Uranus

Uranus

**Uranus** is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both have different bulk chemical composition from that of the larger gas giants Jupiter and Saturn. For this reason, scientists often classify Uranus and Neptune as "ice giants"
 to distinguish them from the gas giants. Uranus's atmosphere is similar
 to Jupiter's and Saturn's in its primary composition of hydrogen and helium, but it contains more "ices" such as water, ammonia, and methane, along with traces of other hydrocarbons. It is the coldest planetary atmosphere in the Solar System, with a minimum temperature of 49 K (−224.2 °C), and has a complex, layered cloud structure with water thought to make up the lowest clouds and methane the uppermost layer of clouds. The interior of Uranus is mainly composed of ices and rock.

Like the other giant planets, Uranus has a ring system, a magnetosphere, and numerous moons. The Uranian system has a unique configuration among those of the planets because its axis of rotation
 is tilted sideways, nearly into the plane of its solar orbit. Its north
 and south poles, therefore, lie where most other planets have their equators.

## Physical Characteristics

Uranus's mass is roughly 14.5 times that of Earth, making it the least
massive of the giant planets. Its diameter is slightly larger than
Neptune's at roughly four times that of Earth. A resulting density of 1.27 g/cm3 makes Uranus the second least dense planet, after Saturn. This value indicates that it is made primarily of various ices, such as water, ammonia, and methane.

The standard model of Uranus's structure is that it consists of three layers: a rocky (silicate/iron–nickel) core in the centre, an icy mantle in the middle and an outer gaseous hydrogen/helium envelope. The core is relatively small, with a mass of only 0.55 Earth masses and a
 radius less than 20% of Uranus's; the mantle comprises its bulk, with
around 13.4 Earth masses, and the upper atmosphere is relatively
insubstantial, weighing about 0.5 Earth masses and extending for the
last 20% of Uranus's radius.

The bulk compositions of Uranus and Neptune are different from those of Jupiter and Saturn, with ice dominating over gases, hence justifying their separate classification as ice giants.
 There may be a layer of ionic water where the water molecules break
down into a soup of hydrogen and oxygen ions, and deeper down superionic water in which the oxygen crystallises but the hydrogen ions move freely within the oxygen lattice.

## Satellites

Uranus has 27 known natural satellites. The five main satellites are Miranda, Ariel, Umbriel, Titania, and Oberon. The Uranian satellite system is the least massive among those of the
giant planets; the combined mass of the five major satellites would be
less than half that of Triton (largest moon of Neptune) alone. The largest of Uranus's satellites, Titania, has a radius of only 788.9 km, or less than half that of the Moon, but slightly more than Rhea, the second-largest satellite of Saturn, making Titania the eighth-largest moon in the Solar System.

### Rings of Uranus

The rings are composed of extremely dark particles, which vary in size from micrometres to a fraction of a metre. Thirteen distinct rings are presently known, the brightest being the ε
ring. All except two rings of Uranus are extremely narrow – they are
usually a few kilometres wide. The rings are probably quite young; the
dynamics considerations indicate that they did not form with Uranus. The
 matter in the rings may once have been part of a moon (or moons) that
was shattered by high-speed impacts.

## Colonization

### Population

329,000 (Miranda and Atmospheric Stations)