Rings and Moons of Uranus

s with the other gas giant planets, many of the discoveries of rings

/ \ and moon have occurred recently. Two of Uranus's moons were discovered more than 200 years ago, but as recently as 1986 only five moons had been discovered. Uranus's rings were discovered in 1977, the first ring system to be discovered during the modern age. Now Uranus is known to have 27 moons (more are likely to be found in coming years) and a set of narrow planetary rings.

In 1977 Jim Elliot, a professor at the Massachusetts Institute of Technology, along with Robert Millis and Edward Dunham of the Lowell Observatory, watched a star move behind Uranus and in that moment discovered the planet's rings. In this technique, called stellar occultation, the quality of the starlight can help identify characteristics of the planet it is passing behind. The observations of Uranus were made using a 36-inch (91-cm) telescope aboard a Lockheed jet, together called the Gerard P. Kuiper Airborne Observatory. The jet flew at about 39,000 feet (12 km) to be above almost all of the Earth's atmospheric water vapor and provide clear measurements of infrared radiation during the occultation. In this case, the star's light blinked as it passed behind Uranus's rings, which were otherwise too small to be seen. Stellar occultation is also an effective technique for measuring a planet's atmosphere. Nine narrow rings were counted using stellar occultation, and then in 1986 Voyager 2 detected two more: a narrow ring and a broad, diffuse ring closer to the planet.

Rings

Uranus has in total 11 rings, all in the planet's equatorial plane, that is, perpendicular to its orbit. They vary from 10 to 100 kilometers in width. The rings are designated using Greek letters, starting from the ring closest to Uranus:

1986 U2R 6

All the rings are listed and described in the table on page 40. The 10 outer rings are dark, thin, and narrow, and the 11th is broad and diffuse, inside the others. The ring 1986 U2R is the broad, diffuse ring.The densest and widest ring is the £ (epsilon) ring, with a width that varies from 12 to 60 miles (20 to 96 km).The other rings are just 0.6 to six miles (1 to 10 km) wide. All the rings are thin, less than 100 feet (a few tens of meters) in thickness perpendicular to the plane of their orbits. The image in the figure on page 39 from the Voyager 2 mission clearly shows the £ ring.

Some scientists think that the dark color of the rings means they are old, perhaps because during aging the rings have lost their water, methane, or ammonia frost. By using occultation profiles at different wavelengths, the sizes of the particles in the rings can actually be determined. Uranus's rings consist mainly of particles between one and several tens of centimeters in diameter, much larger than the particles in Saturn's rings.

Prior to 1977, only Saturn's broad, bright rings were known, and no astronomer seemed to have been thinking about the possibility of narrow, dark rings. How narrow rings are maintained is still not well understood. At first the theory was that small moons, called shepherd moons, orbit near the rings, confining the rings to narrow regions

through gravitational pull from these moons. Uranus's moons Cordelia and Ophelia do indeed shepherd the £ ring. Other rings, though, seem to have no shepherds, and so the theories for how rings remain narrow are not completely delineated.

Strangely, the rings are not perfectly round. They have eccentricities from 0.001 to 0.01. This was another new phenomenon discovered in the Uranian rings: Previously, all physical theories for ring formation required that the rings be almost perfectly circular and that they lie in the equatorial plane of the planet. In addition to their eccentricities, Uranus's rings have inclinations to the planet's equatorial plane that range as high as 0.062 degrees.

Moons

Before 1986, five of Uranus's moons had been observed. William Herschel, the discoverer of Uranus, found Oberon and Titania in 1787. Though Herschel searched for 40 years, he never found another moon of Uranus. In fact, his sightings of Oberon and Titania were not even

This Voyager 2 image of Uranus's rings was taken while the spacecraft was in the shadow of Uranus, about 3.5 hours after closest approach. The short streaks are background stars that appear to be moving because the image required a 96-second exposure. The rings are made up of micrometer-size particles and are very dark.. The frame is about 10,000 kilometers across. (nasa/ Voyager 2/nssdc)

URANIAN RINGS

Distance from Uranus's surface to inner edge of

Name

ring (miles [km])

Width (miles [km])

Eccentricity

1986U2R

23,613 (38,000)

1,550 (2,500)

unavailable

6

25,997 (41,837)

~1 (~1.5)

0.0010

5

26,244 (42,235)

~1.2 (~2)

0.0019

4

26,453 (42,571)

~1.6 (~2.5)

0.0010

a (alpha)

27,787 (44,718)

2.5 to 6.2 (4 to 10)

0.0008

P (beta)

28,373 45,661)

3 to 7 (5 to 11)

0.0004

n (eta)

29,315 (47,176)

1 (1.6)

unavailable

Y (gamma)

29,594 (47,626)

0.6 to 2.5 (1 to 4)

0.0001

5 (delta)

30,015 (48,303)

1.9 to 4.3 (3 to 7)

unavailable

X (lambda)

31,084 (50,024)

~1 (~2)

unavailable

8 (epsilon)

31,783 (51,149)

12 to 60 (20 to 96)

0.0079

confirmed by another astronomer for 10 years. In 1851 William Lassell, a British astronomer who started his professional life as a brewer, found Umbriel and Ariel and reported them to the Royal Astronomical Society. Another century later, in 1948, Gerald Peter Kuiper, the Dutch-born American astronomer (see more about the Kuiper belt later in this volume), found the moon Miranda by using the large, 81-inch (200-cm) reflecting telescope in the McDonald Observatory in Texas.

In 1986 the Voyager 2 mission found 10 more moons (Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, and Puck), all closer to the planet than the original five, and since 1986, five more have been seen from Earth-based observatories (see table on page 41). The first two of these, Caliban and Sycorax, found in 1987, are more than 10 times as far from Uranus as any of the previously known moons. In 1999 the moon 1986 U10 was found sharing an orbit almost identical to that of Belinda. (Though this moon was found in 1999, it was then recognized on images from Voyager taken in 1986, hence its designation.) About once a month, Belinda

URANIAN MOONS

Orbital

Radius

Year

Orbital

Orbital

period

(miles

dis

eccen-

inclina

Orbital

Moon

(Earth days)

[km])

covered

tricity

tion

direction

Regular Group

1. Cordelia

0.335

16 (26)

1986

0.000

0.14

prograde

2. Ophelia

0.376

19 (30)

1986

0.001

0.09

prograde

3. Bianca

0.435

32 (51)

1986

0.001

0.16

prograde

4. Cressida

0.464

50 (80)

1986

0.000

0.04

prograde

5. Desdemona

0.474

40 (64)

1986

0.000

0.16

prograde

6. Juliet

0.493

58 (93)

1986

0.001

0.06

prograde

7. Portia

0.513

84 (135)

1986

0.000

0.09

prograde

8. Rosalind

0.558

45 (72)

1980

0.000

0.28

prograde

9. 2003 U2

0.618

6 (10)

2003

0.000

0.00

prograde

10. Belinda

0.624

50 (80)

1986

0.000

0.03

prograde

11. 1986 U10

0.638

12 (20)

1986

0.000

0.00

prograde

12. Puck

0.762

101 (162)

1985

0.000

0.31

prograde

13. 2003 U1

0.923

6 (10)

2003

0.000

0.000

prograde

14. Miranda

1.414

293 (471)

1948

0.0027

4.22

prograde

15. Ariel

2.520

720 (1,158)

1851

0.0031

0.31

prograde

16. Umbriel

4.144

726 (1,169)

1851

0.0050

0.36

prograde

17. Titania

8.706

981 (1,578)

1787

0.0022

0.14

prograde

18. Oberon

13.46

946 (1,522)

1787

0.0008

0.1

prograde

Irregular Group

19. 2001 U3

266.6

7 (12)

2001

0.146

145.2

prograde

20. Caliban

579.5

61 (98)

1997

0.159

140.9

retrograde

21. Stephano

677.4

12 (20)

1999

0.23

144.06

retrograde

22. Trinculo

759.0

6 (10)

2001

0.208

166.33

retrograde

23. Sycorax

1288.3

118 (190)

1997

0.522

159.4

retrograde

24. 2003 U3

1,694.8

7 (11)

2003

0.661

56.6

prograde

25. Prospero

1,977.3

19 (30)

1999

0.443

151.91

retrograde

26. Setebos

2,234.8

19 (30)

1999

0.588

158.17

retrograde

27. 2001 U2

2,823.4

7 (12)

2001

0.426

167.3

retrograde

Why Are There Rings? ^Galileo Galilei first saw Saturn's rings in 1610, though he thought of them as handles on the sides of the planet rather than planet-encircling rings. After this first half observation, there was a hiatus in the discovery of planetary ring systems that lasted for three and a half centuries, until 1977, when Jim Elliot, an astronomer at the Massachusetts Institute of Technology and the Lowell Observatory, saw the blinking of a star's light as Uranus passed in front of it and correctly theorized that Uranus had rings around it that were blocking the light of the star. Two years later, Voyager 1 took pictures of Jupiter's rings, and then in 1984, Earth-based observations found partial rings around Neptune. Now it is even hypothesized that Mars may have very tenuous rings, with an optical depth of more than 10-8 (meaning that almost all the light that shines on the ring goes straight through, without being scattered or reflected).

There are two basic categories of planetary rings. The first involves rings that are dense enough that only a small percentage of the light that shines on them passes through. These dense rings are made of large particles with radii that range from centimeters to meters. Examples of these dense rings are Saturn's main rings A and B and the Uranian rings. The second involves tenuous rings of particles the size of fine dust, just microns across. In these faint rings the particles are far apart, and almost all the light that strikes the ring passes through. Jupiter and Saturn's outermost rings are of this faint type. Neptune's rings, however, do not fall into either neat category.

In dense rings the constant collisions between particles act to spread out the rings. Particles near the planet lose speed when they collide with other particles, and thus fall closer to the planet. Particles at the outer edges of the ring tend to gain speed when they collide, and so move farther from the planet. In a complex way, the changes in velocity and redistributions of angular momentum act to make the ring thinner and thinner in depth while becoming more and more broadly spread from the planet.

Most dense rings exist within a certain distance from their planet, a distance called the Roche limit. Within the Roche limit, the tidal stresses from the planet's gravity overcome the tendency for particles to accrete into bodies: The gravitational stresses are stronger than the object's self-gravity, and the object is pulled to pieces. The Roche limit differs for every planet, since their gravities vary, and it also differs for each orbiting object, since their densities differ. The Roche limit (R) can be calculated as follows:

* Psatellite '

where p , is the density of the planet, p ,, is the density of the object orbiting the

' planet J ' 'satellite planet, and R is the radius of the planet. Thus, moons that attempted to form within the Roche limit, or were thrown within the Roche limit by other forces, will be torn into rubble by the gravitational forces of the planet and form rings.

All the Uranian rings lie within the Roche limit, but Saturn's, Jupiter's, and Neptune's outer rings lie outside their Roche limits and orbit in the same regions as moons. The moons have important effects on the rings near which they orbit. First, if the moon and particles in the ring share an orbital resonance (when the ratio of the orbital times is an integer ratio, for example, the moon orbits once for every two times the particle orbits), they interact gravitationally in a strong way: The particle, moon, and planet line up regularly, exerting strong forces on the particle and warping its orbit. If the resonances are strong (low integer ratios), a gap can be created in the ring. In other cases the resonance results in a wavy ring.

Moons can also strongly affect ring particles that they orbit next to. Very thin rings that would otherwise be expected to widen with time can be kept thin by moons that orbit closely on either side. These are called shepherd moons, bumping any stray particles back into the rings or accreting them onto the moon's surface.

Intact moons outside the Roche limit may also shed material, forming the source of a ring. Small moons, with low gravity, may allow more material to escape than large moons do. Jupiter's small moons Adrastea, Metis, Amalthea, and Thebe are all thought to create their own rings. In the Saturnian system, by contrast, the large, 250-km radius moon Enceladus is thought to be the creator and sustainer of Saturn's E ring.

Once formed, the ring does not remain forever: Forces from radiation, meteoroid impacts, and drag from the outer parts of the planet's atmosphere (the exosphere) begin to erode the ring. It is estimated that even dense rings can only exist for a few hundreds of thousands or millions of years, and so even the gorgeous rings of Saturn are probably just a fleeting phenomenon in the age of the solar system. Faint rings may disappear within thousands of years, unless replenished by a moon.

A system of rings around a planet can be thought of as a miniature reenactment of the original solar nebula: The planet is the giant mass at the center of a rotating system, much as the early Sun was, and the rings are the material rotating around it. Material is taken from moons to make rings, and other material is swept up by moons, a sort of recycling between moons and rings. This may be the reason that the outer planets have rings and the inner planets do not; the outer planets have great inventories of moons to create and sweep up rings, while the inner planets do not have enough moons.

laps 1986 U10 as they orbit! Since then, a moon (2003 U2) has been found with an orbit even closer to Belinda's than is 1986 U10. Recently discovered moons still have their year and number designations, and have not yet been given their permanent names. Uranus's moons are all named for characters from the writings of William Shakespeare and Alexander Pope. Uranus has, at the moment, 27 known moons, three of which were discovered in 2003. More moons will likely be discovered in the future. Most of the moons are almost perfectly in the plane of Uranus's equator, each within 0.4 degrees of the equatorial plane. Observing the orbits of the moons was the first indication of Uranus's highly tilted axis (Uranus's rotation axis lies 97.92 degrees away from perpendicular to its orbital plane).

Although little is known about many of the Uranian moons, the following is a summary of each moon, numbered in order of its orbit, starting closest to Uranus. When a moon has been given a provisional designation, and then a final name, its provisional designation is shown in parentheses. It is important to keep in mind that new satellites are being discovered at a high rate, and that new information on the sizes and orbits of existing satellites is made available frequently, and so when absolute up-to-date information is needed, one should consult the NASA, Hawaii Observatory, Minor Planet Center, or other Web sites.

Many of Uranus's moons are described here in more detail, but very little is known about some of the smaller moons. A few of the small, irregular moons have virtually nothing known about them and so are left out of this section.

1. Cordelia (1986 U7)

Cordelia is named after one of Lear's daughters in Shakespeare's King Lear. The moon was discovered by Voyager 2 in 1986. Cordelia appears to be the inner shepherding moon for Uranus's £ ring, keeping the ring defined on its inner edge. Cordelia orbits inside the synchronous orbit radius for Uranus, so it orbits Uranus more than once in a Uranian day. Like many moons, Cordelia rotates synchronously, so the same face of Cordelia is always toward Uranus (see the sidebar "What Are Synchronous Orbits and Synchronous Rotation?" on page 51). Cordelia, and all the satellites through Rosalind, were discovered by Voyager 2 in 1986.

2. Ophelia (1986 U8)

Ophelia is named after the daughter of Polonius in Shakespeare's Hamlet. Like Cordelia, the moon was discovered by Voyager 2 in 1986,

Accretion and Heating: Why Are Some Solar System Objects Round and Others Irregular?

T here are three main characteristics of a body that determine whether it will become round.

The first is its viscosity, that is, its ability to flow. Fluid bodies can be round because of surface tension, no matter their size; self-gravitation does not play a role. The force bonding together the molecules on the outside of a fluid drop pull the surface into the smallest possible area, which is a sphere. This is also the case with gaseous planets, like Uranus. Solid material, like rock, can flow slowly if it is hot, so heat is an important aspect of viscosity. When planets are formed, it is thought that they start as agglomerations of small bodies, and that more and more small bodies collide or are attracted gravi-tationally, making the main body larger and larger. The heat contributed by colliding planetesimals significantly helps along the transformation of the original pile of rubble into a spherical planet: The loss of their kinetic energy (more on this at the end of this sidebar) acts to heat up the main body. The hotter the main body, the easier it is for the material to flow into a sphere in response to its growing gravitational field.

The second main characteristic is density. Solid round bodies obtain their shape from gravity, which acts equally in all directions and therefore works to make a body a sphere. The same volume of a very dense material will create a stronger gravitational field than a less dense material, and the stronger the gravity of the object, the more likely it is to pull itself into a sphere.

The third characteristic is mass, which is really another aspect of density. If the object is made of low-density material, there just has to be a lot more of it to make the gravitational field required to make it round.

Bodies that are too small to heat up enough to allow any flow, or to have a large enough internal gravitational field, may retain irregular outlines. Their shapes are determined by mechanical strength and response to outside forces such as meteorite impacts, rather than by their own self-gravity. In general the largest asteroids, including all 100 or so that have diameters greater than 60 miles (100 km), and the larger moons, are round from self-gravity. Most asteroids and moons with diameters larger than six miles (10 km) are round, but not all of them, depending on their composition and the manner of their creation.

There is another stage of planetary evolution after attainment of a spherical shape: internal differentiation. All asteroids and the terrestrial planets probably started out

Accretion and Heating: Why Are Some Solar System Obj ects Round and Others Irregular? (continued) made of primitive materials, such as the class of asteroids and meteorites called CI or enstatite chondrites. The planets and some of the larger asteroids then became com-positionally stratified in their interiors, a process called differentiation. In a differentiated body, heavy metals, mainly iron with some nickel and other minor impurities in the case of terrestrial planets, and rocky and icy material in the case of the gaseous planets, have sunk to the middle of the body, forming a core. Terrestrial planets are therefore made up, in a rough sense, of concentric shells of materials with different compositions. The outermost shell is a crust, made mainly of material that has melted from the interior and risen buoyantly up to the surface. The mantle is made of silicate minerals, and the core is mainly of iron. The gas giant outer planets are similarly made of shells of material, though they are gaseous materials on the outside and rocky or icy in the interior. Planets with systematic shells like these are called differentiated planets. Their concentric spherical layers differ in terms of composition, heat, density, and even motion, and planets that are differentiated are more or less spherical. All the planets in the solar system seem to be thoroughly differentiated internally, with the possible exception of Pluto and Charon. What data there is for these two bodies indicates that they may not be fully differentiated.

Some bodies in the solar system, though, are not differentiated; the material they are made of is still in a more primitive state, and the body may not be spherical. Undifferentiated bodies in the asteroid belt have their metal component still mixed through their silicate portions; it has not separated and flowed into the interior to form a core.

Among asteroids, the sizes of bodies that differentiated vary widely. Iron meteorites, thought to be the differentiated cores of rocky bodies that have since been shattered, consist of crystals that grow to different sizes directly depending upon their cooling rate, which in turn depends upon the size of the body that is cooling. Crystal sizes in iron meteorites indicate parent bodies from six to 30 miles (10 to 50 km) or more in diameter. Vesta, an asteroid with a basaltic crust and a diameter of 326 miles (525 km), seems to be the largest surviving differentiated body in the asteroid belt. Though the asteroid Ceres, an unevenly-shaped asteroid approximately 577 by 596 miles (930 by 960 km), is much larger than Vesta, it seems from spectroscopic analyses to be largely undifferentiated. It is thought that the higher percentages of volatiles available at the distance of Ceres's orbit may have helped cool the asteroid faster and prevented the buildup of heat required for differentiation. It is also believed that Ceres and Vesta are

Radius v. Density, Showing Which Objects Become Round

1,250

1,000

g 600

Moon

^^Triton (Neptune's largest moon)

Moon

^^Triton (Neptune's largest moon)

I Pluto

ISedna

I Charon

Ceres

-Above this line objects are likely to become round simply through self-gravitation (their own mass is high enough that their gravity pulls them into a sphere.)

Pallas

Vesta

2,000

1,750

1,500

1,250

1,000

At a certain mass, solar system bodies should become round due to self-gravitation. This general rule works for some bodies, but others have remained irregular despite what should be a large enough mass.

among the last surviving "protoplanets," and that almost all asteroids of smaller size are the shattered remains of larger bodies.

Where does the heat for differentiation come from? The larger asteroids generated enough internal heat from radioactive decay to melt (at least partially) and differentiate

Accretion and Heating: Why Are Some Solar System Obj ects Round and Others Irregular? (continued)

(for more on radioactive decay, see the sidebar called "Elements and Isotopes" on page 18). Generally bodies larger than about 300 miles (500 km) in diameter are needed in order to be insulated enough to trap the heat from radioactive decay so that melting can occur. If the body is too small, it cools too fast and no differentiation can take place.

A source for heat to create differentiation, and perhaps the main source, is the heat of accretion. When smaller bodies, often called planetesimals, are colliding and sticking together, creating a single larger body (perhaps a planet), they are said to be accreting. Eventually the larger body may even have enough gravity itself to begin altering the paths of passing planetesimals and attracting them to it. In any case, the process of accretion adds tremendous heat to the body, by the transformation of the kinetic energy of the planetesimals into heat in the larger body. To understand kinetic energy, start with momentum, called p, and defined as the product of a body's mass m and its velocity v:

Sir Isaac Newton called momentum "quality of movement." The greater the mass of the object, the greater its momentum is, and likewise, the greater its velocity, the greater its momentum is. A change in momentum creates a force, such as a person feels when something bumps into her. The object that bumps into her experiences a change in momentum because it has suddenly slowed down, and she experiences it as a force. The reason she feels more force when someone tosses a full soda to her than when they toss an empty soda can to her is that the full can has a greater mass, and therefore momentum, than the empty can, and when it hits her it loses all its momentum, transferring to her a greater force.

How does this relate to heating by accretion? Those incoming planetesimals have momentum due to their mass and velocity, and when they crash into the larger body, their momentum is converted into energy, in this case, heat. The energy of the body, created by its mass and velocity, is called its kinetic energy. Kinetic energy is the total effect of changing momentum of a body, in this case, as its velocity slows down to zero. Kinetic energy is expressed in terms of mass m and velocity v:

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