URANUS

Uranus in a Voyager 2 image. The color is due to the methane in its atmosphere. (Calvin J. Hamilton)

Uranus, seventh planet of the Solar System, was not known in ancient times, since it has been  casually discovered by William Herschel in 1781. It is part of the Jovian planets and presents several common characteristics with Jupiter, Saturn and Neptune. Its albedo is 0.51. Up to know it has been visited by just a single probe, Voyager 2, which got close to it in 1986. In Greek mythology, Uranus was the father of Chronos (the Roman Saturn), of the Cyclopes and Titans.

The mass of Uranus is 87,000 billion of billion tons (8,7 10²⁸ g), which is 14 and a half times that of the Earth; its mean density is 1.29, and its diameter is 51,118 km.
It orbits around the Sun at a mean distance of 2,871 million km, that is more than 19 A.U., making one revolution in 84.01 years. Its orbit has an eccentricity of 0.0461 and it is inclined by just 0.77 degrees on the ecliptic plane.
 

A backlight image of Uranus, taken by Voyager 2 in 1986, from a distance of one million km.  (NASA/JPL)

Uranus rotates about itself in 17 hours and 14 minutes. The peculiarity of this planet, is that its rotation axis lays almost on the orbital plane, thus forming an angle of just 8 degrees with it. This causes the fact that, during its rotation around the Sun,  the seasons alternate with marked differences. Indeed, the planet turns alternatively its one pole or the equator to the Sun. The polar regions get  get a larger amount of radiation than the equatorial ones.  No other Solar System planet has this peculiarity: it could be due to a much violent collision with another large body, in very remote epochs.

 
 

Structure and atmosphere
 

Two images of Uranus, in natural colors (left) and false colors (right). They have been obtained by Voyager 2. The right image shows details of the polar region, which is darker and surrounded by lighter and lighter concentric regions. Perhaps they are bands present in its atmosphere.  (NASA/JPL)

Uranus has an atmosphere, composed by hydrogen (83%), helium (15%) and methane (2%), with traces of acetylene and other hydrocarbons.  Even here systems of clouds similar to those of Jupiter and Saturn are present, pushed by strong winds and confined in horizontal bands. However, they are less intense. The winds blow at speeds comprised between 140 and 570 km/h. The blue color of this planet comes from the methane present in the high atmosphere, which absorbs red light and reflects blue light.

 

Uranus too has a magnetic field, but it is very peculiar, if compared to the other planets. In fact, its is not centered on the nucleus of the planet, and it is inclined by almost 60 degrees with respect to its rotation axis. Its origin is unknown; it was believed that it originated from a conductive liquid layer comprised between an hypothetical rocky nucleus and the atmosphere. However Uranus, just like Neptune, does not have the liquid hydrogen layer which Jupiter and Saturn have; even if its internal structure is mostly unknown, it seems that it is different from that of the first two jovian planets. Nevertheless, almost certainly it contains a rocky nucleus.
 
 

Satellites and rings
 

The satellites and rings of Uranus in an image of the Hubble Space Telescope. (HST)

Uranus, like Jupiter and Saturn, is surrounded by a system of rings, although it is very faint and less extended than that of Saturn. The rings of Uranus are 11. The are faint, their albedo is indeed just 0.03. They are composed by rather small dust grains and by larger bodies, whose dimensions are up to 10 meters.

 
 

The rings of Uranus, in false colors. In the upper part  there is the epsilon ring, the brighter one. (NASA/JPL)

 

Two small satellites among the rings of Uranus, Cordelia (1986U7) and Ophelia (1986U8). The epsilon ring can be noticed, brighter than the others. (NASA/JPL)

 

Uranus has at leas 15 satellites which has been discovered up to now. Apart from the five larger ones, all the other ones have been only recently discovered by the Voyager 2 probe. Others likely exist, much smaller, in the ring area. i

Montage of the planet with its five larger satellites: Ariel in the foreground, Umbriel on the left background, Miranda just right of Uranus, Titania on the upper right and Oberon above.. (NASA/JPL)

 
 
 
 
 

ANIMATIONS

  Magnetic field of Uranus AVI, 5.7 Mb (NASA/JPL)