower, grooved terrain and large crater (lower left) 15 miles
across. January 24, 1986. Range, 22,000 miles.
Section 2: The Rings:
Figure 12 : Rings of Uranus, including
newly discovered 10th ring designated 1986U1R (barely visible below
outermost epsilon ring). January 23, 1986. Range, 690,000 miles.
The rings of Pluto are likely to appear as a set of thin circles, as New
Horizons approaches, much like the rings of Uranus appear in this picture.
There are likely to be small, undiscovered sheperd moons,
Figure 13. Two "shepherd" moons, 1986U7 and 1986U8, with epsilon ring.
January 21, 1986. Range, 2.5 million miles.
similar to Uranus' moons 1986U7 and 1986U8. Other small moons that are not
shepard moons are liklely to be discovered, like these, of Uranus.
Figure 14. Three of the moons discovered by Voyager 2: 1986U1, 1986U3,
and 1986U4. January 18, 1986. Range 4.8 million miles.
Only after New Horizons has passed Pluto, will it be apparent what a
hazard the rings might have been. This is a shot of Uranus' rings,
backlit. It is clear that there are many fine particles that could damage
the spacecraft, but which cannot be seen from the sunward side. The short
streaks in this photo are stars, some of them showing through the rings.
Figure 15. Backlit view shows continuous distribution of fine particles
throughout ring system. January 24, 1986. Range, 147,000 miles.
Disclaimer: All information in this article comes from the Voyager space probes, and
the Voyager team, either first, second, or third-hand. I have written some
text, probably too wordy, with my speculations about which moons are the
most likely analogs for Pluto, Charon, and the other small moons of Pluto.
I have also quoted extensively from the very good Wikipedia articles on each moon.
Links are provided to each article, which in turn references the
scientific literature. There is nothing terribly
original in my comments. Any errors are mine.
Funny Pictures from Orbit