The Mysterious Moons of Uranus

With only two planets in our solar system without moons (Mercury and Venus) our curiosity is a little spoilt for choice. Normally when people begin to search about for information on planets with the most interesting moons dominant planets like Jupiter and Saturn tend to win people’s attention while others fall to the side-lines, destined to go unloved and unnoticed. But even the less noticed have a lot to offer so let’s uncover the mysteries of the rather small moons of the Georgian Planet Uranus.

William Herschel: It’s a law of astronomy communication that this picture must be used in any article about things Uranian. (Image credit :via Wikimedia.org)

Uranus itself is a very interesting planet so of course it should have some interesting moons. It actually has the third most moons in the solar system with 27 known natural satellites orbiting around the planet. Now these moons are actually quite small with Uranus’s largest moon Titania having half the diameter of our own moon! Uranus’s moons are also quite light, in fact if you were where to add up the mass of all Uranus’s moons it would account for less than half of the mass of Neptune’s largest moon Triton. But these moons are anything but boring! Uranus has five quite big moons that where obviously the first moons of the planet to be discovered due to their size. Oberon and Titania are the largest of the five and were found by the planet’s discoverer William Herschel in 1787. Then the moon-hunting astronomer William Lassel (1799-1880) who discovered the first moon of Neptune…just 17 days after Neptune was discovered! He discovered Ariel and Umbriel in 1851. It wasn’t until a century later when the final of the five major moons, Miranda was discovered by Gerard Kuiper. The rest of Uranus’s moons were not to be discovered until the Voyager 2’s 1986 fly by and by the Hubble Space Telescope and more advanced ground telescopes. None of the moons have been found to have any form of atmosphere most likely due to their size and lack of ability to have a large enough gravitational force to have one so I don’t think there will be any future plans to set up homes on these chilly worlds.

Uranus and Ariel seen by the Hubble Space Telescope. (Image credit:NASA-Space Telescope Science Institute)

The rest of the moons of Uranus are a lot smaller with many astronomers believing that they are actually captured asteroids and in comparison to some of the other systems of satellites orbiting around the other planets even Uranus’s major moons aren’t exactly big but they still have their peculiarities. In their naming for example, they already differ from other moons, thanks to William Herschel. When Herschel first discovered the planet Uranus in 1781 he originally wanted to name the planet Georgium Sidus (George’s star) or the Georgian Planet after King George III as a tribute to the king but unanimous disapproval across world astronomers meant that Uranus was more suitable in sticking with the tradition of naming planets after gods from Roman and Greek mythology (except for Earth of course). But the Herschel family got a mini celestial coup when William’s son Sir John originated a pattern of naming the Uranian moons after characters from the works of Shakespeare and Alexander Pope (see Comments for more on this).

We all have our faults, but Titania has more than most. (Image credit: NASA)

So as I have stated most of Uranus’s moons are quite tiny and we lack information on many of them but we can look at a few of its most interesting moons, especially the five major moons. Let’s begin with Uranus’s largest two moons, the husband and wife of Uranus’s lunar system, Titania and Oberon. Titania is the largest of the moon of Uranus and is named after the Queen of the Fairies in Shakespeare’s A Midsummer-night’s Dream. Like all of Uranus’s moons it is made of half water ice and half rocky material with some astronomers referring to these types of moons as “dirty ice balls”. The surface of Titania is covered in craters, rift valleys and faults. There are few quite large craters with the rest being quite small but what’s interesting is that many of the craters are half submerged indicating that the surface of Titania is quite young. The craters also have brightly covered ejecta material kicked up from the surface from impact just like other craters. It has a few rift valleys which can be caused by plates moving below the surface. It has one rift valley that stretches across 1000 miles (1600km). Also the existence of faults suggests that the moon of Titania was geologically active in the past as faults are fractures that can be found on planets and moons that allow movement along the surface. It is believed Titania’s surface cooled before its interior which created these many faults and rifts in the surface.

Oberon, spot the the four mile high mountain! (Image credit: NASA)

The second largest moon Oberon is named after Titania’s husband from A Midsummer-night’s Dream and so is the King of the Fairies. Like Titania, Oberon has many faults and rifts that suggest some form of geological activity in the past but it has a few more mysterious features. It has many more larger craters than its ‘spouse’ and there is a mysterious substance that lies on the floor of them with some theories suggesting it to be dirty water that has upwelled from beneath the surface on impact. Like Titania, it is also half rock and half ice but it has a mountain that reaches four miles about its surface in the southern hemisphere but the fact it is so heavily cratered and there is a lack of other surface features indicates that Oberon may have been a little more stable than his tempestuous wife.

Umbriel (Image credit: NASA)

Nest we have the dark ‘dusky sprite’ Umbriel, the third  largest moon, named after the  melancholy sprite from Alexander Pope’s epic poem, The Rape of the Lock. Like the other moons of Uranus it is a dirty ice ball but it is strangely very dark, Umbriel only reflects about half the light of Uranus’s brightest moon, Ariel and astronomers are not fully sure what causes this. Like Oberon it doesn’t have many features which indicate that it has been relatively stable since the moon’s formation. That is not say it doesn’t not have the accessory that accompanies most moons. Umbriel does have craters and a few which stands out from it’s dark surface. Some of Umbriel’s craters are much brighter than the areas around them including a crater near the top of the moon nicknamed the ‘Fluorescent Cheerio’.  Round in shape and extremely bright, this is believed to be the floor of crater which may have been exposed material from deeper under the moon’s surface which has been revealed after impact.

Ariel (Image credit: NASA)

The fourth largest moon is also the brightest, Ariel named after the mischievous sprite from Shakespeare’s, The Tempest.  Ariel is one of the two major moons that may not have had a calm and peaceful past due to the many interconnecting and deep rift valleys that cross it’s surface. Some stretch across hundreds of miles and can reach depths of 6 miles! These rifts also have ridges which are believed to have been created by ice upwelling from the interior of the moon. These deep rift valleys are very smooth on the floor which would lead us to believe some form of liquid eroded them but this could definitely not be water as water has the toughness of steel in these cold temperatures. The liquid was more likely to be liquid ammonia, methane or carbon monoxide, definitely nothing we would wish to take a swim in!

Miranda (Image credit: NASA)

Last but definitely not least of the five major moons is the jigsaw moon of Miranda. This moon is the fifth  largest moon and is the innermost located natural satellite of the five major moons. It is named after the daughter of Prospero in Shakespeare’s The Tempest. Miranda can boast uniqueness compared to other moons in the solar system, unlike any other moon its surface is like an assorted, jumbled up jigsaw, as if someone has glued pieces of a moon together in the wrong order. Like the other moons of Uranus it is made of half water ice and rocky material. It was half believed to lack much activity before its wonders were revealed by Voyager 2. It’s surface is a mixture of massive faults and strange rifts. It has extremely deep canyons measuring nearly 12 miles deep which is massive for a moon that is only 290 miles wide! It’s most stunning feature is what is known as the ‘chevron’, light and dark groves that join to form a ‘V’ shape that dominates the moon’s surface. The faults have also lead to the formation of massive cliffs which include one that is 3 miles high which is three times taller than the Earth’s Grand Canyon is deep. Originally astronomers believed Miranda was shattered multiple times and reformed but others have more recent theories that upwellings of partially melted ice are the root cause of Miranda’s violent past.

The strange terrain of Miranda (Image credit: NASA)

The five major moons of Uranus are dainty but very interesting with some of the most baffling occurrences and features but some of its smaller moons can shine on stage too. Uranus has multiple shepherd moons that help maintain its dark system of rings. Shepherd moons are just ordinary moons that are literally in the right place at the right time. They can orbit on the inner and outer areas of a planet’s ring system and help define it by hovering up any dust and debris that tries to escape. Depending on the location if it’s in the inner or outer area of the ring it will help speed up or slow down particles to aid the retention of the ring. Two of the most well-known shepherd moons in the solar system are Uranus’s moons Cordelia and Ophelia. Cordelia was named after the daughter of Lear in Shakespeare’s King Lear and is only 9 miles wide but it is fast moving and is on the inside of Uranus’s brightest ring. Its pace causes the particles to speed up and thus ‘shepherds’ the particles into a higher orbit. Outside the brightest ring’s orbit we have Ophelia, named after Polonius’s daughter in Shakespeare’s, Hamlet. Ophelia does the opposite of Cordelia with its slow pace causing the particles to slow down and into a lower orbit. This team effort causes Cordelia and Ophelia to create a well-defined, narrow and bright ring around Uranus.

The moons of Uranus are often neglected in studies but despite their lacking in size them definitely makeup for in appearance and purpose. The moons of Uranus have helped us realise that some moons help create the beautiful features of planets. The moons of Uranus still have many features that are unexplainable without future research and many still have very little known about them. There is also a possibility that there are still many more that have gone undetected. Only future observation will perhaps aid the uncovering of the mysteries of the moons of Uranus.

(Article by Kerry Scullion, Education Support Officer)