Uranus / Introduction

The distance between Uranus and the sun is approximately double the distance from the sun to Saturn. The planet was discovered in 1781 by Herschel and is the seventh planet in the Solar System. Due to its immense distance from the sun, only a very small part of the sun’s light ever reaches the planet. Therefore, Uranus is a very cold world with temperatures of around minus 215°C.

The composition and structureof Uranus is very similar to that of Jupiter and Saturn. Its atmosphere consists mainly ofhydrogen (83%), mixed with small amounts of helium (15%) and methane (2%). The methane gas gives the planet its slightly greenish-blue color.

Up to the mid-1980s, only five moons were known. The spacecraft Voyager 2 subsequently discovered ten more satellites that are however considerably smaller.The moons were named after characters in plays by William Shakespeare. Of special interest is the unusual inclination of the axis of Uranus, which is nearly parallel to its orbit plane. Therefore, the south and the north pole of the planet face the sun alternatively.

Orbit of Uranus

Uranus revolves once around the sun every 84 years (= 1 Uranus year). It completes one rotation on its own axis in a period of 17 hours and 15 minutes (= 1 Uranus day). Its mean distance from the sun is 2.88 billion kilometers.

Uranus moves on its orbit at a velocity of 6.81 kilometers persecond. A unique characteristic of the planet is the pronounce dinclination of its rotation axis towards the orbit plane.

The inclination of 98 degrees is much more prominent than that ofother planets (e.g. earth23.5 degrees). The planet therefore ”lies“ on its orbit plane and its northand south pole are temporarily directly facing the sun. Therefore, winter on the northern hemisphere of the planet lasts for 42 years and the summers are equally long. Due to the vast distance of the planet from the sun, the difference in temperature between summer and winter is only 2 °C.

The ring system of Uranus consists of ill-defined dust belts. The fifteen so far known moons revolve around Uranus in independent orbits partly inside and partly outside the ring system.

Composition of Uranus

The equatorial diameter of Uranus is approximately 51,000 kilometers and only slightly larger than the distance from pole to pole. The average density of the planet is equal to 1.3 times that of water. Uranus has therefore a density of between those of Saturn and Jupiter. This implies that Uranus must contain greater amounts of heavier metals.

Uranus is very similar in structure to Saturn. The are though a few distinct differences. The core of Uranus consists of rocky materialat a temperature of approximately 7000 °C. This core is surrounded by a layer of water mixed with methane and ammonia.

Adjacent to this layer there is an atmosphere of hydrogen, helium and methane.One of the characteristics of methane gas is it absorption of red light. Therefore, Uranus appears to us a bluish-green disk.Temperatures in the different locations of Uranus vary. The parts facing the sun during the year are slightly warmer than the equatorial zone, which is rarely exposed to sunlight. Near the equator there are strong winds with velocities of up to 1000 kilometers per hour.

The surface of Uranus is a rather featureless desert. Only a few thin cloud bands were observed by the Voyager 2 spacecraft during its exploratory passage in 1986. Astonishingly, these clouds do not move in the direction which would be the most obvious one, namely from the north to the south pole, following the plane of the orbit. They actually move around the equatorial plane.

Uranus Moons

Uranus has 15 satellites and aring system. The larger five moons have been known for many years while the other ten moons have only beendiscovered in 1986 by theVoyager 2 spacecraft. The moonsand rings of Uranus orbit the planet in a plane through its equator.

Titania and Oberon are the brightest satellites and were discoveredin 1787 by W. Herschel. The discovery of Ariel and Umbriel followed in the mid-18th century and in 1948, Miranda was first detected.

The first five discovered moons of Uranus are the ones most distant from the planet, i.e. its outer moons. Oberon for example is 580,000 kilometers from Uranus. Its neighbor Titania has a diameter of 1580 kilometers and is the largest moon of Uranus. It consists of rock and ice and its surface features a system of valleys. Umbriel has a much darker surface. Its inner neighbors are Ariel and Miranda, which is the smallest of the outer planets.

The small moons of Uranus are Puck, Belinda, Rosalind, Portia, Julia, Desdemona, Cressida, Bianca, Ophelia and Cordelia. They are all named after famous characters in plays by William Shakespeare.

Ophelia and Cordelia are shepherd moons. Cordelia’s orbit is inside the ring system. These small satellites are nothing more than large blocks of rock and ice with diameters of less than 150 kilometers. Their mass is not sufficient to form matter into the spherical shape typical of most moons in the Solar System.

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