Sunday, September 6, 2009

Planet 7. Uranus

Uranus is the seventh planet from the Sun and the third largest (by diameter). Uranus is larger in diameter but smaller in mass than Neptune.

orbit: 2,870,990,000 km (19.218 AU) from Sun
diameter: 51,118 km (equatorial)
mass: 8.683e25 kg

Careful pronunciation may be necessary to avoid embarrassment; say "YOOR a nus" , not "your anus" or "urine us". Evil Evil Evil Evil

Uranus is the ancient Greek deity of the Heavens, the earliest supreme god. Uranus was the son and mate of Gaia the father of Cronus (Saturn) and of the Cyclopes and Titans (predecessors of the Olympian gods).

Uranus, the first planet discovered in modern times, was discovered by William Herschel while systematically searching the sky with his telescope on March 13, 1781. It had actually been seen many times before but ignored as simply another star (the earliest recorded sighting was in 1690 when John Flamsteed cataloged it as 34 Tauri). Herschel named it "the Georgium Sidus" (the Georgian Planet) in honor of his patron, the infamous (to Americans) King George III of England; others called it "Herschel". The name "Uranus" was first proposed by Bode in conformity with the other planetary names from classical mythology but didn't come into common use until 1850.

Uranus has been visited by only one spacecraft, Voyager 2 on Jan 24 1986.

Most of the planets spin on an axis nearly perpendicular to the plane of the ecliptic but Uranus' axis is almost parallel to the ecliptic. At the time of Voyager 2's passage, Uranus' south pole was pointed almost directly at the Sun. This results in the odd fact that Uranus' polar regions receive more energy input from the Sun than do its equatorial regions. Uranus is nevertheless hotter at its equator than at its poles. The mechanism underlying this is unknown.

Actually, there's an ongoing battle over which of Uranus' poles is its north pole! Either its axial inclination is a bit over 90 degrees and its rotation is direct, or it's a bit less than 90 degrees and the rotation is retrograde. The problem is that you need to draw a dividing line *somewhere*, because in a case like Venus there is little dispute that the rotation is indeed retrograde (not a direct rotation with an inclination of nearly 180).

Uranus is composed primarily of rock and various ices, with only about 15% hydrogen and a little helium (in contrast to Jupiter and Saturn which are mostly hydrogen). Uranus (and Neptune) are in many ways similar to the cores of Jupiter and Saturn minus the massive liquid metallic hydrogen envelope. It appears that Uranus does not have a rocky core like Jupiter and Saturn but rather that its material is more or less uniformly distributed.

Uranus' atmosphere is about 83% hydrogen, 15% helium and 2% methane.

Like the other gas planets, Uranus has bands of clouds that blow around rapidly. But they are extremely faint, visible only with radical image enhancement of the Voyager 2 pictures (right). Recent observations with HST (left) show larger and more pronounced streaks. Further HST observations show even more activity. Uranus is no longer the bland boring planet that Voyager saw! It now seems clear that the differences are due to seasonal effects since the Sun is now at a lower Uranian latitude which may cause more pronounced day/night weather effects. By 2007 the Sun will be directly over Uranus's equator.

Uranus' blue color is the result of absorption of red light by methane in the upper atmosphere. There may be colored bands like Jupiter's but they are hidden from view by the overlaying methane layer.

Like the other gas planets, Uranus has rings. Like Jupiter's, they are very dark but like Saturn's they are composed of fairly large particles ranging up to 10 meters in diameter in addition to fine dust. There are 11 known rings, all very faint; the brightest is known as the Epsilon ring. The Uranian rings were the first after Saturn's to be discovered. This was of considerable importance since we now know that rings are a common feature of planets, not a peculiarity of Saturn alone.

Voyager 2 discovered 10 small moons in addition to the 5 large ones already known. It is likely that there are several more tiny satellites within the rings.

Uranus' magnetic field is odd in that it is not centered on the center of the planet and is tilted almost 60 degrees with respect to the axis of rotation. It is probably generated by motion at relatively shallow depths within Uranus.

Uranus is sometimes just barely visible with the unaided eye on a very clear night; it is fairly easy to spot with binoculars (if you know exactly where to look). A small astronomical telescope will show a small disk.

Uranus' Satellites & Rings

Uranus has 21 named moons and six unnamed ones:

Unlike the other bodies in the solar system which have names from classical mythology, Uranus' moons take their names from the writings of Shakespeare and Pope.
They form three distinct classes: the 11 small very dark inner ones discovered by Voyager 2, the 5 large ones (right), and the newly discovered much more distant ones.
Most have nearly circular orbits in the plane of Uranus' equator (and hence at a large angle to the plane of the ecliptic); the outer 4 are much more elliptical.

Distance Radius Mass
Satellite (000 km) (km) (kg) Discoverer Date
--------- -------- ------ ------- ---------- -----
Cordelia 50 13 ? Voyager 2 1986
Ophelia 54 16 ? Voyager 2 1986
Bianca 59 22 ? Voyager 2 1986
Cressida 62 33 ? Voyager 2 1986
Desdemona 63 29 ? Voyager 2 1986
Juliet 64 42 ? Voyager 2 1986
Portia 66 55 ? Voyager 2 1986
Rosalind 70 27 ? Voyager 2 1986
2003U2 75 6 ? Showalter 2003
Belinda 75 34 ? Voyager 2 1986
1986U10 76 40 ? Voyager 2 1986
Puck 86 77 ? Voyager 2 1985
2003U1 98 8 ? Showalter 2003
Miranda 130 236 6.30e19 Kuiper 1948
Ariel 191 579 1.27e21 Lassell 1851
Umbriel 266 585 1.27e21 Lassell 1851
Titania 436 789 3.49e21 Herschel 1787
Oberon 583 761 3.03e21 Herschel 1787
2001U3 4281 6 ? Sheppard 2003
Caliban 7169 40 ? Gladman 1997
Stephano 7948 15 ? Gladman 1999
Trinculo 8578 5
Sycorax 12213 80 ? Nicholson 1997
2003U3 14689 6 ? Sheppard 2003
Prospero 16568 20 ? Holman 1999
Setebos 17681 20 ? Kavelaars 1999
2002U2 21000 6 Sheppard 2003

Uranus' Rings
Distance Width
Ring (km) (km)
------- -------- -----
1986U2R 38000 2,500
6 41840 1-3
5 42230 2-3
4 42580 2-3
Alpha 44720 7-12
Beta 45670 7-12
Eta 47190 0-2
Gamma 47630 1-4
Delta 48290 3-9
1986U1R 50020 1-2
Epsilon 51140 20-100

(distance is from Uranus' center to the ring's inner edge)

Few big Satellites of Uranus 1. Miranda

Miranda ("mi RAN duh") is the eleventh of Uranus's known satellites. Miranda is the innermost of Uranus' large moons.

orbit: 129,850 km from Uranus
diameter: 472 km
mass: 6.3e19 kg

Miranda is a daughter of the magician Prospero in Shakespeare's The Tempest.

Discovered by Kuiper in 1948.

Voyager 2 was forced to fly close to Uranus in order to get the boost it needed to go on to Neptune and due to the orientation of the whole system at almost right angles to the ecliptic only Miranda was approached closely. Before Voyager, of course, little was known about Miranda and as it is not the largest or in any other way remarkable, it probably would not have been chosen as the prime target at Uranus. Voyager's good luck held up, however, as Miranda turned out to be by far the most interesting.

Miranda is about half water ice and half rocky material.

Miranda's surface is all mixed up with heavily cratered terrain intermixed with weird grooves, valleys and cliffs (one over 5 kilometers high; left).

At first, Voyager 2's images of Miranda were a mystery. Everyone had expected that Uranus' moons would show very little history of internal activity (like Callisto). Explaining the bizarre hitherto unknown terrain proved quite an embarrassment to those who had to do it on live TV. Their usual impressive and esoteric technical jargon gave out and they had to resort to using such terms as "chevron" (right), "race track", and "layer cake" to describe Miranda's unique features.

It was initially thought that Miranda had been completely shattered and reassembled several times in its history, each time burying some parts of the original surface and exposing some of the interior. Now, however, a more mundane explanation involving the upwelling of partially melted ices seems to be in favor.

Voyager 2 passed so close to Miranda and the light levels are so low there (almost 3 billion km from the Sun) that special measures had to be employed to avoid smearing the images. This was accomplished by rotating the entire spacecraft while the camera's shutter was open to compensate for its motion. The resulting images have the best resolution of the entire mission.

It is actually possible to see Uranus's 4 largest moons with an amateur telescope, but Miranda is a real challenge. Perhaps with a very dark sky and a telescope with an 18 inch (50 cm) aperture or more it might be possible

Few big Satellites of Uranus 2. Titania

Titania ("ti TAY nee uh") is the fourteenth and largest of Uranus's known satellites:

orbit: 436,270 km from Uranus
diameter: 1578 km
mass: 3.49e21 kg

Titania is the Queen of the Fairies and wife of Oberon in Shakespeare's Midsummer-Night's Dream.

Discovered by Herschel in 1787.

Titania and Ariel appear quite similar though Ariel is 25% smaller. All of Uranus' large moons are a mixture of about 40-50% water ice with the rest rock, a somewhat larger fraction of rock than Saturn's large moons such as Rhea.

Titania's surface is a mixture of cratered terrain and systems of interconnected valleys hundreds of kilometers long. Some of the craters appear to be half-submerged. Titania's surface is clearly relatively young (though older than some such as Enceladus); obviously some sort of resurfacing processes have been at work.

One theory of Titania's history is that it was once hot enough to be liquid. The surface probably cooled first; when the interior froze it expanded forcing the surface to crack and resulting in the valleys that we see today

Few big Satellites of Uranus 3. Oberon

Oberon ("OH buh ron") is the outermost of Uranus' large satellites and the second largest:

orbit: 583,420 km from Uranus
diameter: 1523 km
mass: 3.03e21 kg

Oberon is the King of the Fairies and husband of Titania in Shakespeare's Midsummer-Night's Dream.

Discovered by Herschel in 1787.

Oberon and Umbriel appear quite similar though Oberon is 35% larger. All of Uranus' large moons are a mixture of about 40-50% water ice with the rest rock, a somewhat larger fraction of rock than Saturn's large moons such as Rhea.

Oberon's heavily cratered surface has probably been stable since its formation. It has far more and larger craters than do Ariel and Titania. Some of the craters have rays of ejecta similar to those seen on Callisto.

Some of the crater floors are dark, perhaps covered with darker material (dirty water?) that upwelled into the crater.

Large faults are also seen across the entire southern hemisphere of Oberon. This indicates some geologic activity early in Oberon's history

Few big Satellites of Uranus 4. Umbriel

Umbriel ("UM bree el") is the thirteenth of Uranus's known satellites and the third largest:

orbit: 265,980 km from Uranus
diameter: 1170 km
mass: 1.27e21 kg

Umbriel is a character in Alexander Pope's The Rape of the Lock.
Discovered by Lassell in 1851.

Umbriel and Oberon appear quite similar though Oberon is 35% larger. All of Uranus' large moons are a mixture of about 40-50% water ice with the rest rock, a somewhat larger fraction of rock than Saturn's large moons such as Rhea.

Umbriel's heavily cratered surface has probably been stable since its formation. It has far more and larger craters than do Ariel and Titania.

Umbriel is very dark; it reflects only about half as much light as Ariel, Uranus' brightest satellite.

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