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Uranus

Uranus is the seventh planet from the Sun, one of the giant planets made of gasses. It is the third largest by diameter and the fourth largest by mass (14.536 Earths) planet in our solar system. Its equatorial radius is 4.007 Earths and polar radius 3.93 Earths, and its mass 14.536 Earths that leads to a law mean density of 1.27 g/cm3, and law gravity, close the Earth’s value 0.88 g. Its albedo 0.300 (Bond) or 0.51 (geometric). The so called surface temperature at depths with pressure 1 bar is around 76 K, while at 0.1 bar it varies between  49 K to 57 K. Uranus radiates 1.06 ± 0.08 times more energy than it receives from the Sun hence it produces 6% of its energy.

Earth based observations and information sent to Ea Its orbit has semi-major axis 19.23 AU, aphelion 20.1 AU, perihelion 18.4 AU, eccentricity 0.044 and orbital period 84.3 years. The synodic period is almost a year, as it is very far from the Earth. Its rotation is retrograde with sidereal period 17 h 14 m and axial tilt 97.77°, i.e. the planet spins at an axis tilted sideways, with its axis almost at the plane of its orbit. This was probably the result of collision with a massive object that changed its angular momentum. The upper atmosphere spins in the same direction as the planet but faster. It has many satellites 27 of them known today. Although it is visible to the naked eye, Uranus was first recognized as a planet on 13 March 1781 by William Herschel. Johann Elert Bode, determined its orbit, realized that it was almost circular orbit, noticed that the distance was in agreement with his law on the distance of the planets, hence proved that it was a planet and not a comet as Herschel believed, and named it Uranus, after the ancient Greek deity of the Heavens that was the son and mate of Gaia and the father of Cronus (Saturn). Until then it was mistakenly thought to be a star because of its dimness and slow orbit that made the planet unnoticeable. It is understandable why it passed unnoticed for millennia as its magnitude is between +5.6 and +5.9.

Most information we have for Uranus is by Voyager 2 that visited this remote planet on January 1986, greatly improving our understanding of this asronomical object practically unknown till then. Uranus is believed to consist of three layers. The innermost sphere is the rocky core (iron and silicon) with a radius of about 7.500 km (more than double than the Earth in diameter). It is covered by the middle layer is the mantle with a thickness of about 10.000 km. It is made by ices of water, methane and ammonia. The outer much thinner layer is the thick atmosphere of the planet. It consists mainly of molecular hydrogen (H2 82,5%) and helium (He 15,2%). The chemical composition changes with depth. Around and below 1.3 bar is mainly hydrogen, 83±3% (H2),15±3% helium, 2.3% methane, and very small amounts of hydrogen deuteride (HD), ammonia, water, ammonium hydrosulfide (NH4SH), methane and other. The appearance of methane (CH4 2,3%) in the upper atmosphere is what makes the planet’s color cyan.
Uranus satellites can be classed in three groups. The five of them, Miranda, Ariel, Umbriel, Titania and Oberon, are sufficiently large to be spherical (472 km for Miranda to 1578 km Titania that has a mass close to 1/20th of the Moon), as a result of their gravity, and even four of them, have some activity and signs of volcanoes and canyons The five large moons have all the mass of satellites (99.99%). The thirteen closer to Uranus satellites are small very dark irregular bodies similar in many ways to the rings.

More interesting discovery of Voyager 2 was Uranus magnetic field. It is not yet identified if it is created at the core or at mantle. The magnetic dipole is shifted from the center of the planet at a distance of 0,3 Ru along the rotation axis. This results in a highly asymmetric magnetosphere. Unexpectedly its magnetic poles have an enormous tilt and are 58,6° away from the axis of rotation. The magnetosphere is peculiar as a result the tilt of the rotation axis, that is almost at the plane of the orbit, and the large tilt of the magnetic dipole. Hence the solar wind hits the dipole of the planet at a large variety of angles during the day of the planet and the year of Uranus and the magnetosphere changes drastically with day and season of the year of Uranus and the magnetosphere changes completely every season that lasts around 21 years.

Like most of the planets, Uranus has moons. They are twenty-seven with the two biggest ones, Titania and Oberon, having a diameter of 789 and 761 km respectively. Voyager 2 also confirmed the existence of rings that were first observed on March 10, 1977 by James L. Elliot, Edward W. Dunham, and Douglas J. Mink using the Kuiper Airborne Observatory. There are at least thirteen rings composed by dark particles that vary in size. Most of the rings are a few kilometers wide. The ring ? is the brightest and has a width of 20 to 96 km.

LINKS:

http://hubblesite.org/gallery/album/solar_system/uranus/

http://voyager.jpl.nasa.gov/

PICTURES:

Two "shepherd" moons, 1986U7 and 1986U8, with epsilon ring. January 21, 1986. Range, 2.5 million miles. Voyager 2.

From http://pds.nasa.gov/planets/special/uranus.htm  

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