11. Uranus and Neptune


Uranusí clear atmosphere

Uranusí clear atmosphere

. When viewed through a telescope, Uranus is a clear, featureless ball (left). Its blue-green color results from the absorption of red light by methane gas in the deep, cold atmosphere. An exaggerated false-color image (right) brings out subtle details in the polar regions of Uranus. A thin reddish-brown haze obscures the south pole (right center), perhaps due to chemical reactions of ultraviolet sunlight with methane. Progressively lighter concentric bands may be attributed to zonal motions in the upper atmosphere, which circulates in the same direction as the planet rotates. These Voyager 2 images were taken on 17 January 1986. (Courtesy of NASA and JPL.)


Neptuneís dynamic atmosphere

Neptuneís dynamic atmosphere

. White clouds lie above the swirling Great Dark Spot (right center), and raging winds reach speeds of 300 meters per second, creating a global banding in the atmosphere of Neptune. The faint sunlight at Neptuneís great distance cannot provide the energy of such winds; they are probably energized by heat from the interior of the planet. This image was sent from the Voyager 2 spacecraft on 14 August 1989, after a 12-year journey to the planet, using a 20-watt transmitter with less power than an ordinary light bulb. Traveling at the speed of light, the signals took more than 4 hours to reach Earth. (Courtesy of NASA and JPL.)


Neptuneís stormy disposition

Neptuneís stormy disposition

. The weather on opposite hemispheres of Neptune is recorded in these images, that combine simultaneous observations made with the Hubble Space Telescope and NASAís Infrared Telescope Facility on Mauna Kea, Hawaii. The predominant blue color of the planet is a result of absorption of red and infrared light by Neptuneís methane atmosphere. Clouds elevated above most of the methane absorption appear white. Neptuneís powerful jet stream, where winds blow at nearly 400 meters per second, is centered at the dark blue belt near Neptuneís equator. The Great Dark Spot detected when the Voyager 2 spacecraft visited Neptune in August 1989 has completely disappeared in these images, taken seven year later. (Courtesy of NASA and the Space Telescope Science Institute.)


Neptuneís Great Dark Spot

Neptuneís Great Dark Spot

. The Great Dark Spot of Neptune, rotating in the anti-cyclonic, counterclockwise direction, is as large as the Earth, and about half as large as the Great Red Spot of Jupiter. Unlike the Red Spot, which has lasted for centuries, Neptuneís Dark Spot vanished within a few years after the Voyager 2 spacecraft took this image, in August 1989. (Courtesy of NASA and JPL.)


Neptuneís Small Dark Spot

Neptuneís Small Dark Spot

. Winds swirl around this dark spot in Neptuneís atmosphere, viewed from the Voyager 2 spacecraft in August 1989. Bright, white methane-ice clouds stare out of its center like the pupil of an eye. The spot may be rotating in the cyclonic clockwise direction, opposite to that of the Great Dark Spot on Neptune and the Great Red Spot on Jupiter. If so the material in the dark oval was descending. (Courtesy of NASA and JPL.)


Radius-mass relations

Radius-mass relations

. A liquid body of solar composition describes a radius-mass relation (top) that approximates the mass and radius of Jupiter and Saturn. They consist mainly of the lightest element hydrogen, denoted by H, and next lightest abundant element helium, abbreviated He. Uranus and Neptune contain little hydrogen or helium, for their radii are much too small to be consistent with solar composition. Instead, they lie only slightly above the mass-radius relation for liquid water, so they probably contain large quantities of water. For comparison purposes, the bottom curve describes a giant planet composed of pure rock, as the Earth is but with a much smaller size.


Inside Uranus and Neptune

Inside Uranus and Neptune

. An outer shell of liquid molecular hydrogen covers a thick inner shell of melted ice within the interior of both Uranus and Neptune. Because of their relatively low mass and hydrogen abundance, neither planet contains an inner shell of liquid metallic hydrogen, as Jupiter and Saturn do.


Discovery of the thin rings of Uranus

Discovery of the thin rings of Uranus

. Astronomers recording the light of a star that was expected to disappear behind Uranus, on 10 March 1977, unexpectedly recorded short dips in the starlight before the star passed behind the planet (top). The same pattern was repeated when the star reappeared (bottom), indicating that Uranus is surrounded by narrow rings that briefly block out the light at the same distance on opposite sides of the planet. The strong and abrupt absorption of starlight indicates that the narrow rings are quite opaque and have well-defined edges. (Courtesy of James L. Elliot.)


Uranusí rings

Uranusí rings

. This Voyager 2 image, taken on 23 January 1986, shows five of the nine rings of Uranus that had been previously inferred from Earth-based observations of their brief occultation of a starís light. In this view, sunlight striking the rings particles was reflected back toward the camera, showing that the dense parts of the ring system consist of narrow rings with wide gaps. In contrast, Saturnís main rings are broad with narrow gaps. From bottom to top, the rings are designated by the Greek letters alpha, beta, eta, gamma and delta. (Courtesy of NASA and JPL.)


Between the rings of Uranus

Between the rings of Uranus

. Voyager 2 took this image while in the shadow of Uranus and looking almost directly back toward the Sun. This made the fine dust between the rings appear very bright. The long, 96-second exposure produced streaks due to trailed stars. (Courtesy of NASA and JPL.)


Rings and small satellites of Uranus

Rings and small satellites of Uranus

. Eight of Saturnís small satellites circle the planet just outside its bright epsilon ring. This image, taken with the Hubble Space Telescope on 28 July 1997, is a false-color composite of three images taken at different infrared wavelengths in which Uranus appears relatively dim but the rings and moons do not. The satellites range in size from 40 kilometers across, for Bianca, to 150 kilometers for Puck. The arrows denote their direction of revolution about Uranus. White clouds are seen just above the planetís blue-green methane atmosphere. (Courtesy of NASA and the Space Telescope Science Institute.)


Neptuneís rings

Neptuneís rings

. As Voyager 2 left Neptune in August 1989, the planetís narrow rings were backlit by the Sun, enhancing the visibility of the ringsí dusty particles. The outer ring consists of at least three dense clumps of orbiting debris, named Libertť, Egalitť and Fraternitť, that stand out from the thinner remainder of the ring. Astronomers on the ground had only detected the clumps during some stellar occultations, and assumed that the ring was incomplete. The outermost ring is named Adams, and the innermost is designated Le Verrier. They are named for John Couch Adams (1819-1892) and Urbain Jean Joseph Le Verrier (1811-1877) who independently predicted the existence of the then unknown planet Neptune. A third, fainter ring is located closer to Neptune than the two main rings; it has been named Galle after Johann Gottfried Galle (1812-1910) who found the planet close to both of Adamsí and Le Verrierís predicted positions using a 0.23-meter (9-inch) refractor. (Courtesy of NASA and JPL.)


Uranus from its moon Miranda

Uranus from its moon Miranda

. This montage of Voyager 2 images, obtained in January 1986, shows the blue-green cloud tops of Uranus as they would be viewed from the icy surface of its satellite Miranda. An artist has added the planetís dark rings as they might appear from this vantagepoint. (Courtesy of NASA and JPL.)


Miranda

Miranda

. Two strikingly different types of terrain are found on Miranda, the innermost and smallest of the five major Uranian satellites. It is 480 kilometers in diameter. There is an old, heavily cratered, rolling terrain with relatively uniform reflectivity (left), and a young, complex terrain (right) characterized by sets of bright and dark bands, jagged cliffs, ridges and grooves, as well as the distinctive chevron feature (above and right of center). This image was taken from the Voyager 2 spacecraft on 24 January 1986. (Courtesy of NASA, JPL and the U.S. Geological Survey.)


Four icy satellites of Uranus

Four icy satellites of Uranus

. Uranus has four large ice-covered moons that display impact craters, bright rays of ejected material, smooth regions and large rifts or grooves. They range in size from 580 thousand meters in radius, for Ariel, to a radius of 805 thousand meters for Titania. Numerous valleys and faults cross the terrain of Ariel (top left). The darkest large moon is Umbriel (top right), which reflects only 16 percent of the sunlight striking its surface. Its heavily-cratered surface resembles the lunar highlands, but lacks the numerous bright-ray craters and evidence for geological activity seen on the other large satellites. Titania (lower left) retains the numerous scars of impacts, with bright icy ejected material, as well as a trench-like feature that suggests tectonic activity (right edge). The icy surface of Oberon (lower right) exhibits several large impact craters surrounded by bright rays. One of them has a bright central peak and a floor that is partially covered with very dark, possibly carbon rich, material that may have erupted onto the crater floor sometime after the crater formed. These images were taken from the Voyager 2 spacecraft on 24 January 1986. (Courtesy of NASA and JPL.)


Neptuneís odd satellites

Neptuneís odd satellites

. Nereid, the outermost of Neptuneís satellites, travels in a highly inclined, eccentric orbit, in the same direction as that of the planetís rotation. Triton, the largest satellite of Neptune, travels around Neptune in a circular obit, but, unlike any other large satellite of the giant planets, it travels in the opposite retrograde direction to the rotation of Neptune. In addition, careful analysis of Tritonís motions shows that the satellite is in a decaying orbit and is slowly being pulled toward Neptune.


Neptune on Tritonís horizon

Neptune on Tritonís horizon

. From its satellite Triton (foreground), the planet Neptune would appear to move laterally along the horizon. Terraces on the moonís surface indicate multiple exposures of icy volcanic flooding, while a huge anti-cyclonic storm system, the Great Dark Spot, is visible in Neptuneís atmosphere. (Courtesy of NASA and JPL.)


Triton

Triton

. A mosaic of the south polar cap of Triton, the largest satellite of Neptune, taken in August 1989 from the Voyager 2 spacecraft. At the time of this flyby, Triton was the coldest measured object in the solar system with a surface temperate of 38 degrees kelvin. It is so cold that most of Tritonís nitrogen atmosphere is condensed as frost, making it the only satellite in the solar system known to have a surface made mainly of nitrogen ice. Highly reflective methane ice has been colored pink by the action of energetic radiation. (Courtesy of NASA and JPL.)


Ice lakes on Triton

Ice lakes on Triton

. The smooth, flat surface of this feature on the surface of Triton may have been filled with water ice. It is edged with overlapped terraces, as though the level of the lakes changed as the result of repeated flooding, partial removal and freezing. In Tritonís frigid world volcanoes erupt ice, rather than molten rock. The small pits and finely textured areas near the center of the lake-like features are probably associated with the most recent eruption of ice. This image, taken from the Voyager 2 spacecraft in August 1989, is about 500 thousand meters across. (Courtesy of NASA and JPL.)


Farewell to the planetary system

Farewell to the planetary system

. In parting, Voyager 2 provided this last picture show, with Neptuneís rim in the foreground and Triton appearing as a thin crescent in the distance. The hardy spacecraft, launched in 1977, recorded the swirling face of Jupiter in July 1979, the myriad rings of Saturn in August 1981, pale Uranus in January 1986 and stormy Neptune in August 1989 before heading out of the solar system. (Courtesy of NASA and JPL.)


Summary Diagram

Summary Diagram

. Summary Diagram.