Uranus

The ice gi­ant that is shrouded in mys­tery has fas­ci­nated ex­plor­ers for decades

All About Space - - Contents -

All About Space's lat­est re­port on the ice gi­ant

The sev­enth planet from the Sun, Uranus be­came the first planet to be found with the aid of a te­le­scope on 13 March 1781 by British as­tronomer Wil­liam Her­schel. On that fate­ful night, he de­scribed ob­serv­ing a “neb­u­lous star or per­haps a comet”. Lit­tle did he know that he had just dis­cov­ered Uranus – named after the the Greek god of the sky – a name pro­posed by Jo­hann Elert Bode in 1783.

Over 230 years later, Uranus still re­mains to be a puz­zle. What is known is that Uranus is lo­cated about 2.9 bil­lion kilo­me­tres (1.8 bil­lion miles) from the Sun, about 19-times the dis­tance from Earth to the Sun, mean­ing that one or­bit of the Sun takes 84 Earth years. The planet is enor­mous, with a di­am­e­ter of 50,724 kilo­me­tres (31,518 miles) – four­times wider than the Earth.

Uranus has a com­po­si­tional mass that is 80 per cent a fluid mix­ture of wa­ter, am­mo­nia (NH3) and methane (CH4) ices. It is the methane in the outer at­mos­phere that gives it its blue-green colour, but the thick cloud cov­er­age does not al­low our in­stru­ments to peer down any fur­ther, and is one rea­son why Uranus re­mains an enigma.

What as­tronomers strongly sug­gest is that below the planet’s cloud tops is a main at­mos­phere which con­tains mostly hy­dro­gen and helium by com­po­si­tion and has traces of methane and other volatiles. Below that is the fluid icy man­tle, which makes up most of its com­po­si­tion by mass, but it is also the­o­rised that the pres­sure and tem­per­a­tures are enough to make it ‘rain di­a­monds’ at that depth. Fi­nally at the cen­tre is the sil­i­cate iron-nickel core, thought to be between half to just over three-times the mass of the Earth.

Al­though there has only been one mis­sion to visit the ice gi­ant close up – Voy­ager 2 in 1986 – the planet has long been stud­ied by ground and space-based tele­scopes such as Hub­ble and the Keck Ob­ser­va­tory in Hawaii, United States. Ob­ser­va­tions through­out the years have re­vealed more sub­tle, yet sur­pris­ing de­tails about the planet. These in­clude the planet’s thin rings, which con­firmed that Saturn is not an out­lier and rings can form around any planet. As­tronomers have also been able to de­duce that Uranus’ plan­e­tary ro­ta­tional tilt is off by a no­table 97.77 de­grees, which im­plies there was a col­li­sion in its early age that knocked it over.

Sim­i­lar to the larger gas giants, Jupiter and Saturn, storms have been ob­served brew­ing in the cloud tops of Uranus. In Novem­ber 2014 the planet was ex­tremely ac­tive; storms raged on Uranus that were even ob­served by am­a­teur as­tronomers. These ob­ser­va­tions caused an­other dilemma in re­gards to Uranus. As there seems to be no in­ter­nal heat source and it’s a huge dis­tance from the Sun’s heat, as­tronomers ques­tion what’s go­ing on in­side Uranus to make such storms arise. “The colours and mor­phol­ogy of this cloud com­plex sug­gest that the storm may be tied to a vor­tex in the deeper at­mos­phere sim­i­lar to two large cloud com­plexes seen dur­ing the equinox," said Larry Sro­movsky, a plan­e­tary sci­en­tist at the Univer­sity of Wis­con­sin, Madi­son, United States at the time of its dis­cov­ery.

This false-colour im­age shows the many bright clouds on Uranus

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