ASTR-201 Lessons

2.7: The Moons of Mars and Jupiter

In this, our year about the Moon, I would be remiss not to take the time to mention the celestial bodies that revolve around our neighbours in the solar system. So, we are taking time to do just that! While knowledge of the facts and effects of these moons may not feel as important (since they impact planets that are tens of millions of kilometres away from us), they are interesting nonetheless. I would also hasten to remind you that “nothing exists in a vacuum,” or that everything around us has an effect on us, even if we are not sure how. But enough contextualising; let’s get started.

A natural division when talking about the planets is to split things into the inner and outer planets, so you might wonder why we are not discussing Mercury, Venus, or Earth today. However, if you recall from Lesson Seven of Year One, Mercury and Venus have no moons, and we have already discussed Earth’s one Moon at great length.  By contrast, the other planets have many moons between them, too many of them to discuss in one lesson. Therefore, in this lesson, to split it up more evenly, we will discuss the moons of Mars and Jupiter, and in the following week we will discuss the moons of Saturn, Uranus, and Neptune, as well as Saturn’s rings.

The Moons of Mars

We’ll start right off with our neighbour to the outer part of the solar system, Mars. Its two moons are called Phobos and Deimos, which mean fear and panic or terror in Greek, respectively. They were named after the twin sons of Ares, the Greek god of war, whom the ancient Romans later called Mars.  It’s not entirely clear just how much their discoverer, Asaph Hall, knew about the astronomical effects these two bodies are purported to have, but more on their magical implications later! 

For now, let’s get to know a little bit more about their physical composition. The basics are not too surprising. Both moons are made of rock and are heavily cratered, which I imagine sounds very similar to our own Moon. But let’s take a close look. If you direct your attention to the front of the room, you will see images of Phobos and Deimos, both taken by the Mars Reconnaissance Orbiter (a satellite launched in 2005 to study the geology of Mars). How are these two moons different from the Earth’s moon? The most noticeable difference that can be seen from these pictures is that both Phobos and Deimos are not round; rather, they are shaped like potatoes.

One thing you can’t tell from the images is actually something quite interesting - their relative size.  Not only is Phobos much closer than Deimos to Mars, but it is also much bigger. In terms of size, Phobos is almost twice as big as Deimos in diameter (22.2 km compared to 12.4) and about seven times larger in terms of mass. Although Phobos (the moon closer to Mars, remember, and thus with a shorter path to travel) orbits the planet approximately four times in the time that Deimos orbits Mars once, both Phobos and Deimos are tidally locked to the planet just as the Moon is tidally locked to the Earth. This, of course, means the same portions of both moons face Mars during the entirety of the moons’ orbits and makes the reflection of magic much more predictable.

Speaking a little bit more about their astronomical paths, Phobos orbits Mars only about 6,000 km above the surface of the planet in about seven and a half hours, which is less than the time it takes Mars to rotate about its axis (about 24 and a half hours). As such, it appears to rise in the west and set in the east from the surface of the planet.  To put it in perspective, Phobos is the lowest orbiting known moon in the solar system, and it is slowly getting closer and closer to Mars, to the point that it is expected to crash into the planet in about 50 million years.  Deimos orbits Mars just over 20,000 km from the surface of the planet in 30.3 hours.

Phobos.                                        Deimos.  

Source: here                                  Source: here

And now, as I alluded to earlier, we finally get to the magic and the reason these moons are included in the curriculum here at Hogwarts. Let us begin by talking about the magical effect that Phobos and Deimos have on Mars, rather than immediately jumping to our own planet. While we haven’t yet been able to observe the effect of their magic on Mars from Earth, we are actually very certain that these moons have a magical effect on the Red Planet owing to the many similarities between the pair and our Moon, namely their rocky composition and their tidal lock. It’s also worth noting that they are much closer to their planet than our Moon is to ours, which greatly increases the amount of magic successfully reflected (though this is counteracted by the fact that they are significantly smaller than our Moon).  This means, of the two moons, Phobos - being the larger and closer one to Mars - has a larger magical impact on Mars than Deimos does.  

However, there is something unique to Mars, its moons, and their magical ecosystem. While we only have one Moon, Mars has two. What might happen because of this? Well, because they do not have identical orbits, nor are their orbital periods the same, they are very rarely in sync with each other. When the moons appear close together from a viewpoint on Mars, the magic they reflect will interfere constructively, significantly amplifying their magical effect. However, in cases when they get close enough that  Phobos occults Deimos, the amount of magic reflected will lessen, as Deimos’ magic is blocked by Phobos. On the other hand, when they appear far apart (again, from a viewpoint on Mars’ surface), their magic will interfere destructively, significantly decreasing the overall magical energy available. That is, as long as both moons are above the horizon at that location. I bet you’re glad Hogwarts isn’t on Mars! We haven’t even got into the effects the phases have. Suffice it to say that a two-moon system (when both moons reflect magic) is vastly more complicated than ours, which is fortunately very predictable in comparison.

Now, that brings us to the effects of these moons’ magic (or, more accurately, how they modify the Sun’s magic that they reflect) on us here at Hogwarts and in the rest of the world. By and large, because of their small size and their immense distance from Earth, it’s nearly impossible to study their magical fluctuations with any certainty. Moreover, it is incredibly difficult to isolate the magical effects of any one body on the Earth, as there are often multiple things influencing us at once. However, to gain a little insight into their possible effects, we can go back to various ancient societies to examine their naming conventions. In almost every culture that studied or named Mars and left records, the planet has been named after gods of death, war, disease, or all three. Sadly, as far as we can tell, none of these civilizations were aware of Mars’s moons, owing to how close they are to the planet and how difficult it is to see anything when obscured by the planet’s reflected light. Regardless, many view these associations, as well as Mars’s own effects, as a sign that these moons likely have similar effects on the magic they reflect. Some even point to the moons as the very reason Mars has so strong an effect. Their true effects are likely something we will never know until we can survive on Mars and study the magic up close and personal, but I can say with certainty that I wouldn’t like to be standing on Mars’ surface when the two are in syzygy!

The Moons of Jupiter

Next we move into the outer planets to study the four largest moons of Jupiter, called the Galilean moons after their discoverer. These are, in increasing order of distance from Jupiter: Io, Europa, Ganymede, and Callisto. With so many to cover, I’ll do my best to keep this brief, but informative.

Io

Our first stop, Io, is just a mere 421,800 km from Jupiter’s surface. Named after one of the many lovers of Zeus (or Jupiter, as the Romans called him) and the mother of many famed heroes you might one day hear of in Mythology class, Io is a bit bigger than the Earth’s Moon – about 3,640 km in diameter - and completes its tidally locked orbit in only about one day and 45 minutes. Interestingly, this puts Io at a two-to-one ratio of rotations when compared to its neighbour Europa, and a four-to-one ratio when compared to Ganymede. The forces that these moons exert on Io, when in concert with the forces Jupiter itself exerts, keeps Io’s orbit eccentric (or non-circular), which in turn causes a lot of interesting things to happen on the surface. The most important and pertinent of these are the many active volcanoes on the moon. In fact, Io has the most active volcanoes of any planet or moon in the entire solar system! To get an idea of how active, take a look at the image at the front of the class, where you can see Io in true colour.  The red parts of the picture are active volcanoes.  

Io’s magical effects are unique among those of any planet or moon in the solar system. Apart from reflecting a significant amount of the Sun’s magic onto Jupiter, Io is a rare beast in that it releases some magic of its own because of its incredible volcanic activity. As we learned in Year One, active volcanoes speed the release of magical energy from the interior of a planet (or moon). All this means that, even though Io is quite distant, the total amount of magical energy coming from it is enough to be noticed from Earth. It has been found to increase the unpredictability of magic, as well as backfires, when its AME is high. If we can feel its effects here, you can only imagine the powerful effect it must have on the surface of Jupiter itself! 

Io.  

Source: here

Europa

Next, at 671,100 km from the planet, we come to Europa, named after another of Jupiter’s conquests - this time a Phoenician princess. Europa is the smallest of the four Galilean moons with a diameter of about 3,120 km.  As mentioned earlier, it has a slightly longer tidally locked rotation than Io at about three and a half days.  It is also geologically active, causing any mountains or craters that form to be quickly eroded.  It is covered with water ice (recall that in space many things can be frozen into ice, so it is important to distinguish between the various things that have become frozen and turned into what we would consider “ice”), and scientists believe that Jupiter’s tidal forces melt that ice, meaning that there is likely an ocean of salt water beneath it.  The possible existence of liquid water makes Europa one of the prime candidates in the search for extraterrestrial life in the solar system, but any human explorer had better be shielded from Jupiter’s radiation, as it’s strong enough to kill you in only one day!  

Now onto the magic! You may think that because this moon is very tectonically active (and volcanoes themselves are a type of tectonic activity) that Europa is also a prime candidate for magical reflection onto both Jupiter and Earth, but that’s not actually the case. Recall that I mentioned the large amounts of water ice and likely liquid water all over the moon’s surface - this is the reason why Europa reflects much less magic than expected or than its rocky counterpart, Io. We briefly discussed the “wildness” of the Sun’s magic back in your first year and how the water in Earth’s atmosphere “tames” it, or makes it more predictable and manageable. Well, this too also dampens the intensity of the magic (pun intended). Therefore, water is not a terribly good surface for reflecting magic, making Europa a small fry in terms of the magical side of things.

Europa.  

Source: here

Ganymede

The third moon in this list is, you guessed it, named after yet another one of Jupiter’s many dalliances. While the prince of Troy was not unique in attracting Jupiter’s interest, Ganymede is unique in quite a few ways. For starters, Ganymede is Jupiter’s largest moon and indeed the largest moon in the solar system with a diameter of about 5,260 km, making it even larger than Mercury.  It orbits Jupiter at a distance of about 1.07 million km in about seven days and six hours, which is the same as its period of rotation, as it too is tidally locked to the planet.  Like Europa, it is covered with water ice, so it also reflects less magic onto Jupiter than another surface would. Its last feature of note before we move on is its magnetic field. While most planets have their own magnetic field, no other moons in the solar system do, making Ganymede an interesting study, even if its field is quite small in comparison to Jupiter’s. It is believed that this magnetosphere has less to do with the moon’s hefty size, and more to do with the composition of its core (which is liquid iron) as well as the forces exerted on it by Jupiter.

Ganymede.  

Source: here

Callisto

After all that, we finally reach Callisto. In size, it’s a bit smaller than Mercury – about 4,820 km in diameter - making it the second largest of Jupiter’s moons.  It orbits in a tidally locked rotation around Jupiter at a distance of 1.88 million km in about 16 days and 17 hours.  Its surface is made of water ice and rock, and the ice and rock are peppered with so many craters that almost every inch of its surface is covered.

Sadly, while this moon is interesting to learn about and look at, it too has only a little impact on the magic reflected onto Jupiter, at least in comparison to the showy Io. While its surface contains less water, it does comprise a large portion of it, and it is also quite far away from Jupiter in comparison to the others. 

Callisto.  

Source: here

Jupiter’s Other Moons and Conclusion

Phew, that was quite the marathon! However, if you thought four was a lot, it bears mentioning that, as of the day of this lesson, Jupiter has a whopping 53 moons that have been officially confirmed. If we are speaking of the number of suspected moons (which includes celestial bodies that have yet to be proved/confirmed), that number jumps to 79!

However, conveniently enough, the four we talked about are the most important from both a magical and astronomical standpoint. The others by and large are too small and too far away to have much, if any, of a magical effect on the planet. For the sake of completeness, though, I’d like to just briefly mention a few: Metic, Adrastea, Amalthea (the next largest of any of Jupiter’s moons), and Thebe are closer to Jupiter than Io is, and they all revolve in the same direction as the Galilean moons.  The other small moons are much farther away from Jupiter than Callisto, and some of them revolve in the opposite direction from the Galilean moons. They all have highly eccentric orbits, some of which are greatly inclined to the orbits of the Galilean moons, so if they did have a larger magical impact, you can only imagine the complexity of their magical interactions!  

And that’s the end of another lesson, which, while jam-packed, will only be tested with a ten-question quiz.  But be warned, the number of questions does not necessarily correlate with the ease of the assignment! The next lesson, which will deal with the moons of Saturn, Uranus and Neptune, will be a treat to look forward to, I assure you!.

Original lesson written by Professor Turing

Part of this lesson written by Professor Plumb