Uranus: a story of planetary catastrophe – 24 July 2014

A talk by Greg Smye-Rumsby.

The planet Uranus is named after the Greek god Ouranos, meaning ‘sky’ or ‘heaven’. It is the coldest of the planets, an ice giant with a tumbling orbit which takes 84 years to orbit the Sun. A ‘day’ on Uranus lasts 17¼ hours, so it has very long seasons.

Planets only take a few hundred years to form from proto-planetary discs with a star at the centre. All planets have moons and rings, and if they do not, they have been tampered with.

William Herschel identified a possible ring of Uranus in 1789, but this sighting is generally considered doubtful. The rings of Uranus are very dark and were only confirmed in 1977 with two more discovered from Hubble images in 2005.

The big question about Uranus is why it is tilted on its side. Was this the result of an impact, or did Uranus tip itself over? The magnetic poles are asymmetric and offset from the centre and the magnetosphere is greatly tilted.

The heat generated by the core of Uranus is almost equal to the heat it receives from the Sun, about 3 billion km away.

The atmosphere of Uranus is very active despite being so cold. The Keck telescope has observed bright clouds and high altitude haze. The clouds are made up of methane, ammonia, hydrogen and helium.

The core of Uranus is similar in size to the Earth and is composed of molten rock.

The rings are not very dense so there must be many shepherding moons keeping them in position. The ring system is also asymmetric with many tiny rings.

The moons of Uranus are all medium sized or tiny and dark, therefore they are very old. Their characteristics are quite bizarre. Miranda appears to be inside-out, apparently having been torn to pieces. Its orbital inclination buckles the rings of Uranus. The moon Ariel seems to have been re-surfaced, with the crust cracking open them refreezing.

There is plenty going on in this strange world, and many questions remain to be answered.

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