ASTR-201 Lessons

2.8: Moons of Saturn, Uranus and Neptune

Having discussed the moons of Mars and Jupiter, we now turn to the moons of the remaining outer planets in the solar system. Also, while we’re in the neighbourhood, we’ll make it a point to talk about Saturn’s rings as well. Of course, this means these moons and other features are even farther from Earth, but you’d be surprised by what they can affect, even subtly and slightly. And, of course, knowledge for its own sake is a worthy pursuit as well!

Moons of Saturn

We’ll kick off today’s lesson with Saturn, named after the Greek god of the harvest. This giant has an ongoing competition with its neighbour, Jupiter, for the greatest number of moons. Currently, the gas giant has 53 confirmed moons, with 29 that have yet to be confirmed! With such a large assortment, there’s almost guaranteed to be something of interest to everyone! 

The biggest is Titan, a moon whose size completely dwarfs most other moons and even planet Mercury, though it just barely comes in second place to Jupiter’s Ganymede.  It orbits Saturn at an average distance of 1.22 million km, which it finishes in nearly 16 days. As you’re likely tired of hearing by now, its rotational period is also the same, as it too is tidally locked to its planet.  Its atmosphere, however, is quite unique.  Titan is the only moon in the solar system with a substantial atmosphere; it exerts 1.45 times as much pressure on the surface as the Earth’s atmosphere does.  It is composed mostly of nitrogen with some methane and traces of other gases, which makes the moon appear orange and opaque, giving Titan a very striking appearance. 

Titan in true colour.  

Source: here

However, despite its exotic beauty, we understandably wanted to know more about what it looks like underneath the vivid atmosphere. And in 2004, our wish was granted! A spacecraft named Cassini was finally able to fly close  enough to discover that Titan’s surface is mainly rocky but also contains some water ice, lakes and rivers of liquid methane, and  clouds that rain methane onto the surface.  Quite the unique environment, no? A year later, a probe was even able to land on its surface and take pictures, one of which I have reproduced below.

Surface of Titan.

Source: here

Now, that’s all very interesting, but what about magic, you may be asking? Well, magic can’t pass through Titan’s clouds, because methane, as concentrated as it is in those clouds, actually blocks it! Because of this, Titan reflects some of the Sun’s magic onto Saturn’s atmosphere (though not as much as our Moon reflects to us, because Titan is much farther from the Sun than our Moon is).  Similarly, Titan’s internal magic also can’t escape through the clouds, meaning there’s plenty of it on the surface, and it’s all wild.  If you were to ever go there, you wouldn’t be able to cast any spells reliably.  On the other hand, Cassini has observed anomalies in one of Titan’s lakes - bright spots about an inch in diameter that come and go.  Muggle astronomers jokingly call them magic islands.  They insist that the anomalies are only bubbles, but some wizarding astronomers have reason to believe that there may be life on Titan in the form of a magical plant or fungus that can vanish and reappear and that feeds on methane, so magic islands may just be the right name for them. 

While you all know I’d love to stay here all evening and discuss each and every one of Saturn’s moons with you in as much detail as Titan, the headmistress has insisted that you apparently need sleep more than random moon facts! So, in an effort to condense some of the lesson, I’ve provided you with a chart with the seven biggest of Saturn’s moons. All of these are more or less spherical in shape, so the table includes the diameter, orbital radius, and orbital period for each.

Additionally, because many of us also learn visually, here is a picture of those moons (and some of the smaller ones in the background for scale).  From left to right, the bigger moons are Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus (as well as the giant orange Titan in the background).  If you find yourself intrigued by these beguiling moons and would like to know more, this image was taken from a text in the library full of fascinating facts. I’ll include just a few highlights to whet your appetite. Iapetus, as you can see on the far right hand side, has a dark-coloured side and a light-coloured side, which makes it look quite different from the rest of its siblings. Enceladus, aptly named after the Greek giant responsible for volcanic eruptions, has incredible tectonic activity, much like other moons mentioned last week. Of particular interest is the fact that it shoots geysers of liquid water up from its south pole, making it another prime candidate for life under the ice!  Also, interestingly enough, Enceladus has the highest albedo of any body in the solar system, coming in at 0.81.

Saturn’s biggest moons and some smaller ones.  

Source: here

Saturn’s Rings

As mentioned in Lesson Seven of last year, Saturn is surrounded by magnificent rings, each 200 to 3,000 metres thick.  These rings are composed of chunks of water ice and rock, which range in size from that of a grain of sand up to that of a house! You may be wondering, however, just what they have to do with the various moons that orbit the planet. I assure you, there is quite the overlap. Indeed, the rings would likely not still be around if there were no moons - in particular, if not for the “shepherd” moons. Despite being on the much smaller side, these moons are very important. They are named as such because they orbit between or around the rings, and their gravitational influence helps shape, or "shepherd," the rings into the perfect orbits that we recognise. These moons prevent the individual particles that make up the ring system from drifting off into an unorganised mess and help to keep the rings separated in their individual paths.

Saturn’s Rings

Source: here

These five shepherd moons include Atlas, Pan, Daphnis, and Prometheus and Pandora. Each has their own specific job, namely ushering the A ring (the largest and first discovered of Saturn’s rings), Encke Gap, Keeler Gap, and F ring, respectively. How do they do this, you might ask? Well, it differs slightly from ring to ring (and moon to moon), but when it comes down to it, the main player is gravity. As you can imagine, there are a lot of gravitational forces acting in a confined space when you consider Saturn itself has a gravitational force, as do all of its 53+ moons and the larger chunks in its rings. Suffice it to say that it’s a delicate gravitational ecosystem, which makes the beauty of the rings all the more breathtaking! 

The impact that these moons have on the magic that passes to and from the Ringed Planet is quite interesting as well. While they are too small and much too far from the surface of Saturn to reflect any significant magic themselves, the way they shape the rings influences how the rings affect the magic reflecting off of the planet. We’ll discuss this some more in Year Four when we talk about Saturn more in depth, but depending on your viewing angle to the planet, the magic reflected onto and off of Saturn may need to pass through the rings. As you can imagine, this pass through the rings disrupts the flow of magic, some of it getting scattered off by bits of rock and some of it passing through pieces of ice. Fascinating, isn’t it?

Moons of Uranus

Like Jupiter and Saturn, the next two planets we will discuss also have many moons as well as rings. However, these rings are quite faint, much more like Jupiter’s than those of Saturn. In fact, most people do not realise that these planets even have rings, thinking that this honour is solely reserved for Saturn. But, enough of my blathering, let’s get to it. 

We’ll start with Uranus, which has 27 known moons. Its two largest, and as you know by now, the most likely to have a magical impact on us or the planet they orbit, are Titania and Oberon. And the name Titania isn’t just for show! This large celestial body is the eighth largest moon in the solar system (though only half the size of our Moon). These two moons were actually so large, they were discovered far back in the 18th century, without the need for the advanced technology we have today. The smaller ones, however, were discovered much more recently, owing to the need for more advanced telescopes (or even probes) to see them.

Uranus's six largest moons

Source: here

Above is a picture of a bit of Uranus and its six largest moons to scale. Helpfully, they are also shown from left to right in increasing order of their distance from Uranus. Starting at the left, we have Puck, Miranda, Ariel, Umbriel, Titania, and Oberon.  Their surfaces are composed of rock and water ice.  They all reflect a bit of magic onto Uranus’s atmosphere, proportional to their size, composition, and distance from the planet (with larger and closer moons reflecting more and smaller and more distant moons reflecting less). However, because of a variety of factors, including most of the larger moons being further away, none of these moons are predicted to reflect as much magic as our Moon does.

Another interesting thing to keep in mind about the various celestial satellites and their ability to affect the magical ecosystem is their distance from the Sun. As we have been moving further and further out into the solar system, the moons orbiting the planets overall have less and less effect on the magic systems of these planets. Of course, being that they are devoid of life, calculating how much magic would be reflective is purely hypothetical, but still interesting nonetheless. Also, you never know what we could discover tomorrow. But for now, back to the present and concrete facts. Yes, the moons of the outer planets are of less importance to the overall magical scheme of things here on Earth, so oftentimes they are completely discounted, but never forget that these planets have many more moons than we do; what they may lack in size and proximity, they make up for in numbers. It certainly makes calculations more complicated!  

Ah, but look at the time! If I have any hope of covering Neptune before our time is up, I’d better wrap up our discussion of Uranus’s moons. I’ve handed out a table of the key details of the six most important (and largest) of the moons covered. 

Moons of Neptune

Finally, we come to Neptune and its modest 14 moons. Of course, that’s far more than we have, but in comparison to the heavy hitters like Jupiter, Saturn, and Uranus, this number may look small in comparison. We’ll only be covering one of its moons today, but it certainly packs a punch. Its largest moon, Triton, is more than 99% as massive as all of its other moons combined!  Its diameter is about 2,705 km - nearly as large as our Moon - and more than six times as big as the planet’s second-largest moon, Proteus.  Its orbital period is actually quite short, coming in at only 5 days and 21 hours. But, that’s not the most interesting thing about it… it orbits Neptune backwards! That is to say, it orbits in the opposite direction of Neptune’s own rotation, which is quite unique. Its surface is about 2/3 rock and 1/3 ices of various sorts: nitrogen, methane, carbon dioxide, and water. And, as if that wasn’t enough, its surface is dotted with geysers that spray liquid nitrogen! 

Aside from all these very interesting astronomical facts, it’s also pertinent to note that we’ve finally ended with a moon that’s big enough to reflect some magic onto Neptune! While we just covered the fact that this is highly unlikely due to its great distance from the Sun, Triton makes up for it by virtue of its sheer size. Sadly, Neptune’s other moons are too small to reflect a significant amount and we are short on time, so I’ll wrap up our discussion here. 

Triton. 

Source: here

Concluding Remarks

And with that, another Astronomy lesson comes to an end. I hope you enjoyed broadening your horizons and learning about the moons and rings of the other planets of the solar system.  It’s been quite the marathon, hasn’t it? In good news, next week will be a bit of a break from the barrage of facts, names, and numbers. Instead, during the next lesson, we will return to Earth’s Moon and discuss the many stories from various cultures that exist to explain its origin.  However, the bad news is that you will sit for your final exams for the year! I hope this news isn’t too shocking, as I’m sure you knew it was coming, but in case it was a surprise, consider yourself warned and given an opportunity to study! To aid you in reviewing the material from this lesson, I’ve included our typical ten-question quiz.

Original lesson written by Professor Turing

Part of this lesson written by Professor Plumb