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This means that the impactor can melt about 25 times its own mass (4.5 x 10'4/1.7 x 10'3 = 26). Of course this is a rough calculation, but it does show how effective accretion can be in heating up a growing body, and how it can therefore help the body to attain a spherical shape and to internally differentiate into different compositional shells.

and it too orbits Uranus more than once per Uranian day. It appears to be the outer shepherding moon for Uranus's 8 ring, keeping the ring defined at its outer edge.

Bianca is named for the sister of Katherine in Shakespeare's The Taming of the Shrew. Like many other Uranian moons, its density and mass are unknown, so its composition cannot be accurately guessed.

What Are Synchronous Orbits and Synchronous Rotation?

^Synchronous rotation can easily be confused with synchronous orbits. In a synchronous orbit, the moon orbits always above the same point on the planet it is orbiting (this section uses the terms moon and planet, but the same principles apply to a planet and the Sun). There is only one orbital radius for each planet that produces a synchronous orbit. Synchronous rotation, on the other hand, is created by the period of the moon's rotation on its axis being the same as the period of the moon's orbit around its planet, and produces a situation where the same face of the moon is always toward its planet. Tidal locking causes synchronous rotation.

Gravitational attraction between the moon and its planet produces a tidal force on each of them, stretching each very slightly along the axis oriented toward its partner. In the case of spherical bodies, this causes them to become slightly egg-shaped; the extra stretch is called a tidal bulge. If either of the two bodies is rotating relative to the other, this tidal bulge is not stable. The rotation of the body will cause the long axis to move out of alignment with the other object, and the gravitational force will work to reshape the rotating body. Because of the relative rotation between the bodies, the tidal bulges move around the rotating body to stay in alignment with the gravitational force between the bodies. This is why ocean tides on Earth rise and fall with the rising and setting of its moon, and the same effect occurs to some extent on all rotating orbiting bodies.

The rotation of the tidal bulge out of alignment with the body that caused it results in a small but significant force acting to slow the relative rotation of the bodies. Since the bulge requires a small amount of time to shift position, the tidal bulge of the moon is always located slightly away from the nearest point to its planet in the direction of the moon's rotation. This bulge is pulled on by the planet's gravity, resulting in a slight force pulling the surface of the moon in the opposite direction of its rotation. The rotation of the satellite slowly decreases (and its orbital momentum simultaneously increases). This is in the case where the moon's rotational period is faster than its orbital period around its planet. If the opposite is true, tidal forces increase its rate of rotation and decrease its orbital momentum.

Almost all moons in the solar system are tidally locked with their primaries, since they orbit closely and tidal force strengthens rapidly with decreasing distance. In addition, Mercury is tidally locked with the Sun in a 3:2 resonance. Mercury is the only solar system body in a 3:2 resonance with the Sun. For every two times Mercury revolves around the Sun, it rotates on its own axis three times. More subtly, the planet Venus is tidally locked with the planet Earth, so that whenever the two are at their closest approach to each other in their orbits, Venus always has the same face toward Earth (the tidal forces involved in this lock are extremely small). In general any object that orbits another massive object closely for long periods is likely to be tidally locked to it.

4. Cressida (1986 U3)

Cressida is named for the daughter of Calchas in Shakespeare's Troilus and Cressida. All the moons through Cressida orbit closer to Uranus than the Roche limit. Will they be torn apart over time? (For more on the Roche limit, see the sidebar "Why Are There Rings?" on page 42) These moons are probably small enough to avoid destruction through tidal forces within the Roche limit, but they are also too small for clear observation and discovery of their compositions or surface characteristics.

5, 6, 7, 8. Desdemona (1986 U6), Juliet (1986 U2), Portia (1986 Ui), Rosalind (1986 U4)

As with the inner satellites, almost nothing is known about these four small bodies. Desdemona is named for the wife of Othello in Shakespeare's Othello. Juliet was, of course, named after the heroine in Shakespeare's Romeo and Juliet. Portia was named after an heiress in Shakespeare's The Merchant of Venice. Rosalind was named after a daughter of the banished duke in Shakespeare's As You Like It.

### 9. 2003 U2

Mark Showalter, a research scientist at Stanford University, and Jack Lissauer, a research scientist at the NASA Ames Research Center, discovered 2003 U1 and 2003 U2 with the Hubble Space Telescope.

10. Belinda (1986 U5)

Belinda is named after the heroine in Alexander Pope's The Rape of the Lock and was discovered by Voyager 2 in 1986.

### 11. 1986 U10

This tiny moon was imaged by Voyager 2 in 1986, but not noticed in the images until 1999, by Erich Karkoschka of the Lunar and Planetary Lab of the University of Arizona in Tucson.This moon's orbit is nearly identical to that of Belinda, about 47,000 miles (75,000 km) from Uranus. For several years the moon's existence was in doubt because it was so small that it could not be seen with Earth-based telescopes. In fact, after initially being given its temporary designation and announced as a new moon of Uranus, the International Astronomical Union decided there was not enough data on the object, and stripped it of the title "moon." In 2003 it was finally seen again in an image made by the newest camera on the Hubble Space Telescope, its existence confirmed, and its status as a moon of Uranus renewed.

Puck is a playful fairy in Shakespeare's A Midsummer Night's Dream. Of the 10 new Uranian moons discovered by Voyager 2, only Puck was discovered soon enough for the mission's observation schedule to be adjusted to get images. Puck is irregularly shaped, with a mottled surface, and it and the other small moons are very dark, each with an albedo less than 0.1.

13, 14. 2003 Ut and 2003 U2

Mark Showalter and Jack Lissauer discovered 2003 U1 and 2003 U2 with the Hubble Space Telescope.

### 15. Miranda

Miranda is named after the daughter of the magician Prospero in Shakespeare's The Tempest. In 1948 Gerald Kuiper, the Dutch-born American astronomer, discovered the moon. Miranda was the last moon of Uranus discovered before Voyager 2. Voyager 2 flew close to

Uranus's moon Miranda displays its distinctive chevron feature above and to the right of center in this image centered on the south pole. (nasa/jpl/usgs/Voyager)

Uranus to get the boost it needed to go on to Neptune, and happened to pass close to Miranda, obtaining good images of the moon.

Miranda is the smallest of Uranus's major moons, but it has the most complex and interesting surface. At 290 miles (470 km) in radius, it seems to be at size interface between planets that differentiate (and turn into proper spheres) and small, irregular planets. Miranda's surface is heavily cratered, as expected for an old surface on a small body, but the old surface is covered with unusual patterns. Miranda has lines and grooves (called "ovoids"), as well as a white chevron-shaped cracks, and three strange areas called coronae that have been geologically active. Miranda's image on page 53 shows both old, heavily cratered, rolling terrain and young, complex terrain containing bright and dark bands and ridges.

Each of these three coronae has clearly defined edges and lacks cra-tering, showing that they formed after the heavy period of cratering early in the solar system.The largest corona, Arden, is a giant oval of concentric light and dark stripes, bounded by a deep canyon.The second-largest corona, called Inverness, covers Miranda's south pole and is rectangular, though also covered with stripes and ridges and troughs of dark and light material. Its stripes are in the shape of a right angle, filling in the corona from one side.The third corona, Elsinore, is oval, like Arden, but filled with troughs and ridges, like Miranda, though it lacks the color striping and appears plain gray.

Canyons hundreds of kilometers long and tens of kilometers wide score other parts of Miranda's surface. These canyons appear to have been formed by stretching Miranda's crust, perhaps because buoyant upwelling material from deep inside Miranda rose to the bottom of the crust and lifted and stretched it. There are many theories about the formation of the coronae, as well. Some researchers propose that the coronae are sequential outpourings of lava from these hot upwellings. Others argue that the coronae look very much like oceanic crust on Earth and are the frozen remnants of plate tectonics on Miranda. Voyager 2 acquired the image of Miranda on page 55 from a distance of 19,400 miles (31,000 km) but achieved a high resolution of about 2,000 feet (600 m) per pixel.The grooves reach depths of a few kilometers.The image area is about 150 miles (240 km) across.

All these theories have a fundamental mystery: Where did the heat come from to cause buoyant upwellings, volcanic activity, and plate tectonics? Miranda is too small to have built up any significant heat by

The close-up image of Uranus's moon Miranda shows a complex array of fractures, grooves, and craters. (NASA/JPL/Voyager 2/nssdc)

accretion, and all its heat would have been lost very quickly, also because of its small size. Miranda appears to have a density of only 70 lb/ft3 (1,115 kg/m3), indicating that it consists largely of water ice. Its crust may indeed have lava flows and plate tectonics, but the molten material would have to consist mostly of water. Unfortunately, the formations that are thought to be lava are hundreds of meters thick and have sharp ridges. Both these features indicate that the lava had to have a high viscosity (that is, be a thick and slowly flowing liquid), which liquid water would not be. If the lava consisted of water, it would be thin and run out into sheets, not able to build up into thick flows and ridges. It is possible, though, that the lava consisted of a mixture of water and ammonia or methanol, which at the very low surface temperatures of Miranda (around —310°F or —190°C) would have a reasonably high viscosity and would freeze like rock.

All these theories about Miranda are currently unsubstantiated and await more space missions to gather more data. Miranda's very

The close-up image of Uranus's moon Miranda shows a complex array of fractures, grooves, and craters. (NASA/JPL/Voyager 2/nssdc)

Ariel's complex surface is thought to have been made by a combination of interior and surface processes along with cratering. (NASA/JPL/Voyager 2)

strange surface, apparently so active in the past, may have been created by processes so unlike those on Earth that scientists have not yet imagined the right hypotheses.

t6.Ariel

Ariel is named for an ethereal spirit in Shakespeare's The Tempest and was discovered by William Lassell, a British astronomer who started his professional life as a brewer, in 1851. Ariel has the youngest surface of the five major moons, and is the brightest. Its appearance is similar to Titania's. It bears many deep, flat-floored canyons crossing back and forth over an older, cratered surface. The largest canyon is called Kachina Chasmata and is 390 miles (622 km) long.The floors of the canyons appear to have volcanic activity in them, and the surface also bears thin, winding, raised ridges called rilles, which on Earth's Moon have been identified as lava tubes formed during spreading volcanic activity.

Ariel's complex surface is thought to have been made by a combination of interior and surface processes along with cratering. (NASA/JPL/Voyager 2)

The images for the montage of Ariel on page 58 were taken by Voyager 2 on January 24, 1986, at a distance of about 80,000 miles (130,000 km). Ariel's many small craters are close to the threshold of detection in this picture. Numerous valleys and fault scarps crisscross the highly pitted terrain.The largest fault valleys, at the right, as well as a smooth region near the center of this image, have been partly filled with younger deposits that are less heavily cratered than the pitted terrain. Narrow, somewhat sinuous scarps and valleys have been formed, in turn, in these young deposits.

On Ariel, the volcanic activity is thought to be not the hot, silica-based molten rocks found on Earth, Io, and Mars but a type of cold volcanism (cryovolcanism) created by a flowing mixture of ammonia and water at very low temperatures. Though Ariel currently has no tidal resonance with other moons, it is thought that the cryovolcanism was caused in the past by tidal heating, and that the deep canyons formed when Ariel cooled from its initial formation.The interior is thought to be water-rich, so scientists hypothesize that when the liquid interior cooled and froze, it expanded (water is one of the very few materials that actually expands when it freezes), splitting Ariel's crust.

### 17. Umbriel

Umbriel, named after a character in Alexander Pope's The Rape of the Lock, was discovered by William Lassell in 1851. Umbriel is thought to be in its primitive state, never having heated enough to differentiate into a core and a mantle. Its surface is very dark (only half as bright as Ariel), heavily cratered, and appears never to have been resurfaced. Its dark, cratered surface appears similar to Oberon's and very different from Titania's and Ariel's. Its outstanding feature is the crater Wunda, 50 miles (80 km) in radius, which has a conspicuously bright floor.

### 18. Titania

Titania is named after the Queen of the Fairies, wife of Oberon, in Shakespeare's A Midsummer Night's Dream. The largest of Uranus's moons (see image on page 58), it was discovered by William Herschel in 1787.Titania is an icy moon with relatively few craters on its surface, indicating that it may have been resurfaced at some point in its history. Its appearance is similar to Ariel's. At 100 miles (163 km) in radius, Gertrude is the moon's largest crater. Large, interconnected canyons, seemingly caused by faulting, cover Titania's surface. The

Uranus's five largest satellites, Miranda, Ariel, Umbriel, Titania, and Oberon, are shown in this montage from left to right (also in order of increasing distance from Uranus). The moons are presented at the same size and brightness scales to allow comparisons. Photographic coverage is incomplete for Miranda and Ariel, and gray circles depict missing areas. (nasa/]pl/ Voyager 2)

largest canyon is Messina Chasmata, 940 miles (1,500 km) long and 60 miles (100 km) across at its widest point.

### 19. Oberon

Oberon is named after the King of the Fairies, husband of Titania, in Shakespeare's A Midsummer Night's Dream. Discovered by William Herschel in 1787, Oberon is a midsize moon and one of the original five discovered before the Voyager 2 mission. Its surface is heavily cratered with what may be volcanic flows on some large crater floors. Its dark, cratered surface is similar to Umbriel's. Some craters have light-colored ejecta rays radiating from them. Oberon's largest crater is Hamlet, at 60 miles (100 km) in radius. The images from Voyager 2 were not as good as those for other moons, but scientists did make out a 310-mile (500-km)-long linear feature, Mommur Chasma, that appears to be created by a fault.The Voyager 2 image shown on page 59, taken from distance of 1.72 million miles (2.77 million km), clearly displays large craters.

The surface of Oberon seems to be very old, not resurfaced with water-rich lavas as Miranda and Ariel seem to have been. Oberon's leading hemisphere appears redder than its other hemisphere (like Earth's Moon, its rotation is synchronous, that is, the same side of the moon faces Uranus at all times, so one hemisphere of Oberon always leads in its orbit). The reddish color may be due to dust from other moons swept up by Oberon during orbit.

All the moons with orbits outside Oberon are irregular:They have large orbits at significant inclinations, and almost all of these moons orbit in a retrograde sense.

Uranus's moon Oberon displays some bright patches suggestive of crater ejecta in an icy surface. (nasa/jpl/ Voyager 2)

20. 2001 U3

Matthew Holman and J. J. Kavelaars first observed this tiny moon using the 13-foot (4-m) Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile. Although it was first seen in 2001, it was not confirmed until 2003.

21. Caliban (1997 Ui)

Caliban is named after a savage slave from Shakespeare's play The Tempest, the son of the witch Sycorax who imprisoned the fairy Ariel for disobedience. In 1997 Brett Gladman, a scientist at the Canadian Institute for Theoretical Astrophysics and Cornell University, and his colleagues Phil Nicholson, Joseph Burns, and J. J. Kavelaars discovered Caliban using the 200-inch (5-m) Hale telescope. The first images of the moon were taken on September 6 and 7 of that year. Prior to the discovery of Caliban and Sycorax, all of Uranus's moons orbited directly (in the same sense as the Earth orbits the Sun) and in planes close to the planet's equator.

All the other gas giant planets were known to have irregular moons with unusual orbits, and now so did Uranus: Both Caliban and Sycorax have highly inclined, retrograde orbits, and both are assumed to be captured asteroids. Both are reddish in color, similar to some Kuiper belt objects. At the time of their discovery, they were the dimmest moons ever to be discovered by a ground-based observatory; Caliban is slightly dimmer than Sycorax. Almost all the moons exterior to Caliban have retrograde orbits and are likely candidates to be captured asteroids.The moons of all the gas giants fall into this pattern: Near moons have more circular, less inclined, prograde orbits, and farther moons have inclined, eccentric orbits and are candidates to be captured asteroids.

22. Stephano (1999 U2)

Almost nothing is known about this small moon; it was found at the same time as Setebos.

23. Trinculo (2001 Ui)

Trinculo is named for the jester in Shakespeare's play The Tempest. The productive team of Brett Gladman, Matthew Holman, J. J. Kavelaars, and Jean-Marc Petit found this tiny moon, dimmer than many of those that were found by Voyager 2 when it was near Uranus.

24. Sycorax (1997 U2)

Sycorax is named after the witch in Shakespeare's play The Tempest who is also the mother of Caliban. In 1997 Phil Nicholson, Brett Gladman, Joseph Burns, and J. J. Kavelaars discovered Sycorax using the 200-inch (5-m) Hale telescope. See the entry on Caliban for more details.

### 25. 2003 U3

Scott S. Sheppard and Dave Jewitt at the University of Hawaii discovered two new irregular moons of Uranus, 2001 U2 and 2003 U3, from images obtained by the Subaru 326-inch (8.3-m) telescope at Mauna Kea in Hawaii, on August 29, 2003.Additional observations by the Hawaii team using the Gemini 322-inch (8.2-m) telescope allowed Brian Marsden at the Minor Planet Center to correlate one of the moons to an independent discovery made in 2001 by a group led by Matthew Holman and J. J. Kavelaars.The 2001 observations were not sufficient on their own to determine if the objects were satellites of Uranus; no reliable orbits were found. They were then lost until discovery in 2003 by the Hawaii team. The moon 2003 U3 has the first direct (prograde) orbit of any of Uranus's irregular satellites.

26. Prospero (1999 U3)

Almost nothing is known about this small moon; it was found at the same time as Setebos.

27. Setebos (1999 Ui)

Kavelaars, Gladman, Holman, Petit, and Scholl reported the discovery of three new satellites (1999 U1, 1999 U2, and 1999 U3) of Uranus in images obtained with the 138-inch (3.5-m) Canada-France-Hawaii telescope at Mauna Kea and at the Palomar Observatory. These moons have since been observed by other researchers, their orbits refined, and their names given.Very little is known about these additional irregular moons. ♦ ♦ ♦

As of the writing of this chapter, Uranus is known to have 27 moons. This number is almost certain to rise with time. Jupiter is known to have 63 moons, and Saturn is known to have 33 moons. Neptune has only 13 known moons.The most recent moons found for each of these planets are irregular captured asteroids, often with radii of just a few kilometers. Their small size makes discovery difficult. This list of 27 moons (located on page 41), then, can only be thought of as a provisional list.

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