So – You’re Still Interested in Astronomy?
1. So – which particular moons are we looking at today?
Today we’ll take a look at all 27 of the known moons of the seventh planet out from the Sun, Uranus. You will note that all of them are named after characters in plays by William Shakespeare and Alexander Pope. And the reason we can cover them all is that not a great deal is known about quite a few of them. They are divided into three distinct groups. The Inner Moons, the 5 Major moons, and the Irregular moons.
2. The Inner moons
There are 13 inner moons: Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Cupid, Belinda, Perdita, Puck and Mab.
I mention all of these moons together because they are so small, and so dark, that even now we don’t know much about them apart from the fact they are all intimately connected to the rings surrounding Uranus. For example, Cordelia and Ophelia serve as shepherds to the ɛ ring, whereas Mab is thought to be the source material for the outermost µ ring.
At 162km (100 miles) in diameter, Puck is the first of Uranus’s larger moons, although it is classed among this first group.
3. What is a Shepherd Moon?
A planetary ring is a disc or ring of dust, moonlets or other small objects that orbit a planet. Sometimes, those rings have shepherd moons, small moons that orbit near to the outer edges of the rings or within the gaps between rings. The gravity of the shepherd will ensure each band remains clearly defined and distinct. So, there you go…you know what a shepherd moon is.
4. Irregular Outer Moons
All of them have a retrograde orbit – except for Margaret – and Sycorax is by far the largest of these distant moons, and lies more than twenty times further away from Uranus than the outermost major moon, Oberon.
Miranda is 470km (293 miles) is diameter, and is the smallest and innermost of the 5 main satellites. She orbits along Uranus’s equatorial plane (which lies on its side, so it looks as if things orbit from north to south over the poles). (See main Uranus pic & her rings)
As you can see from the picture, Miranda has the most extensive and varied topography of all the Uranian moons. She also possesses a 20km high cliff – the highest in the Solar System – and strange chevron shaped tectonic features called coronae
With a diameter of 1,158km (719miles) Ariel is the 4th largest Uranian moon and is thought to have formed from the accretion disc that surrounded the planet following its formation. Like her parent, she orbits on her side, (relative to rotation) and as such, experiences severe seasonal variation.
Umbriel is 1,170km (727 miles) in diameter and is covered in impact craters. So much so, that she is the 2nd most heavily cratered moon after Oberon. A feature that sets this moon apart from all others, is that from space it looks as if she’s wearing little cap and you can easily spot the a ring of bright material lining the bottom of the Wunda crater along the equator – which, if you remember, because she orbits on her side, looks as if it sits on the north pole.
See? A true world of Wunda!
Titania is the largest moon belonging to Uranus, possession a diameter of 1,578km (981 miles) and she is named after the fabled queen of the fairy folk.
Titania is an interesting place, and has numerous craters measuring up to 326km (203 miles) in diameter. She also lies within Uranus’s magnetosphere. This is important, because the trailing hemispheres of satellites orbiting inside planetary magnetospheres are struck by magnetospheric plasma, which co-rotates with the planet itself. This bombardment is thought to lead to the darkening of the trailing hemisphere.
Titania has a rocky core and ice covering. It is thought she possess a 50km thick liquid ocean between the core/mantle boundary.
At 1,523km (946 miles) this outermost moon of Uranus is the second largest and is named after the king of the fairies. It formed from the accretion disc surrounding Uranus as it coalesced. With a rocky core and ice covering, the surface is covered by craters measuring up to 210km in diameter, and spends a significant amount of its time outside Uranus’s magnetosphere. As a result the surface is scoured by the solar winds, creating the second darkest surface after Umbriel. Oberon has the reddest surface, thought to be due to a combination of source material, and the constant bombardment of micrometeors.
Oberon possesses two main surface features. Besides cratering, it is covered by chasmata features – very large canyons.
10. Any other interesting facts?
There certainly is. The five largest moons are massive enough to have achieved hydrostatic equilibrium, and four of them show definite signs of internally driven processes such as continued canyon formation and volcanism.
And you might have remembered me mentioning that Uranus and her moons orbit on their sides? Let me explain what that means.
When you look at Uranus, the line running from North to South Pole actually runs through what we would call the equator. So, instead of spinning round and round like other planets, Uranus spins – for want of a better way of explaining it – up and over. Her moons follow that equatorial plane, so it looks like they’re going up and down instead of round and round.
Now, what this means that the planet and moons are subject to extreme seasonal cycles. What do I mean? Well, imagine a summer 42 years long. And when I say summer, I mean constant sunlight. Some might like that, but remember the downside…summer is followed by winter. (Perhaps that’s what Ned Stark was warning us about? 42 years of winter darkness – ouch!)
So there we go. That’s our look at the moons of Uranus.
Next time, will take a look at Neptune
See you then…