The Ice Giants Uranus and Neptune

The Discovery of Uranus Discovered in 1781 by Herschel; first planet to be discovered in more than 2000 years Little detail can be seen from Earth; arrows point to three of Uranus’s moons: Details are virtually invisible on photographs of Uranus made with large Earth-based telescopes. (Arrows point to three of the planet's moons.) (UC/Lick Observatory)

The Discovery of Neptune Neptune was discovered in 1846, after analysis of Uranus’s orbit indicated its presence Details of Neptune cannot be made out from Earth either; arrows again point to moons: Nereid Triton Neptune and two of its moons, Triton (left arrow) and Nereid (right), imaged with a large Earth-based telescope. (UC/Lick Observatory)

Physical Properties of Uranus and Neptune Uranus and Neptune are very similar: Jupiter, Saturn, Uranus, and Neptune, drawn to scale and compared with Earth. Uranus and Neptune are quite similar in their bulk properties. Each probably contains a core about 10 times more massive than Earth. Jupiter and Saturn are each substantially larger, but their rocky cores are probably comparable in mass to those of Uranus and Neptune. (NASA)

Physical Properties of Uranus and Neptune Mass 14.5 × Earth 17.1 × Earth Radius 4.0 × Earth 3.9 × Earth Density 1300 kg/m3 1600 kg/m3

Uranus, Close Up This image of Uranus, taken from a distance of about 1 million km, was sent back to Earth by the Voyager 2 spacecraft as it whizzed past the giant planet at 10 times the speed of a rifle bullet. The image approximates the planet's true color, but shows virtually no detail in the largely featureless upper atmosphere, except for a few wispy clouds in the northern hemisphere. (NASA)

Physical Properties of Uranus and Neptune Peculiarity of Uranus: axis of rotation lies almost in the plane of its orbit. Seasonal variations are extreme. The 98° axial tilt of Uranus places its equator almost perpendicular to the ecliptic. As a result, the planet is characterized by the most extreme seasons known in the solar system. The equatorial regions have two "summers" (warm seasons around the times of the two equinoxes) and two "winters" (cold seasons at the solstices) each year; the poles are alternately plunged into darkness for 42 Earth years at a time.

The Atmospheres of Uranus and Neptune Outer atmospheres of Uranus and Neptune are similar to those of Jupiter and Saturn Uranus and Neptune are cold enough that ammonia freezes; methane dominates and gives the characteristic blue color

The Atmospheres of Uranus and Neptune Uranus is very cold; clouds only in lower, warmer layers. Few features are visible. (a), (b), and (c) These computer-enhanced Hubble Space Telescope images, taken at roughly four-hour intervals, show the motion of a pair of bright clouds (labeled A and B) in the planet's southern hemisphere. (The numbers at the top give the time of each photo.) (d) A near-infrared image of Uranus, also from the Hubble instrument, shows the planet's ring system, as well as a number of clouds (pink and red regions) in the upper atmosphere. The clouds circle the planet at speeds of up to 500 km/h. (NASA)

Neptune close up Neptune as seen by Voyager 2, from a distance of roughly 1 million km. A closer view (b), resolved to about 10 km, shows cloud streaks ranging in width from 50 km to 200 km. (NASA)

The Atmospheres of Uranus and Neptune Band structure of Neptune is more visible, and Neptune has internal heat source of unknown origin:

The Atmospheres of Uranus and Neptune Neptune has storm systems similar to those on Jupiter, but fewer:

Magnetospheres and Internal Structure Uranus and Neptune both have substantial magnetic fields, but at a large angle to their rotation axes. The rectangle within each planet shows a bar magnet that would produce a similar field. Note that both Uranus’s and Neptune’s are significantly off center. A comparison of the magnetic field strengths, orientations, and offsets in the four jovian planets: (a) Jupiter, (b) Saturn, (c) Uranus, and (d) Neptune. The planets are drawn to scale, and in each case the magnetic field is represented as though it came from a bar magnet (a simplification, for purposes of illustration only). The size and location of each magnet represent the strength and orientation of the planetary field. Notice that the fields of Uranus and Neptune are significantly offset from the center of the planet and are inclined considerably to the planet's axis of rotation. Earth's magnetic field is shown for comparison. One end of each magnet is marked N to indicate the polarity of Earth's field.

Magnetospheres and Internal Structure Magnetic fields of Uranus and Neptune must not be produced by dynamos, as the other planets’ fields are. Interior structure of Uranus and Neptune, compared to that of Jupiter and Saturn: A comparison of the interior structures of the four jovian planets. The planets drawn to scale. (b) The relative proportions of the various internal zones.

The Moon Systems of Uranus and Neptune The five largest moons of Uranus, and Proteus, the sole midsized moon of Neptune, are shown to scale, with part of Earth's Moon (also to scale) for comparison. In order of increasing distance from the planet, the Uranian moons are Miranda, Ariel, Umbriel, Titania, and Oberon. The appearance, structure, and history of Titania and Oberon may be quite similar to those of Saturn's moon Rhea. The smallest details visible on both moons are about 15 km across. Umbriel is one of the darkest bodies in the solar system, although it has a bright white spot on its sunward side. Ariel is similar in size, but has a brighter surface, with signs of past geological activity. The resolution is approximately 10 km. (NASA; Lick Observatory)

Table 13-1 The Major Moons of Uranus

The Moon System of Uranus Uranus has 27 moons, five of which are major: Miranda, Ariel, Umbriel, Titania, and Oberon Very similar to Saturn’s medium-sized moons, except that all are much less reflective. Umbriel is the darkest:

The Moon System of Uranus Miranda is the most unusual; origin of the cracks and grooves is unknown: The asteroid-sized innermost moon of Uranus, photographed by Voyager 2. Miranda has a strange, fractured surface suggestive of a violent past, but the cause of the grooves and cracks is currently unknown. The resolution in the inset is about 2 km. The long "canyon" near the bottom of the inset is about 20 km deep. (NASA)

The Moon System of Neptune Neptune has 13 moons, but only two can be seen from Earth: Triton and Nereid Triton is in a retrograde orbit; Nereid’s is highly eccentric Triton’s surface has few craters, indicating an active surface

The Moon System of Neptune : Triton Nitrogen geysers have been observed on Triton, contributing to the surface features: The south polar region of Triton, showing a variety of terrains ranging from deep ridges and gashes to what appear to be lakes of frozen water, all indicative of past surface activity. The pinkish region at the right is nitrogen frost, forming the moon's polar cap. The resolution is about 4 km. The long black streaks at the bottom left were probably formed by geysers of liquid nitrogen on the surface. (NASA)

The Moon System of Neptune Also, there appear to be ice volcanoes: Scientists think that this roughly circular lake-like feature on Triton may have been caused by the eruption of an ice volcano. The water "lava" has since solidified, leaving a smooth surface. The absence of craters indicates that this eruption was a relatively recent event. The frozen lake is about 200 km in diameter; its details are resolved to a remarkable 1 km. The inset is a computer-generated view along Triton's surface, illustrating the topography of the area. (NASA)

Occultation of Starlight By carefully watching the dimming of distant starlight as a planet crosses the line of sight, astronomers can infer fine details about that planet. The rings of Uranus were discovered with this technique.

The Rings of the Outermost Jovian Planets Uranus and Neptune have faint ring systems, recently detected. Uranus’s rings are narrow: The main rings of Uranus, as imaged by Voyager 2—all nine of them known before the spacecraft's arrival—can be seen in this photo. From the inside out, they are labeled from 6 to Epsilon. The resolution is about 10 km, which is just about the width of most of these rings. The two rings discovered by Voyager 2 are too faint to be seen here. (NASA)

Figure 13-17 Epsilon Ring A close-up of Uranus's Epsilon ring reveals some of its internal structure. The width of the ring averages 30 km; special image processing has magnified the resolution to about 100 m. (NASA)

The Rings of the Outermost Jovian Planets Two shepherd moons keep the epsilon ring from diffusing: These two small moons, discovered by Voyager 2 in 1986 and now named Cordelia (U7) and Ophelia (U8), tend to "shepherd" the Epsilon ring, keeping it from diffusing. (NASA)

The Rings of the Outermost Jovian Planets Neptune has five rings, three narrow and two wide: In this long-exposure image, Neptune (center) is heavily overexposed and has been artificially blocked to make the rings easier to see. One of the two fainter rings lies between the inner bright ring (Leverrier) and the planet. The others lie between the Leverrier ring and the outer bright ring (known as the Adams ring). (NASA)

Summary Uranus, Neptune, and Pluto were all discovered in the last 350 years Uranus and Neptune are similar: gaseous and cold Uranus’s spin axis is almost in the plane of its orbit Surface features are hard to discern on Uranus, but more obvious on Neptune Uranus has no excess heat emission, but Neptune does

Summary Uranus’s midsized moons are similar to those of Saturn Neptune’s moon Triton has a retrograde orbit Uranus and Neptune both have faint ring systems