11. Uranus and Neptune

Fundamentals

Planetary twins

Saturn was the most distant planet known to the ancients. Uranus and Neptune are both so far away, and so faintly illuminated by the Sun, that telescopes were required to discover them. Uranus is about 19 times as far away from the Sun as the Earth is, and Neptune is about 30 times as distant. As a result, it takes 84.0 Earth years for Uranus to complete one revolution about the Sun and nearly twice that for Neptune. With an orbital period of 164.8 years, Neptune has not yet completed its first full orbit since discovery.

These two distant planets remain little more than dim, fuzzy spots of light in even the most powerful telescope. One can still infer enough about Uranus and Neptune from telescopic observations to know that they have similar physical properties. The size, mass, composition and rotation of Uranus and Neptune are in fact so similar that they are often called planetary twins. These parameters lie between those of the Earth and the giants Jupiter and Saturn.

Some comparisons of Uranus and Neptunea

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UranusNeptune
Mass (Earth-mass)14.5317.14
Radius (Earth-radius)3.983.91
Density (kilograms per cubic meter)12401670
Rotation period (hours)17.2416.11
Orbital period (Earth-years)84165
Mean distance from Sun (AU)19.1930.06
Atmosphere83 percent hydrogen
15 percent helium
79 percent hydrogen
18 percent helium
Energy balanceless than 1.42.7Ī 0.3
Effective temperature59.3 degrees kelvin59.3 degrees kelvin
Temperature at one-bar level76 degrees kelvin73 degrees kelvin
Central temperature5,000 degrees kelvin5,000 degrees kelvin
Magnetic dipole moment.50 DE25 DE
Equatorial magnetic field strength0.23 x 10-4 tesla0.14 x 10-4 tesla
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a The Earthís mass is 5.9743 x 1024 kilograms, the Earthís equatorial radius is 6.378 x 106 meters, the astronomical unit, denoted AU, is the mean distance between the Earth and the Sun with a value of 1.496 x 1011 meters. The energy balance is the ratio of total radiated energy to the total energy absorbed from sunlight, the effective temperature is the temperature of a black body that would radiate the same amount of energy per unit area, and a pressure of one bar is equal to the atmospheric pressure at sea level on Earth.

Uranus is tipped on its side and has no strong source of internal heat

Blue-green Uranus lies sideways, with its poles where its equator should be. This knowledge comes not from watching the small, featureless ball rotate, but instead from observing the orbits of its major moons. The orbits are all circular, and they lie in one plane, which is turned at right angles to the plane of Uranusí orbital motion about the Sun. As a result, the moons form a bulls eye pattern, revolving around Uranus like a Ferris wheel. Since these satellites should be orbiting within the plane of Uranusí equator, the entire planet has to be tipped on its side. One speculation is that Uranus was knocked sideways during a massive collision, perhaps when the planet was still forming.

When Voyager 2 arrived at Uranus, on 24 January 1986, the spacecraftís infrared detectors found that the planet is radiating about as much energy as it receives from the Sun. This means that Uranus lacks a strong internal heat source, in contrast to Jupiter and Saturn that produce heat in their centers. These two giants each radiate away about twice as much energy as they receive from the Sun. But like Jupiter and Saturn, and unlike Uranus, the planet Neptune glows in the infrared with its own internal heat discovered when the hardy spacecraft flew past Neptune in August 1989, twelve years after launch.

Storm clouds on the outer giants

Mild weather on Uranus

With no appreciable heat rising from the interior to drive the weather system, Uranus presents a dull and placid face to the world. In addition, a cold and hazy atmosphere obscures our view. Even at close range, looking at Uranus is something like gazing down into a bottomless ocean.

Fig. .. 

Yet, some zonal banding was extracted from the Voyager 2 images. The clouds are arranged in bands that circle the planetís rotation axis, running at constant latitudes parallel to the equator like the more vivid bands seen at Jupiter and Saturn. The features at different latitudes on Uranus move in the same east-west direction as the planet rotates, but at faster speeds. The difference is greatest at high latitudes, where the clouds circle the poles in 14 hours, and it gets progressively smaller toward the equator, closer to the internal rotation period of 17.24 hours.

Stormy weather on Neptune

Fig. .. 

The Voyager 2 flyby in 1989 forever changed our view of Neptuneís weather. Despite its great distance from the Sun, the dimly lit atmosphere of Neptune is one of the most turbulent in the solar system, with violent winds, large dark storms and high-altitude white clouds that come and go at different places and times.

The wind pattern on Neptune lacks Jupiterís multiple zonal winds that flow in opposite directions. Neptune has just one westward air current at low latitudes, like the Earthís trade winds, and one meandering eastward current at mid-latitudes in each hemisphere, resembling the Earthís jet streams. And like Uranus, the polar and equatorial temperatures on Neptune are nearly equal.

Fig. .. 

You wouldnít want to forecast the lively, variable and unpredictable weather on Neptune. When the Hubble Space Telescope took another look at the planet, in 1994 to 1996, the violent storms seen by the Voyager 2 cameras had vanished without a trace, and other storms had appeared.

Fig. ..  Fig. .. 

The largest dark storms on Neptune are probably high-pressure systems that come and go with atmospheric circulation. The most prominent one was the Great Dark Spot, a vast, circulating storm almost as large as Earth. 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 south pole, both rotate counterclockwise, in the direction of high-pressure anticyclones, and both are about the same size relative to their planet. As on Jupiter, some of the small dark spots on Neptune may be whirling in the opposite direction to the bigger one, perhaps indicating that they are little cyclones with descending material at their centers.

There are some important differences between the two Great Spots. The Jovian red spot has survived for centuries, while Neptuneís dark one disappeared from view within a few years of its sighting from Voyager 2. And Jupiterís whirling storm lies above the clouds while Neptune's seems to form a deep well in the atmosphere, providing a window-like opening to the deeper, darker clouds below.

White, fleecy cirrus-like clouds cast shadows on the blue cloud deck below, indicating that they are high-altitude condensation clouds that rise about 100 kilometers above the surrounding ones. They form as atmospheric gas flows up, over and around the storm center, without being consumed by it. When the rising methane gas cools, it forms white clouds, fashioned from crystals of frozen methane. Water in the Earthís atmosphere freezes in a similar way into ice crystals that form cirrus clouds. When strong upwelling carries the wispy white clouds to great heights in Neptuneís atmosphere; they are sheared out like anvils of terrestrial thunderstorms.

Although the global wind pattern on Neptune resembles the Earthís trade winds and jet streams, they cannot be energized in the same way. Solar heating of the atmosphere and oceans drives the terrestrial winds. At Neptuneís distance the Sun is 900 times dimmer and the winds should be correspondingly weaker if they are driven by the feeble sunlight. The fast winds on Neptune and the planetís complex stormy weather must instead be energized by heat generated in the planetís core. The internal heat warms Neptune from the inside out, producing convecting currents of rising and falling material, somewhat like a pot of boiling water on a stove. Uranus, on the other hand, shows no signs of substantial internal heating, and this may explain why its atmosphere is relatively benign and inactive.

Interiors and magnetic fields

Water worlds

The atmosphere above the cloud tops of Uranus and Neptune consists mainly of molecular and atomic hydrogen, warmed by the Sunís ultraviolet rays. The tenuous gas forms an extensive hydrogen corona around Uranus, but is held closer to the cloud tops above Neptune. The overwhelming abundance of hydrogen in the outer atmospheres of Uranus and Neptune resembles that in Jupiter, Saturn and the Sun.

Fig. .. 

Unlike Jupiter and Saturn, however, Uranus and Neptune cannot consist mostly of the lightest element hydrogen, or they would have a lower mean mass density then observed. For their size, Uranus and Neptune are too massive for hydrogen to be their main ingredient, and their bulk must instead be composed of heavier abundant elements. To put it another way, both planets are too small for their mass to be mainly composed of hydrogen and helium, and must consist mainly of heavier material.

Fig. .. 

Since Uranus and Neptune have similar mass, size, composition and rotation, their interiors are also expected to be alike. But they must be quite different from Jupiter and Saturn inside. The hydrogen in Uranus and Neptune is confined within a thin atmosphere and liquid molecular shell that do not extend to great depths and contribute only about 15 percent of the planetary mass. These two planets do not have enough hydrogen, or sufficient mass and internal pressure, to squeeze the hydrogen into a metallic state. So there is no internal shell of liquid metallic hydrogen inside Uranus and Neptune.

Most of their interior probably consists of a vast internal ocean of water, H2O, methane, CH4, and ammonia, NH3. Although customarily denoted as ices, since they would be frozen at the cloud tops of these planets, these substances are kept liquid by the high temperatures, up to 8,000 degrees kelvin, deep in the planetary interiors. These molecules will form from atoms of hydrogen, H, oxygen, O, carbon, C, and nitrogen, N, the most abundant heavy elements in the material from which the Sun and giant planets originated.

Uranus and Neptune are not unlike the cores of Jupiter and Saturn, which similarly contain 10 to 20 Earth masses of melted ice and molten rock. But Uranus and Neptune are almost all core, without the deep envelope of hydrogen and helium that make up most of the mass of Jupiter and Saturn. The differences between these four planets apparently derive primarily from the amounts of hydrogen and helium that they were able to attract and hold as they formed.

Tilted magnetic fields

Like the Earth, Jupiter and Saturn, both Uranus and Neptune have strong magnetic fields. But the resemblance ends there. Here on Earth our magnetic pole is very near our geographic pole, which is very useful for navigation with a compass. The magnetic and rotational axes of Jupiter and Saturn are also closely aligned. But they are way off kilter on both Uranus and Neptune.

It is almost certain that the same dynamo process as that responsible for Earthís magnetic field generates the magnetic fields of Uranus and Neptune. In this mechanism, swirling currents in a fluid conduct electricity, generating and sustaining a planetís magnetism. This happens in Earthís molten metallic core, and it occurs within the liquid metallic hydrogen inside Jupiter and Saturn. Unlike these two giants, there is no shell of liquid metallic hydrogen inside Uranus and Neptune, but electrical currents within their vast internal oceans might generate the magnetic fields. It is probable that the electrical conductivity within Uranus and Neptune is provided by water-rich material that has a conductivity that is about two orders of magnitude less than that of metals. It is also likely that this conductivity comes from protons, not electrons, within the ionized waters.

Rings of Uranus and Neptune

Narrow, widely spaced rings around Uranus

Astronomers have had a history of happy accidents concerning Uranus, starting with William Herschelís (1738-1822) serendipitous discovery of the planet in 1781. Another lucky incident occurred on 10 March 1977, when the planet was scheduled to pass in front of a faint star. By observing such a stellar occultation, astronomers hoped to determine properties of the planetís atmosphere, and to accurately establish its size from the duration of the starís disappearance behind it.

Fig. .. 

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 the star blinked on and off several times before and after the planet covered it. Moreover, each dip on one side of Uranus was matched by another one on the other side, at the same distance from the planet. The symmetrical, brief dips indicated that Uranus is surrounded by a family of narrow rings that blocked out the starís light but could not be seen directly from the Earth.

During the next few years, observations of more than 200 stellar occultations by Uranus revealed the details of nine narrow rings. In order of increasing distance from Uranus, the rings are named 6, 5, 4, a, &beta, &?,?, d, and e, following the differing notation of the discoverers. From the brief duration of the dips of blocked starlight, astronomers concluded that all but one of the individual rings could be no wider than 10 kilometers. The relatively long time between the dips indicated that the threadlike rings are separated by hundreds of kilometers of nearly empty space. These skeletal, web-like rings are unlike any seen before, all very narrow and widely spaced from each other.

Fig. .. 

When Voyager 2 arrived at Uranus in 1986, nearly a decade after the discovery of its narrow rings, instruments on the spacecraft confirmed all the known rings, and added at least two. They found the ? ring, a narrow strand between the d and &epsilon: rings, and another one interior to ring 6. The spacecraft also discovered at least 10 small moons that are located just outside the ring system.

The particles in the main narrow rings of Uranus are both dark and large. They range between a softball and an automobile in size, or between 0.1 and a few meters across. And they contain very few smaller particles in the millimeter to centimeter, or 0.001 to 0.01 meter, range, with surprisingly small amounts of micron-sized dust about 10-6 meters across.

Fig. .. 

Broad sheets of dust were nevertheless detected in the wide gaps between the rings. When Voyager 2 entered Uranusís shadow and looked back at the rings, about 100 very diffuse, nearly transparent bands of microscopic dust were seen with sunlight streaming past them. The dust is lit up when the Sun shines through the rings, in the same way that grime on a car's windshield becomes visible when struck by the lights of an oncoming car.

Fig. .. 

The irregular orientation and shapes of the Uranian rings are attributed to small moons that lie just outside them. The repeated gravitational tugs of two of them, Cordelia and Ophelia, pull the epsilon ring into its oval shape and restrain its edges. These tiny moons flank the ring, controlling its shape in much the same way that the shepherd satellites, Pandora and Prometheus, constrain Saturnís F ring. Nearby moons probably sharpen the edges of the other rings, keeping them from spreading out as the result of particle collisions, but many of the expected moons have not been found. They may have been too dark or too tiny for Voyagerís cameras to record.

Neptuneís sparse thin rings and arcs

After the discovery of the rings of Uranus by watching a distant star pass behind the planet, astronomers hoped to repeat the achievement by observing stellar occultations by Neptune, but the results were inconclusive. Sometimes the starlight would remain unchanged before and after the planet directly occulted the star. At other times the star would blink on and off, but always on just one side of the planet. Because the brief dimming of starlight was not symmetrical about the planet, and not all stellar occultations produced a blinking signal, the hypothetical rings became shortened, in the minds of the astronomers, to ring-arcs that only reached part way around the planet. Chance might then dictate which astronomers would detect the obscuration.

Fig. .. 

Voyager 2 clarified the problem. Neptune's ring-arcs turned out not to be isolated segments, but rather three thicker portions of one very thin ring. The ring is narrow and continuous, stretching all the way round the planet just like any well-behaved ring. Its material is generally spread so thinly that it does not noticeably dim a starís light. The ring is only dense enough to hide a star in three arc-like concentrations, subsequently named Libertť, Egalitť and Fraternitť after the French revolutionary slogan. It was these high-density clumps that had been detected from Earth, blocking starlight and giving the impression of disconnected arcs. The rest of the ring couldnít be seen from Earth because it is so transparent, and hence below the threshold of detectability.

Formation and evolution of the rings of Uranus and Neptune

It is now thought that all the planetary rings are younger than the age of the solar system, so they cannot be permanent features dating back to its origin. And the present rings are now viewed as a passing stage in an ongoing process of creation and loss.

The austere rings that now circle Uranus and Neptune may have had a violent and chaotic past, arising from catastrophic collisions of moons or when one larger satellite moved inward by tidal interaction with the planet until it was close enough to be ripped into pieces. The inner small moons and larger particles in the rings were then probably gradually broken up by collisions into smaller ones. And all the ring particles we see today will eventually be eroded away by meteoritic bombardment, ground into fine dust by particle collisions, or displaced by gravitational interaction with neighboring satellites.

Thus an entire ring system will eventually be turned into dust. And because all the dust is dragged into the planet's atmosphere or ejected from the system, the rings will inevitably decay and disappear over astronomical times.

The large moons of Uranus and Neptune

Five major moons of Uranus

Uranus possesses five major satellites discovered telescopically from Earth before the space age, and named Miranda, Ariel, Umbriel, Titania and Oberon. As a group, they are similar in size to the mid-sized satellites of Saturn. The two largest and outermost Uranian moons, Titania and Oberon, are roughly half the size of the Earthís Moon; the smallest and innermost, Miranda, is about one-seventh the lunar size. Infrared spectroscopy from Earth indicated that they all have water ice on their surfaces, but their icy surfaces are darker and less reflective than Saturnís moons.

Accurate masses for the largest moons of Uranus were obtained by observing their gravitational effects on the trajectory of the Voyager 2 spacecraft during its flyby on 24 January 1986. Combined with the size of the moons, these masses yield mean mass densities for the four largest ones of between 1400 and 1700 kilograms per cubic meter, higher than their Saturnian counterparts, between about 1100 to 1400 in the same units.

The high mass density implies that the large moons of Uranus are about half rock and half water ice. Thus, with the exception of Miranda, the major moons of Uranus are rocky on the inside, as well as dirty on the outside. Their dark surfaces and rocky interiors may be related to an ancient collision that might have knocked Uranus on its side before the satellites were fully formed.

Fig. ..  Fig. .. 

The landscape on Miranda is one of the most amazing yet observed in the solar system. It includes old cratered plains, bright younger terrain, and an eclectic mixture of ridges, grooves, mountains, valleys, fractures and faults.

Fig. .. 

The four largest moons of Uranus can be divided into two pairs, Ariel and Umbriel and Oberon and Titania. The members of each pair have similar mass and size, but very different surfaces. The reason why they have similar bulk properties and look so different may be related to the differences in the way that the moons produced internal heat and eventually froze inside, but the details remain a perplexing mystery.

Neptuneís Triton, a large moon with a retrograde orbit

Jupiter, Saturn and Uranus have a flock of satellites whose orbits mimic those of the planets around the Sun. Their larger moons revolve in regularly spaced; circular orbits in the same direction as the rotation of the planet and close to the planet's equatorial plane, presumably because they share the rotation of the nebular disk from which the planet and its satellites formed. The radii, orbital distances and other characteristics of these regular satellites also tend to differ in smooth progression.

Fig. ..  Fig. .. 

In sharp contrast to the other major outer planets, Neptune lacks a system of regular larger satellites. Its largest moon, Triton, is the only large satellite in the solar system to circle a planet in the retrograde direction, opposite to the planetís direction of rotation. This oddity is compounded by the high orbital inclination. The satelliteís orbital plane is titled at an enormous 157 degrees from the planetís equator. The tilted orbit gives rise to dramatic seasonal variations, for each pole of Triton in turn faces the Sun for nearly half of Neptune's 165-year orbit about the Sun, and the planet appears to move along the satelliteís horizon.

Nereid, the outermost moon of Neptune, adds to the mayhem, with the most elongated orbit of any planetary satellite, seven times as distant from the planet at its farthest compared with its closest approach.

Tritonís frozen surface, thin atmosphere and geyser-like eruptions

Measurements from Voyager 2 indicated that Triton is the ultimate icebox, with a daytime surface temperature of 38 degrees kelvin at the time of encounter. That is only about three dozen degrees above absolute zero, when nothing can move, not even an atom. In fact, 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.

Fig. .. 

A frosty coating of nitrogen and methane ices overlies all of the surface features, reflecting the incident sunlight. 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.

Although nitrogen and methane frosts are apparently the dominant constituents of Triton's visible disk, water ice is needed to support and preserve the observed topography, including cliffs and ridges that exceed one kilometer in height. At the frigid 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, thin, brilliant veneers of nitrogen and methane ice apparently overlie a rigid crust of water ice.

Even in the middle of southern summer, a bright ice cap extends from the south pole three-quarters of the way to Triton's equator. It is so cold that some of Tritonís air freezes out at its poles, coating them with a huge ice cap of frozen nitrogen. By way of comparison, the Earthís polar caps contain frozen water ice, for it is too warm for nitrogen to freeze at our planetís poles, and it is too cold for water ice to vaporize from Tritonís surface and enter its atmosphere.

On a world that is literally frozen solid, astronomers were amazed to find at least four erupting plumes near the center of Tritonís sunlit polar cap. These plumes rise in straight columns to an altitude of 8 kilometers, where dark clouds of material are left suspended and carried downwind for over 100 kilometers, like smoke wafted away from the top of a chimney. Since the active plumes occur where the Sun is directly overhead, the solar heat might energize them.

Scientists have not reached a consensus about what produces the plumes, but one likely explanation is that geysers are sending up plumes of nitrogen gas laced with extremely fine dark particles. Triton is far too cold for geysers to spout steam and water, like geysers on Earth, but Sun-powered geysers might expel dark material when pent-up nitrogen gas becomes warm and breaks through an overlying seal of ice.

Fig. .. 

Long cracks or faults on Triton seem to have been partially filled with oozing ice, as they are on Ariel, moon of Uranus. Vast frozen basins found within Triton's equatorial regions have apparently been filled by icy extrusions flowing out from the warm interior, like a squeezed slush cone. These frozen lakes of ice look like inactive volcanic calderas, complete with smooth filled centers, successive terraced flows and vents. The warm liquid or slushy ice on Triton apparently acted like lava on Venus, resurfacing the globe and filling low flat areas with smooth deposits that subsequently froze.

Origin and evolution of Triton

Tritonís retrograde and inclined orbit suggests that it is not a true satellite of Neptune, dating back to the planetís formation, but that Triton was once a separate world, which was captured into an eccentric, backwards and tipped orbit in the remote past. Neptune's lack of an ordered family of large satellites might be also be explained if Triton was born in its own independent orbit around the Sun, and was subsequently captured by Neptune, destroying any regular satellite system the planet may have had in the process.

Fig. .. 

All in all, Voyager 2's journey past Neptune was a brilliant success. As it drew away, the intrepid explorer took one last picture of Neptune and Triton as neighboring crescents; continuing out to where Triton might have originated and billions of comets now hibernate.

Fig. ..summary 

(Summary Diagram)