Uranus and Neptune

The planets from Mercury to Saturn were already known in antiquity. Uranus and Neptune can only be observed with a telescope. Uranus and Neptune are giants, similar to Jupiter and Saturn.

Uranus. The famous German-English amateur astronomer William Herschel discovered Uranus in 1781.

Herschel himself first thought that the new object was a comet. However, the extremely slow motion revealed that the body was far beyond the orbit of Saturn. Based on the first observations, the Finnish astronomer Anders Lexell calculated a circular orbit. He was one of the first to propose that the newly discovered object was a planet. Johann Bode of the Berlin Observatory suggested the name Uranus but more than five decades passed before the name was unanimously accepted.

The mean distance of Uranus is 19 AU, and the orbital period 84 years. The inclination of the rotation axis is 98°, which is totally different from the other planets. Due to this uncommon geometry, the poles are either lit or in darkness for decades. The rotation period, confirmed by the Voyager 2 magnetometric measurements in 1986, is 17.3 hours; the exact period had been uncertain prior to the fly-by.

Uranus is greenish, as viewed through a telescope. Its colour is due to the strong methane absorption bands in the near-infrared. A part of the red light is also absorbed, leaving the green and blue part of the spectrum untouched. Uranus is almost featureless (Fig. 7.47) because its clouds are below a thick haze or smog.

The strong limb darkening makes the terrestrial determination of the Uranus' size difficult. Therefore, the radius was not accurately determined until 1977 during a stellar occultation caused by Uranus. The rings of Uranus were discovered at the same time.

The internal structure of Uranus is thought to be slightly different from that of other giant planets. Above the innermost rocky core, there is a layer of water, which, in turn, is surrounded by a mantle of hydrogen and helium. The mixture of water and ammonia and methane therein are dissociated to ions under the heavy pressure. This mixture behaves more like a molten salt than water. The convection flows in this electrically conductive "sea" give rise to the Uranian magnetic field. The strength of the magnetic field at the cloud tops is comparable to the terrestrial field. However, Uranus is much larger then the Earth, so the true strength of the field is 50 times greater than that of the Earth. The Uranian magnetic field is tilted 60° with respect to the rotation axis. No other planet has such a highly inclined magnetic field.

The Uranian rings (Fig. 7.48) were discovered in 1977, during a stellar occultation. Secondary occultations were observed before and after the main event. A total of 13 rings are known, nine of which were discovered in the occultation. The innermost ring is broad and diffuse. All other rings are dark and very narrow,

Neptune Ring

Fig. 7.47. Two views of Uranus. The left picture shows Uranus as it would appear to the naked eye. (NASA). At the right there is a Hubble Space Telescope view of Uranus surrounded by its rings. Also 10 satellites are visible in the original picture. (Seidelmann, U.S. Naval Observatory, and NASA)

Fig. 7.47. Two views of Uranus. The left picture shows Uranus as it would appear to the naked eye. (NASA). At the right there is a Hubble Space Telescope view of Uranus surrounded by its rings. Also 10 satellites are visible in the original picture. (Seidelmann, U.S. Naval Observatory, and NASA)

Fig. 7.48. Left: The rings of Uranus are very narrow and composed of a dark material. Nine rings are visible in the picture of Voyager in 1986. Right: Rings seen in the light scattered

only a few hundred metres or a few kilometres wide. The Voyager 2 results showed that the rings contain very little dust, unlike those of Jupiter and Saturn. The mean size of the ring particles is more than 1 metre. The ring particles are darker than practically any known material in the solar system; the cause of this dark colour is unknown.

There are 27 moons (2006 number) orbiting around Uranus, ten of which were discovered by Voyager 2. The geological history of some moons is puzzling, and many features reminiscent of an active past can be found.

The innermost of the large moons, Miranda, is one of the most peculiar objects discovered (Fig. 7.49). It has several geological formations also found elsewhere (but here they are all mixed together), in addition to the quite unique V-shaped formations. It is possible that Miranda's present appearance is the result of a vast collision that broke the moon apart; some pieces may have later settled down, inside out. Another peculiar object is Um-briel. It belongs to the ever increasing family of unusual dark bodies (such as the Uranian rings, one side of Ia-petus and Halley's comet). The dark surface of Umbriel is covered by craters without any traces of geological activity.

forward when the Voyager spacecraft was in the shadow of the planet. (NASA)

Neptune. The orbit of Uranus was already well known in the beginning of the 19th century. However, some unknown perturbations displaced Uranus from its predicted orbit. Based on these perturbations, John Couch Adams, of Cambridge, and Urbain Jean-Joseph Le Verrier, of Paris, independently predicted the position of the unknown perturbing planet.

The new planet was discovered in 1846 by Johann Gottfried Galle at the Berlin Observatory; Le Verrier's prediction was found to be only 1° off. The discovery gave rise to a heated controversy as to who should be given the honour of the discovery, since Adams' calculations were not published outside the Cambridge Observatory. When the quarrel was settled years later, both men were equally honoured. The discovery of Neptune was also a great triumph of the Newtonian theory of gravitation.

The semimajor axis of the orbit of Neptune is 30 AU and the orbital period around the Sun 165 years. The internal rotation period, confirmed by Voyager 2 in 1989, is 16 hours 7 minutes and the rotation period of the outer layers of the clouds is about 17 hours. The obliquity of the rotation axis is 29° but the magnetic field is tilted some 50° with respect to the rotation axis. The mag

7. The Solar System

184

Fig. 7.49. Four Uranian moons (from top left to lower right): Miranda, Ariel, Titania and Umbriel. (NASA)

netic field is tilted like in Uranus, but the field strength is much smaller.

The density of Neptune is 1660 kg m-3, and the diameter 48,600 km. Thus the density of Neptune is higher than that of other giant planets. The internal structure is quite simple: The core, composed of silicates (rocks) is about 16000 km in diameter. This is surrounded by a layer of water and liquid methane and the outermost gaseous layer, atmosphere, is mainly composed of hydrogen and helium, methane and ethane being a minor components.

Uranus Bilder Voyager

Fig. 7.50. (Left) Neptune shows more features than Uranus. In the picture of Voyager 2 the Great Dark Spot, accompanied by bright, white clouds is well visible. Their appearance is changing rapidly. To the south of the Great Dark Spot is a bright feature and still farther south is another dark spot. Each feature moves eastward at a different velocity. (Right) Details of the Southern Dark Spot. The V-shaped structure near the right edge of the bright area indicates that the spot rotates clockwise. Unlike the Great Red Spot on Jupiter, which rotates counterclockwise, the material in the Neptune's dark oval will be descending. (NASA/JPL)

Fig. 7.50. (Left) Neptune shows more features than Uranus. In the picture of Voyager 2 the Great Dark Spot, accompanied by bright, white clouds is well visible. Their appearance is changing rapidly. To the south of the Great Dark Spot is a bright feature and still farther south is another dark spot. Each feature moves eastward at a different velocity. (Right) Details of the Southern Dark Spot. The V-shaped structure near the right edge of the bright area indicates that the spot rotates clockwise. Unlike the Great Red Spot on Jupiter, which rotates counterclockwise, the material in the Neptune's dark oval will be descending. (NASA/JPL)

Cloud structures are more complicated than on Uranus, and some dark spots, like in Jupiter, were visible during the Voyager fly-by (Fig. 7.50). The speed of the winds are high, up to 400 m/s.

Like other giants, Neptune also has rings (Fig. 7.51). The rings were discovered by Voyager 2, although their existence was already expected prior the fly-by. Two relatively bright but very narrow rings are at a distance of 53,000 and 62,000 km from the centre of the planet. Moreover, there are some faint areas of fine dust.

There are 13 known moons, six of which were discovered by Voyager 2. The largest of the moons, Triton, is 2700 km in diameter, and it has a thin atmosphere, mainly composed of nitrogen. The albedo is high. Triton reflects 60-80% of the incident light. The surface is relatively young, without any considerable impact craters (Fig. 7.52). There are some active "geysers" of liquid nitrogen, which partly explains the high albedo

Fig. 7.51. The rings of Neptune. Ring particles are small and best visible in the forward scattered light. There are several brightenings in the outermost ring. One of the rings appears to have a twisted structure. Neptune at left is overexposed. (NASA/JPL)

Fig. 7.52. The southern hemisphere of Triton, Neptune's largest satellite in a picture taken in 1989 by Voyager 2. The dark spots may indicate eruptions of "icy volcanoes". Voyager 2 images showed active geyser-like eruptions spewing nitrogen gas and dark dust particles several kilometres into the atmosphere. (NASA)

Fig. 7.52. The southern hemisphere of Triton, Neptune's largest satellite in a picture taken in 1989 by Voyager 2. The dark spots may indicate eruptions of "icy volcanoes". Voyager 2 images showed active geyser-like eruptions spewing nitrogen gas and dark dust particles several kilometres into the atmosphere. (NASA)

and the lack of the craters. The low surface temperature of Triton, 37 K, means that the nitrogen is solid and covers the surface like snow. It is the lowest surface temperature known in the solar system.

Telescopes Mastery

Telescopes Mastery

Through this ebook, you are going to learn what you will need to know all about the telescopes that can provide a fun and rewarding hobby for you and your family!

Get My Free Ebook


Post a comment