13. Uranus

The discovery brought fame to Herschel and since the distance fit Daniel Titius' (1772) progression which relates the distances of the planets from the sun in AU, led Johanne Bode to publish the progression in his introductory astronomy text "Bode's law" and predict the existence of a planet at 2.8AU.

A false-colour image of Uranus taken by the Hubble Space Telescope show clouds in its atmosphere. Also visible are some of Uranus' moons as well as numerous bright clouds in its atmosphere. The orange-coloured clouds form a circle of latitude around the planet. It seems that a cloud's colour depends upon its height in the atmosphere. Green and blue regions show where the atmosphere is clear and where sunlight can penetrate deep into the planet. The dark rings are made of boulder-sized chunks of rock and possibly ice.

Here from earth we see a featureless blue green world about 3.87" across.

Uranus lies very close to the plane of the ecliptic and has a small eccentricity (0.047), but has an obliquity (polar axis tilt) of 98 degrees! Like Venus, the planet rotates (and its moons revolve) "backwards". More curious, the magnetic field is offset from the centre by about a third of the planet's radius and inclined 60 degrees to the rotation axis (so is more nearly perpindicular to the ecliptic).

So Uranus has extreme seasons, each about 21 years in length. Southern summer occured around 1986 when Voyager 2 flew past. Despite the extreme seasons, the temperature below the haze layer near the tropopause (P=0.1 bar) does not vary much from about 52K (-221C).

Even though the pole is tilted right over (due to accretion of planetismals?) the rotation is rapid, 17h14m. (-17h14m actually.)

The telescopic appearance of Uranus is unremarkable. A small greenish disk os observed under excellent seeing conditions, probably the result of scattering by methane which absorbs red light. Voyager 2 images show bands similar to but with less contrast than those of Saturn.

The atmosphere is dominated by H and He but the He, about 15%, is rather less than the 40% expected from earth based studies. Methane, acetylene and other hydrocarbons exist in low concentrations and over the sunlit south pole Voyager found a layer of --petrochemical smog.

Uranus has five moons, thirteen large rocks and nine known rings. As expected, Uranus has coaxed these satellites into reasonably circular orbits around the equator with orbital motion in the direction of the planet's rotation. 1999 is not the "eclipse season", the south pole is now swinging away fron the sun and around 2005 the moons will enjoy eclipse phenomena like the Galilean satellites.

The Major Uranian Moons Ariel, Umbriel, Titania, Oberon, Miranda
Named not after Greek and Roman mythological characters but rather after Shakesperian characters and characters from Alexander Pope's Rape of the Lock (Belinda had a lock of her hair stolen...).
The outrmost moons Titamia and Oberon are about 45% the Moon's size and have old, cratered surfaces:
Titania was discovered by Herschel in 1787, six years after he discovered Uranus, and named it after the queen of the faries in A Midsummer night's Dream. Titania, 1580km across, 435,840km, period 8.706 days, is marked by huge fault systems and canyons.
Oberon has some unknown dark material covering the floors of many craters.
Ariel discovered by Wm. Lassell in 1851 and named after the captive spirit in the Tempest is the brightest (14th magnitude) Uranian moon. Diameter=1160km, period 2.52 days, distance a=191242km. Ariel is geologically young. The surface is dark overall with bright patches surrounding some of the craters and is devoid of craters more than 50km in diameter. Ariel must have undergone intense activity leaving many fault valleys and ice flows which show up as a series of groves. Ariel is odd like Uranus since it is tipped on its side.
Umbriel (name derived by Pope from umbra=shadow) is a very dark ancient world diameter=1170km period=4.144 days some 266,000 km from Uranus dominated by impact craters. About a third the size of Luna with no signs of geologic activity.
Miranda was discovered by Kuiper in 1948 and named after the heroine in The Tempest. The smallest and innermost of the five, is -strange-. An irregular shape (480x466km) the moon is too small for gravity to crush it into a good sphere. There are huge 20 km deep fault canyons, terraced layers and a mixture of old and new surfaces. Sort of like an old moon pulled apart then not put back together the right way!

The Rings were discovered by chance when Uranus occulted a star. The rings caused unexpected dropouts in the starlight and by studying these dropouts observed at different locations on Earth (parallax) the surprising ring structure was deduced.

The rings are dark and narrow and even the largest "E" ring would be invisible to an astronaut who visited the system.

Voyager 2 observed the nine rings and discovered a few more rings and some incomplete "ringlets". Radio observations of the largest outermost epsilon ring showed it to be composed of meter size ice boulders, nothing much smaller than a soccer ball. Very different from the rings of Saturn and Jupiter. These rings are thought to be the result of a recent collision or tidal breakup of an inner moon.
There is evidence for some dust spread through the ring system but the Voyager photographs show the rings to be really skinny, as thin as 50m up to the big epsilon ring 150m (metres, not km!).
The Epsilon ring is shepherded by Cordelia and Ophelia. Shepherds for the other rings are suspected to exist (like if our theory of shepherding satelites is correct!) but have not yet been found.

Uranus and Neptune are only about 5% as massive as Jupiter but with relatively high mean density indicating a composition with a higher proportion of heavier materials (ie less H and He) than Jupiter or Saturn. This is a real puzzle, where did the H and He go?

Note the relative size and mass of Uranus and Neptune: Neptune is smaller and more massive. For some reason much of the expected primordial H and He seems to be "missing" from these two planets. Another curiosity is that Uranus does not seem to generate any internal heat while Neptune, like Jupiter and Saturn, does. This results in Uranus and Neptune having the same effective temperature even though Neptune is half again as far from the sun as Uranus. It also may account for the fact that Neptune does have surface features (no convection) while Uranus is featureless.