Uranian Moons

Uranus has 27 known moons which are divided into three categories. Closest to the planet are 13 inner moons. Next from Uranus are five major, large moons - Miranda, Ariel, Umbriel, Titania and Oberon. Outermost moons are also irregular moons and there are nine of them. Combined mass of these moons is 10,000 times smaller than the mass of Uranus, making the planet’s satellite system the least massive of four gas planets. All 27 of theme were named after characters from the works of William Shakespeare and Alexander Pope.

Uranus and its moons and rings. Credit: NASA

Large Uranian Moons

All of five large Uranian moons are massive enough to be spherical. They were formed from an accretion disc believed to have existed around the planet after another planet collided with Uranus early during its formation, forming both the large moons and the planetary ring. This is why these five moons orbit Uranus on its equatorial plane.

Since the planet itself is tilted by almost 98 degrees, meaning it orbits the Sun on its side, the large moons have orbits perpendicular to the ecliptic. Also, their rotational axes are parallel to the planet. Hence, like Uranus, five major moons have one side where Sun doesn’t set for approximately 42 years and one where it never rises in that time period. For more on this, check this article where Uranian orbit and axial tilt is explained.

All five of the major Uranian moons are tidally locked to the planet, meaning they always face Uranus with the same side. They orbit the planet at distances from 130,000 to 584,000 kilometres. Most of our knowledge about these moons comes from the Voyager 2 probe as it flew by Uranus in 1986. They are made mostly of ice and rocky core.

Of the five major planets, Miranda is the smallest one and closest to the planet. It is only 470 kilometres in diameter, making it the second smallest body to be spherical, after Saturn’s moon Mimas. It was also the most recently discovered of the five, in 1948, almost 200 years after Ariel and Umbriel and more than 260 years after Titania and Oberon. Miranda has very strange and extreme topography, with the Verona Rupes cliff being the tallest in the Solar System at 5 to 10 kilometres in height.

Ariel is the second smallest and second closest to Uranus. It is 1158 kilometres in diameter. Due to tidal heating, it shows signs of more recent geological activity than any other Uranian moon, as evident from its complex surface. It was discovered in 1851 by William Lassell, along with Umbriel.

Speaking of Umbriel, it’s the third closest large Uranian moon and third largest, with diameter of 1170 kilometres, only 12 km more than Ariel. It’s the darkest Uranian moon with surface covered with impact craters. Being similar in size but lighter than Ariel suggests that Umbriel is made mostly of water ice.

The largest Uranian moon is Titania with the diameter of 1577 kilometres, making it the eighth largest moon in the Solar System. Titania’s surface is cratered by impacts, similar to Umbriel but less cratered than Oberon. It was discovered by William Herschel in 1787, only six years after he discovered Uranus and at the same time as Oberon.

Oberon is the outermost large Uranian moon and the second largest. Its surface is heavily cratered while between its mantle and core there could be liquid water.

Inner Uranian Moons

Uranus has 13 inner moons with orbits inside the orbit of large moon Miranda. They are closely connected with the planet’s ring system, which originated from one or more of these inner moons that fell apart some time in the past. They have diameters ranging from only 18 kilometres to 162 kilometres. Puck is the one with 162 km in diameter and it was the first inner moon discovered, in 1985 by the Voyager 2 probe. Additional nine inner moons were discovered by Voyager 2 in 1986 and the remaining three were discovered in 1999 and 2003. 

Interestingly, a moon called Perdita was pictured by Voyager 2, but only noticed in 1999 on the old photographs taken by the probe. It took Hubble Space Telescope to confirm the existence of Perdita and find another two moons. They are Mab with diameter of 25 kilometres and Cupid with 18 km, making them two smallest inner moons of Uranus. Mab is actually a source of material for one of the rings

The two innermost moons, Cordelia and Ophelia, are shepherds of another planetary ring. The whole system of inner moons is unstable and it could easily happen they throw each other out of their orbits and collide, providing materials for future rings.

Irregular Uranian Moons

There are nine irregular moons around Uranus, all of them probably being objects that the planet captured after its creation, meaning they did not form from the same material as the planet and other moons. All but one, Margaret, orbit the planet in the opposite direction of the planet’s rotation around its axis. They all have large inclination, meaning they don’t orbit Uranus above equator like major and inner moons do. Their orbits are also very elliptical, meaning the distance from the planet varies to high degree. For reasons above, they are called irregular.

They range from 18 to 165 kilometres in diameter. They were discovered only recently, from 1997 to 2003, since they are too distant from Uranus and Voyager 2 wasn’t able to spot them. The closest one is Francisco 4.2 million kilometres from the planet and the most distant is Ferdinand, almost 21 million kilometres from Uranus.

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