The known size of the solar system doubled when Uranus was discovered in 1781.

\Uranus is tipped on its side and rotates in the retrograde direction. Although weakly heated by the Sun, Neptune's atmosphere is surprizingly active with a large storm system and high-speed winds that may be driven by internal heat.

The magnetic fields of Uranus and Neptune are tilted from their rotation axes and offset from their centers.
Miranda looks as if it were broken apart and reassembled again. Why is the Uranian ring system mostly empty space?

Triton may have formed in orbit around the Sun and was subsequently captured by Neptune, whose tidal forces kept Triton molten for much of its early history.
Volcanoes of ice once flooded lakes on Triton, and geysers may now be erupting on its surface.
Triton has a thin, nitrogen-rich atmosphere and polar caps of nitrogen and methane ice.

Pluto is a frigid, rocky world with an oversized moon and a wispy atmosphere.
There is a growing suspicion that thousands of small frozen worlds remain to be discovered in the outer darkness of the solar system.

(a) An unusual comet During the night of March 13, 1781, a professional musician and obscure amateur astronomer, William Herschel, discovered the planet Uranus. At first, he did not realize what he had found - he mistook it for a comet.
Herschel had come across an unusual faint "star" while surveying the heavens with his home-made, 15-centimeter (6-inch) reflector. When he increased the magnification of his telescope, the unusual object increased in size, and Herschel had the impression of seeing its body, while the neighboring stars still appeared as undefined points of light.
Another clue to the nature of the object was provided during the next few nights when he found that it was slowly moving across the background of distant stars. The object was clearly not a star; it belonged to our solar system, and the most natural explanation was a new comet. Herschel presented his discovery to England's Royal Society in a paper with the unexciting title "Account of a Comet".

(b) The first new world, Uranus
The object was soon lost in sunlight, but it was picked up a few months later. Its orbit was nearly circular, at twice the distance of Saturn, and this proved the object was no ordinary comet. It was quickly recognized as a new planet, and Herschel became world famous - almost overnight - as the first man in recorded history to have discovered a new planet. After some controversy, the new planet was named Uranus, after the Greek god of the heavens. (See Focus 10A, Naming a new world.) As we move outward from planet to planet, we also move upward among the generations of the gods: Mars' father is Jupiter, and Jupiter's is Saturn, while Saturn's is Uranus. Saturn's mother and subsequent mate was Gaea (Earth) who was born of Chaos.
In retrospect, orbit calculations proved that Uranus had been detected and mistaken for a star, on no fewer than 22 occasions during the century that preceded the realization that it was a planet. It is actually recorded as a star on several charts.
Uranus orbits the Sun at 19.2 times the Earth's distance, or twice Saturn's distance from the Sun. As predicted by Kepler's law, Uranus requires 84 years to complete a round trip. Thus, Uranus has only completed two circuits around the Sun since it was discovered. The day on Uranus is only 17.24 Earth-hours, so the Uranian year is more than 45000 Uranian days long.

This portrait shows Sir William Herschel thirteen years after his discovery of the first new planet in recorded history. He is holding a drawing with a Latin designation of the new planet as the "Georgian Planet," a name proposed by Herschel in honor of King George III, England's reigning monarch and a patron of the sciences. Eventually, the classicists prevailed, and the planet was named for Uranus, first ruler of Olympus, father of Titans and the grandfather of Jupiter.
One consequence of this naming was that the name of a newly-discovered, heavy element was changed to uranium in honor of the discovery of a new world. A product of radioactive uranium is plutonium, which powers spacecraft traveling in remote regions where there isn't enough sunlight to operate solar cells.

(c) The discovery of Neptune
Click here for an image of Neptune

John Couch Adams' speculations about a remote planet began in 1841 when he was 22 years old. In 1845, Adams presented his results to two prominent astronomers, James Challis and George Biddell Airy, England's Astronomer Royal. Although he reported the location of the unknown world to both Challis and Airy, neither of these astronomers felt compelled to look for it. In November 1845, the young French astronomer, Urbain Jean Joseph Leverrier, published his own investigation of Uranus' motion, showing that an unknown planet was influencing Uranus (Fig. 10.1). When his solution for the location of the planet reached England, Airy compared Leverrier's work with Adams' solution, which Airy had set aside eight months earlier. The two solutions were almost identical!

On August 31, 1846, Leverrier presented a revised location to the French Academy of Sciences. Ironically, Leverrier was unable to convince his French colleagues to search for the suspected planet. He then wrote to Johann Galle at the Berlin Observatory, urging him to look for it. Within a night of receiving Leverrier's letter on September 23, 1846, Galle and his student Heinrich D'Arrest located the planet about one degree from Adams' and Leverrier's predicted locations. It was subsequently named Neptune after the Roman god of the sea. In hindsight, the name is appropriate, for current models suggest that a deep sea of liquid makes up the bulk of the planet's interior.

Not only had the discovery of Neptune increased the radius of the solar system by a factor of 1.6, it was also acclaimed as the ultimate triumph of Newtonian science, having resulted from mathematical calculations based on Newton's theory of gravitation. If proof were needed, this achievement certified the validity of Newton's theory.

Neptune requires 165 years to revolve once around the Sun. It has not made a full orbit since it was discovered in 1846. Neptune's mass is 17 times the mass of the Earth; its mean density is about 1.7 times that of water. Both Neptune and Uranus have radii that are about four times that of the Earth. In fact, the mass, radius and mean density of Uranus and Neptune are so similar that they are often called planetary twins (Table 10.1).

Table 10.1. Some comparisons of Uranus and Neptune

(d) Discovering Pluto
Click here for an image of Pluto

(a) Uranus is tipped on its side
Uranus is a sideways world with a spin axis that lies almost in the plane of its orbit. In fact, the planet's equatorial plane is inclined 97.9 degrees from its orbital plane so it spins backwards compared to most of the other planets (Fig. 10.3). Because the planet rotates on its side, with the south pole now facing the Sun and Earth, its moons form a bull's eye pattern revolving about the planet like Ferris wheels. One speculation is that Uranus was knocked on its side during a collision with another planetary object early in the planet's history.
Because the rotational poles of Uranus lie near its orbital plane, the planet also has very strange seasons. First one pole faces the Sun and then the other during its long 84-year orbit about the Sun. During the northern summer, the north polar region receives more heat than the equatorial regions, while the south pole remains in darkness. Then the north pole enters winter, and the south pole is bathed in sunlight for the next 42 years.

(b) Uranus' atmosphere and clouds At the low temperatures prevailing near the top of Uranus' atmosphere, methane gas freezes and forms a cloud deck; haze particles are also formed here due to the action of ultraviolet sunlight on methane. The haze and methane clouds give the planet a bland appearance (Fig. 10.4), and hide the lower atmosphere from view. Methane also selectively absorbs reddish sunlight to give the planet its pale greenish-blue color. Ammonia and water clouds form deeper in the atmosphere and are difficult to see. (On warmer Jupiter and Saturn the topmost light-colored clouds that we see are composed of ammonia ice crystals.) The methane clouds form where the atmospheric pressure is about the same as that at sea level on Earth (1 bar). At this level, the planet's equatorial radius is 25559 kilometers. Rapid internal rotation produces an equatorial buldge, so the polar radius is 586 kilometers shorter.
Above the clouds there is a warmer atmosphere consisting primarily of hydrogen molecules, with some atomic hydrogen and helium. Observations of ultraviolet emissions from hydrogen indicate that a dense hydrogen corona extends thousands of kilometers above the visible clouds. The planet is small enough, and the atmospheric gas warm enough, that hydrogen can escape the planet's gravitational pull and accordingly blow off the planet into space. In contrast, more-massive Jupiter and Saturn hold their hydrogen atmospheres close to the cloud tops, as does the more remote, colder planet Neptune.
When the Voyager 2 spacecraft sped past Uranus on January 24, 1986, several streaks were detected in its otherwise featureless cloud layer. Careful charting of the motion of these streaks showed that the equatorial winds blow parallel to the planet's equator, but in a direction opposite to that of the planet's rotation. (Variations in the strength of Uranus' radio emission indicate that the magnetic field rotates with a period of 17.24 hours. The magnetic field is probably connected to the body of the planet, revealing the motion of the deep interior.) So the equatorial clouds on Uranus tend to lag behind the rotation of the interior, as they do on Earth.
Uranus emits just as much heat as it receives from the Sun. If Uranus has an internal heat source, it has not been detected and is very much weaker than those of Jupiter and Saturn. The helium abundance in the outer atmosphere of Uranus is considerably greater than that in Saturn's outer envelope, indicating that helium droplets do not rain with Uranus as they do in Saturn. (This is consistent with the low internal heat on Uranus, as well as the probable absense of a liquid metallic hydrogen core in which the helium would accumulate - see Chapter 9.) Deep convection within Uranus' atmosphere probably limits access to any internal heat created at the time of its formation.
Uranus lacks a large internal heat source, and it is tipped on its side (by 97.9 degrees) so that its south pole now receives more sunlight than the equator. Yet, the temperatures at the poles and equator are essentially the same. Atmospheric circulation evidently distributes the polar heat planet-wide. But the winds blow parallel to the equator of Uranus! Although the Sun has to provide most of the energy that heats the outer atmosphere and drives its circulation, it does not determine the pattern of circulation. Instead, the weather pattern is determined chiefly by the effects of the planet's rotation. (Jupiter and Saturn have a zonal circulation that is symmetric about the axis of rotation, but they have large internal heat sources and stand upright.)

(c) Stormy weather on Neptune
The warmer upper regions of Neptune's atmosphere consist mainly of molecular and atomic hydrogen, with smaller amounts of helium, but the thin atmosphere does not extend out into an extensive hydrogen corona as it does on Uranus. A blue cloud deck of methane ice is seen through the exceptionally clear atmosphere. At the one bar level, the equatorial radius is 24764 kilometers, with an equatorial bulge of 424 kilometers, which are quite similar to those of Uranus.
Like Jupiter and Saturn, Neptune glows in the infrared with its own internal heat. The cloud temperature on Neptune is 59.3 degrees Kelvin, which is about the same as Uranus (58.2); but Neptune is 50 percent farther from the Sun and the temperature that would result from sunlight alone is 46 degrees Kelvin. This difference implies that Neptune radiates 2.7 times as much heat as it absorbs from the Sun, and its outer atmosphere must therefore receive heat from the interior. As on Uranus, there is no helium depletion in Neptune's atmosphere, so its internal heat is presumably left over from the days of the planet's formation. Uranus, on the other hand shows no signs of internal heating, and this may explain why its atmosphere is relatively benign and inactive, in contrast with the dynamic stormy atmospheres found on the other giant planets.
The first close-up look at the stormy patterns of Neptune's dimly lit atmosphere were provided on August 25 1989 by the Voyager 2 spacecraft. It revealed a turbulent world, with giant storms, high-altitude clouds, and violent winds.
The largest storm system seen on Neptune is as broad as the Earth (Fig. 10.5). It is called the Great Dark Spot because it resembles the Great Red Spot of Jupiter. Both storms are found in the planetary tropics - at about one-quarter of the way from the equator to the pole, both rotate counterclockwise (high pressure anticyclones), and both are about the same size relative to their planet. The main difference is that Jupiter's spot lies above the clouds while Neptune's seems to form a deep well in the atmosphere. The Great Dark Spot is also more variable in size and shape than its Jovian counterpart; Neptune's vortex is soft, not rigid, and stretches in and out.
White, fleecy cirrus-like clouds cast shadows on the blue cloud deck below, indicating that they are high-altitude (50 to 100 kilometers) condensation clouds. Some of these bright clouds form and dissipate above the Great Dark Spot (see Fig. 10.5), in much the same way that clouds form over terrestrial mountains where local winds force moist air to rise to cooler altitudes. However, the cirrus clouds on Neptune consist of frozen methane, while those in Earth's atmosphere are water ice crystals.
Although weakly heated by the Sun, Neptune is the windiest planet in the solar system. Its Great Dark Spot is blown westward at more than 325 meters per second relative to Neptune's interior (the planet's periodic radio emission implies an internal rotation period of 16.11 hours), and smaller clouds with twice this speed overtake the larger spot, moving close to the speed of sound (about 560 meters per second). Like Uranus, the strong equatorial winds on Neptune blow opposite the direction of rotation, which means the storm clouds lag behind the planet's interior. (In contrast, Venus, Jupiter and Saturn all have equatorial clouds that move around the planet at a faster speed than the interior rotates). And like Uranus, the polar and equatorial temperatures on Neptune are nearly equal, and zonal winds blow parallel to the planet's equator, in spite of the different internal energy sources and orientation with respect to the Sun. Rotation therefore appears to play the dominant role in atmospheric circulation on both planets.

(a) Inside Uranus and Neptune
The planets Uranus and Neptune are outwardly very similar (see Table 10.1). They have comparable masses (14.53 and 17.14 times that of the Earth, respectively); they differ in radius by only 4 percent; and their rotation periods are also similar (respectively 17.24 and 16.11 hours). So one would expect their interiors to be similar too.
Although the outer atmospheres of Uranus and Neptune are primarily hydrogen and helium, they contain a larger proportion of heavier material inside. The interiors of Uranus and Neptune probably contain no solid rocky core, but instead a hot, global sea of water and melted rock and ice. The "ice" is a mixture of water, methane and ammonia; substances that would be frozen at the cloud tops, but are liquified by high temperatures and pressures inside. Unlike Jupiter and Saturn, there is no liquid metallic hydrogen inside Uranus and Neptune, so ionized water is required to generate their magnetic fields. If this fluid is heated from below, it will rise and create electrical currents, because ionized water is electrically conducting. The currents would generate a magnetic field.

(b) Tilted offset magnetic fields
Uranus is tipped on its side, and its magnetic pole is close to the equator, having a tilt of 58.6 degrees from the rotational pole. And although Neptune stands upright, its magnetic field is similarly askew, with a tilt of 46.8 degrees between the rotational and magnetic axes (Fig. 10.6). This would make compass navigation a rather awkward process on these two planets, and it also provides a mystery. Theoreticians expected a closer alignment between rotation and magnetism. (On the Earth the disparity is only 11.7 degrees, and on Jupiter 9.6 degrees. On Saturn there is no detectable displacement between the rotational and magnetic axes.)

Every other planetary magnetic field is dominated by a dipole component, the equivalent of placing a small and powerful bar magnet at the center of the planet. But on Uranus and Neptune the dipolar magnet has to be displaced toward the surface, by about one-third the radius of Uranus and more than half Neptune's radius (Fig. 10.6). The offset dipole results in a variable magnetic field strength at the cloud tops, ranging from more than 1 Gauss in one hemisphere to less than a tenth of this value in the other.

The large offsets of Uranus' and Neptune's magnetic dipole suggest that the fields are generated at an intermediate depth. Convection and rotation probably cause currents of ionized (electrically-conductive) water within a spherical shell far removed from the planet's center, rather than deep within a core as is believed to occur on Earth.

(c) Magnetospheres of Uranus and Neptune
Uranus and Neptune have fully developed magnetospheres that are well-represented by the offset, tilted dipoles. Interaction with the solar wind results in a bow shock on the sunward side and a stretched-out magnetotail downwind.
Moons act as efficient collectors of radiation belt particles and sweep wide swaths out of the magnetospheres. Because of the large dipole tilt, the satellites move across a broad range of magnetic latitudes, effectively eradicating the trapped energetic particles. As a result the inner magnetospheres of Uranus and Neptune are relatively empty, in contrast with the case at Jupiter and Saturn. And because Neptune's atmosphere is too cold to have an extended hydrogen corona to supply its magnetosphere, Neptune's magnetosphere is the emptiest yet seen among the planets.
The tilted, rotating magnetic field complicates Neptune's magnetosphere, and its interaction with the solar wind. Once every 16 hours, the southern magnetic pole points directly into the solar wind; half a rotation later the magnetic equator faces the incoming solar wind. The result is a wagging magnetotail whose shape changes as the planet rotates.

(a) Regular satellites of Uranus
Uranus' five large satellites, discovered telescopically from Earth, form a regular satellite system with circular orbits in the planet's equatorial plane, sharing Uranus' tilt with respect to the ecliptic and moving in the direction the planet rotates. They have radii between 200 and 800 kilometers (Table 10.2), so they are similar in size to Saturn's icy satellites. However, the mean density of Uranus' large moons is greater than that of Saturn's icy satellites, suggesting that the satellites of Uranus contain more rocky material.

Radioactive elements in the rocks could have heated and melted Uranus' satellites early in their history, leading to internal differentiation and volcanic flows of water ice on their surface. In fact, reflectance spectroscopy indicates the presence of water ice on all of the five largest satellites.

Table 10.2. Uranus' large moons

The equatorial radius of Uranus, RU, is 25,559 kilometers. The large moons are named for characters in literature: Oberon and Titania are named for the king and queen of the fairies in Shakespeare's A Midsummer Night's Dream. Inside their orbits is Umbriel, named after a "dusky, melancholy sprite" in Alexander Pope's Rape of a Lock. Closer to the planet is Ariel, described by Shakespeare as "an airy spirit" in The Tempest. Closer yet is a moon called Miranda, for Prospero's daughter in The Tempest. It was she who proclaimed,

O, Wonder! How many goodly creatures are there here! How beautious mankind is! O, brave new world, That has such people in't . The Tempest, V, i, line 183

Small satellites were found in abundance by Voyager 2, bringing the grand total of moons and moonlets to 15 (Fig. 10.7). Nine of the newly discovered satellites have circular orbits between the outer edge of the ring system and Miranda, the innermost of Uranus' major satellite, and the tenth is within the ring system. The small moons have radii between 20 and 85 kilometers, and they are all dark. (In contrast, Saturn's small satellites are bright.) But it was the closeup pictures of the largest moons that really captivated astronomers.

(b) Satellite sculpture
Because they are small, the moons of Uranus were expected to be heavily cratered iceballs, devoid of any signs of internal activity. But Voyager 2 surprised nearly everyone. The five previously known moons have, with the exception of Umbriel, surfaces that have been warped and twisted by internal upheavals. Oberon and Umbriel contain large impact craters that probably date back to an intense bombardment about 4 billion years ago. But Oberon also has a mountain several kilometers high, and the dark, patchy floors of its craters suggest that dirty water has seeped through cracks in the crust.
Titania's bright icy surface reveals few large impact craters, suggesting a younger surface. Its surface is cut by rift valleys and linear faults, up to 1600 kilometers long, that may have been formed when subsurface water froze and expanded, shattering the overlying crust (Fig. 10.8).
Ariel has the brightest and apparently youngest surface in the Uranian satellite system, suggesting that it has been resurfaced by icy extrusions. Its surface is criss-crossed with huge canyon systems, perhaps as a result of repeated expansion and contraction (Fig. 10.9). The canyon floors bulge at the center, as if ice welled up through the long cracks. The ice-filled valleys resemble the mid-ocean rifts on Earth (see Chapter 5). But, unlike terrestrial volcanoes that explosively emit liquid lava, Ariel's volcanoes oozed a solid icy mixture that crept outward like a glacier.
Miranda, the smallest and innermost of Uranus' major satellites, has the most amazing surface of all (Fig. 10.10). Icy volcanoes also seem to have been active on its surface, which is marked with a bizarre variety of grooves, valleys, mountains and cliffs, as though it were the work of a combination of faultings and eruptions. Some astronomers have argued that Miranda was once broken apart by a collision but that it managed to pull itself together again into a single body. It may even have been shattered and reassembled many times.
Another possibility is that Miranda froze while it was still in an embryonic stage of differentiation, when rock masses were sinking to the interior and lighter chunks of ice were rising to the surface. Heat generated by the decay of radioactive elements may have once risen up and expanded under the cold ice at the surface, stretching it into long parallel grooves. In any case, Miranda has all the earmarks of a "brave new world."

(c) Rings around Uranus
Astronomers have had a history of happy accidents concerning Uranus, starting with Herschel's initial discovery. Another lucky incident occurred on March 10, 1977, when the planet was scheduled to pass in front of a faint star. Because of uncertainties in the predicted time of the star's disappearance, one telescope was set into action about 45 minutes early. Soon after the recording began, the starlight abruptly dimmed but then it almost immediately returned to normal, producing a brief dip in the recorded signal. At first, the dip was attributed to a wisp of cloud on Earth or to an unexpected change in the telescope's orientation. But more dips occurred before the star disappeared behind the planet, and the pattern was repeated after the star had re-emerged. This symmetry indicated that Uranus was surrounded by narrow rings, and eventually the count rose to nine rings! Voyager 2 confirmed them all, and added at least two.
The Uranian ring system has a skeletal appearance compared with the Saturnian one. The concentric, web-like rings are all very narrow (typically less than 10 kilometers), and widely spaced from each other. There is so little material in the rings around Uranus that their total mass is probably less than that of the low-density gaps in Saturn's rings.
The rings are extremely difficult to see from Earth, because the wide spaces between the rings appear dark and the narrow rings reflect little sunlight. The ring particles are very dark and colorless - quite unlike the bright particles found in Saturn's rings. In fact, the particles of Uranus' rings reflect only about 2 percent of the sunlight falling on them, making them as dark as charcoal, or the blackest carbon-enriched meteorites and asteroids known. The dark rings might contain primordial organic material rich in carbon, or they could consist of frozen methane that has been blackened by energetic particles trapped in the planet's radiation belts. Saturn's bright rings apparently consist of purer water ice with little methane or dark carbonaceous material. Images taken from behind the rings, looking back toward the Sun, showed the presence of numerous dust rings in addition to those visible in reflected sunlight (Fig. 10.12). But the material in the dusty rings is extremely diffuse, and the main rings are comparatively free of dust. (Less than 0.1 percent of the epsilon ring's opacity comes from dust particles, for example, and the dust that exists between the main rings is even more transparent.) Particles in the main rings of Uranus are actually dark and large, and lack dust.
Small particles created by collisions are rapidly swept out of Uranus' ring system and fall into the planet. Because the planet's upper atmosphere, or hydrogen corona, extends outwards from the planet into the rings, gaseous molecules collide with the small dust-sized particles and drag them down. The smaller particles therefore spiral into the lower atmosphere where they eventually burn up, but this gas drag has little effect on large particles. As a result, Uranus' rings consist mainly of meter-sized and larger particles. The slender rings are not circular and symmetric, but elongated and thickened in places. Some of the rings vary by more than a factor of two in width, and part of one ring disappears in places. So, the material in the rings is not uniformly distributed, and nearby moons are required to shape the rings and define their irregular structure.
Unconstrained rings will broaden due to particle collisions, but satellites can confine the ring particles to a narrow track. One satellite on each side of a ring can use gravity and speed differences to confine ring particles, just as two shepherd satellites control the F-ring of Saturn (see Chapter 9). Satellites can also raise waves within the rings. Resonances that amplify these waves can create sharp edges to rings. The outer edge of Saturn's B ring is believed to be confined by such a resonance with the planet's moon Mimas. This process is much more likely to take place in Uranus' ring system than in Saturn's because the Uranian rings are much closer to their planet.
Two of the small, newly-discovered moons, Cordelia and Ophelia, flank Uranus' outermost epsilon ring (Fig. 10.13), controlling its shape. The inside satellite, Cordelia, resonates at the rings inner edge, and the outside one, Ophelia, resonates with the outer edge. Cordelia also shapes the outer edge of the delta ring, while Ophelia resonates with the inner edge of the gamma ring. The moons confining the other rings may have been too dark or too tiny for Voyager's cameras to record, but the observations of ring gaps provide indirect evidence for unseen satellites that mold and shape the structure of the rings. These invisible moons plow through the rings, creating gaps in the surrounding ring material.
After transmitting a fascinating sequence of images and radio signals, Voyager 2 left Uranus and headed for its rendezvous with Neptune.

With a temperature of 38 degrees Kelvin (-391 degrees Fahrenheit), Triton has the coldest measured surface of any natural body in the solar system! It is so cold because it is so far away from the Sun, therefore receiving little sunlight, and also because Triton reflects more of the incident sunlight than most satellites (only Enceladus and Europa are comparable). As a result, the total amount of sunlight absorbed by Triton's surface is less than that of any other planet or satellite.
Even in the middle of southern summer, a bright, white ice cap extends from the south pole three-quarters of the way to Triton's equator (Fig. 10.14). Northward of the equator, somewhat darker plains contain pits criss-crossed with ridges, like the skin of a cantaloupe, and much smoother features, suggesting multiple epochs of surface flooding. All of the surface features are apparently overlain by a frosty coating of nitrogen and methane ices and their derivatives. The brilliant ice has a salmon-pink tint with peach hues, possibly due to organic compounds derived from methane by the bombardment of energetic particles from the solar wind and Neptune's radiation belts.
Voyager's cameras showed that Triton's southern polar cap is in the process of dissipating. Nitrogen ice was sublimating, or changing directly from the solid to vapor form, and supplying the atmosphere with gas. The vaporized nitrogen gas probably was carried by atmospheric winds to the dark hemisphere, where it condensed to form a winter deposit of frost. Thus, the mass of the atmosphere is expected to vary with the long seasons (41 years each) as the polar caps wax and wane. (On Mars, carbon dioxide is similarly recycled between the polar caps and atmosphere during its seasons - see Chapter 6.)
Triton's atmosphere consists mainly of nitrogen, with a trace of methane, and it is exceedingly cold and thin. Over 99 percent of its tenuous atmosphere is nitrogen, the same gas that makes up most of the atmospheres of Earth and Saturn's moon Titan. In the height of Triton's 41-year-long summer, the atmosphere's surface pressure was 16 microbars, or only 1/62500 times that of the Earth at sea level, and the surface temperature was only 38 degrees above absolute zero. In spite of the low pressure, Triton's thin atmosphere can support clouds and haze, but the surface remains visible. In contrast, Titan has a denser nitrogen-rich atmosphere with a surface pressure of 1.5 times that at Earth, and its surface is invisible.

Although nitrogen and methane ice are apparently the dominant constituents of Triton's surface, water ice is needed to support and preserve the observed topography, including cliffs and ridges that exceed one kilometer in height. At the temperature of Triton, water ice is as strong as steel, and behaves like hard rock on Earth; but the methane and nitrogen ice do not have sufficient strength to support the elevated features, which would deform and collapse under their own weight. Thus, a rigid crust of water ice is apparently overlain by thin, brilliant veneers of nitrogen and methane ice.

Triton maintains a neutral hydrogen torus comparable to that of the Titan torus around Saturn (see Chapter 9). The doughnut-shaped torus is fed by hydrogen liberated from Triton's atmosphere by the action of energetic sunlight on methane.
Triton also has a substantial ionosphere with a peak density of about 50 thousand electrons per cubic centimeter at 350 kilometers altitude. In contrast, Titan with a similar nitrogen-rich atmosphere and closer to the Sun has no detectable ionosphere. So energetic sunlight cannot be the dominant factor in producing Triton's ionosphere, as it is on Earth. Apparently interactions between Triton's atmosphere and charged particles in Neptune's magnetosphere generate and maintain the ionosphere of Triton.

Gravitational dance
Pluto is a small frozen world at the outer fringe of the known planetary system. It has a diameter of about 2300 kilometers, which makes it smaller than our Moon (3476 kilometers) and the smallest planet in the solar system. It orbits the Sun on a tilted path (inclined by 17 degrees) that rises above and drops below the plane of the solar system and is also more elliptical than the orbit of any other planet. Pluto's highly-elongated orbit carries it between 29.7 and 49.3 A.U. from the Sun, resulting in a large temperature variation during a Plutonian year. At 50 degrees Kelvin, Pluto is slightly warmer than Triton, not only because it is now closer to the Sun, but also because its surface absorbs more sunlight than Triton's bright reflective surface.

Pluto's elliptical orbit carries the tiny world inside Neptune's orbit, where it currently resides. This cross-over of the orbits of Pluto and Neptune led to speculations that Pluto might be an escaped satellite of Neptune. This might account for the odd orbits of Neptune's moons. But the idea that Pluto might be an escaped satellite of Neptune has become unlikely with the discovery that Pluto has a satellite. Pluto and its moon seem too small and loosely bound to each other for such a scenario to be likely.

Pluto's equatorial plane is inclined 122 degrees from its orbital plane, so its north pole lies just below the plane of its orbit. Like Uranus, the planet Pluto spins on its side. Each pole alternates every 124 years between pointing toward the Sun and away from the Sun. (Venus and Uranus are similarly tipped, with respective inclinations of 177 and 98 degrees; all three planets therefore spin backwards, rotating in the opposite, retrograde direction to that of their orbital motion.) Charon's orbit around Pluto is also inclined at 122 degrees to the plane of Pluto's orbit around the Sun, so Charon revolves in Pluto's equatorial plane).

Pluto and Charon are locked in a gravitational dance so Charon keeps the same face to its partner, as does our Moon. But unlike the Earth, Pluto also keeps the same face toward Charon.

Astronomers were offered a rare opportunity to observe Pluto and Charon's pirouette edge-on between 1985 and 1991, an event that only occurs twice in a Plutonian year - that is, every 124 Earth-years. When Charon's orbit narrows to a straight line as seen from Earth, we see a series of mutual eclipses as Charon and Pluto alternately pass directly in front of one another. Timing the starts and ends of such occultations and measuring the amount by which light decreases permitted an accurate mapping of the outlines of this intriguing pair (see Table 10.3). As it turned out, Charon is about half the size of Pluto, making it the largest known satellite relative to its planet in the solar system.

Table 10.3. Parameters of the Pluto Charon-System

         

Semimajor axis 19130 kilometers        

Orbital period 6.38718 days        

Orbit tilt 122 degrees        

Mass of system 1.36 x 10^25 grams (0.0023 Earth masses)        

Mass of Pluto 1.25 x 10^25 grams (0.0021 Earth masses)        

Pluto's radius 1123 kilometers        

Charon's radius 560 kilometers        

Mean Density of System 1.99 g/cm^3        

Pluto's Density 1.84 to 2.14g/cm^3        

Charon's Density 1 to 3 g/cm^3                    

(b) Worlds of ice and rock

The series of Pluto-Charon eclipses also provided information about the composition of the two bodies. The spectrum of Pluto's reflected sunlight was obtained when Charon was hidden behind Pluto. It revealed a feature due to absorption by methane, either in the atmosphere or trapped on the surface as snow. The strength of the features vary as Pluto rotates, so some of the absorption is probably due to surface ice.

Like Triton, much of Pluto is reddish in color, perhaps because energetic particles have transformed some of the methane ice into organic substances. In contrast, Charon is grayish and its spectrum (which was obtained by subtracting that of Pluto from the combined light of the two bodies) reveals the spectral signature of water ice. Charon's smaller size and weaker gravity probably allowed methane to escape, exposing a layer of water ice.

Pluto's apparent brightness changes periodically as the planet rotates, suggesting that there are lighter and darker patches on its surface. The planet also dimmed gradually when its slow orbital motion about the Sun turned its pole out of view and exposed the equatorial regions to sunlight. Thus, Pluto probably has bright polar caps and a darker equatorial region.

Although most of Pluto's surface is bright and icy, the planet is not just a ball of ice. The occultation data indicate that Pluto's density (determined from its mass and size) lies between 1.84 and 2.14 g/cm^3. This means that Pluto is an extremely rocky planet containing as much as 80 percent rock by mass. In many ways Pluto is a twin of Triton. They have almost exactly the same size and density, and they also both have methane ice on their surface. This suggests that they both formed as independent bodies in the outer precincts of the solar system. Pluto's evolution may have been different, for it was not subjected to tidal heating as Triton probably was; but both objects most likely have an icy mantle covering a rocky core, and the surface ice on both of them has sublimated to form a tenuous atmosphere.

Table 10.4. Triton and Pluto Compared

Triton Pluto

Present distance from Sun 30 A.U. 29 A.U.

Rotation period (in Earth days) 5.9 days 6.4 days

Rotation direction retrograde retrograde

Diameter (in kilometers) 2705 km 2330 km

Density 2.07 g/cm^3 1.84 to 2.14 g/cm^3

Albedo (percent) 70 40

Temperature (degrees Kelvin) 38 K 50 K

Atmospheric pressure (bars) 0.00001 0.00001

Surface composition nitrogen and methane methane ice ice + ?

Atmosphere composition nitrogen and methane methane gas gas + ?

(c) Pluto's thin atmosphere Definite evidence for an atmosphere on Pluto was provided during a stellar occultation. If the planet passed behind an airless planet, it would blink out abruptly, and then quickly switch on when it reappeared on the other side. Instead, the star dimmed gradually when it passed behind Pluto, indicating that greater amounts of sunlight were being absorbed as it passed through progressively deeper layers of the atmosphere (see Fig. 10.19). When the star emerged, it slowly brightened, so an atmosphere had to envelop Pluto. The bulk of the atmosphere might consist of methane gas, but a predominantly nitrogen atmosphere, like that of Titan or Triton, cannot be ruled out by the observations.

Pluto's atmosphere is very thin, and it might not always be present. The atmospheric pressure of Pluto is about the same as that on Triton, or about 1/100,000 that at sea level on Earth. When the planet is closest to the Sun, as it is now, solar radiation heats and vaporizes the surface ice to release gas and form an atmosphere. But when the planet moves farther from the Sun in the remoter parts of its eccentric orbit, much if not all of the atmosphere is expected to freeze out and collapse to the surface, covering Pluto with a fresh layer of frost or snow.

(d) Are there unknown planets beyond Pluto?

It seems likely that all the sizeable planets inside the orbit of Pluto have been discovered, and the possibility of planets beyond Pluto is an open question. Pluto is now known to be too small to have caused the unexplained waverings in the motions of Uranus and Neptune. In fact, the early-recognized difference between the observed and predicted properties of Pluto had led to a continuation of the Lowell Observatory search for a trans-Neptunian planet, but after examining 90 million star images, Clyde Tombaugh felt he could rule out the presence of a Neptune-sized body in the ecliptic within a range of 270 times the Earth's distance from the Sun. Scientists at the United States Naval Observatory are nevertheless searching for a presently unknown planet in the extreme southern sky out of the ecliptic and not visible from mid-northern latitudes where the Lowell Observatory is located.

Evidence for past collisions or capture suggest that the outer solar system once teemed with icy worlds the size of Pluto and Triton. Uranus may have been knocked on its side during an off-center collision with such an object, and Triton may be a relic of the once-formidable population. It may have been a free-ranging object that passed close to Neptune, and was snared by its gravity, thereby ending up in its odd backward orbit about Neptune. The oversized moon Charon may have joined Pluto during a past collision or very close encounter; this might also account for Pluto's large tilt and the substantial eccentricity and inclination of its orbit. Each of these events is highly improbable and their presumed occurence implies the existence of hundreds of thousands of similar-sized bodies in their region of formation.

Although many of these objects would have been swept up by the giant outer planets, a large number of them were probably ejected into outer space in slingshot fashion. Several astronomers have therefore proposed that there may be a swarm of small planets in the outer reaches of the solar system or interstellar space. Such planets would be in the realm of the comets, to which we now turn.