Caoimhín's Content

In this video, although we begin where we left off last time, floating high in the atmosphere of Saturn, we’re going to quickly transfer ourselves to Jupiter. Jupiter is that bit closer to the Sun than Saturn, which should give us a slightly better view of the inner planets. The inner planets are Earth, Mercury, Venus, and Mars, they’re all considered the inner planet even though they’re not all inside of our orbit, they are all rocky planets and they are all inside the orbit of the gas planets. Viewing the night sky from Jupiter, ther first thing you may notice is a bunch of moons, but no ring. Jupiter does have a ring, but it is certainly too faint and thin to be visible from the Earth, and there is a good chance that Jupiter’s ring wouldn’t even be visible from the atmosphere of Jupiter. We are again on a gas giant, so we need to be high in the atmosphere to see space, without too much gas getting in the way. This also gives us a similar view of the daytime sky as we would get on our Moon, allowing us to see the Sun and the planets and stars all at the same time.

It is certainly a lot easier to see the inner planets from Jupiter than from Saturn, but it can still be tricky, for example Mars still looks very very faint. Mars is not only smaller than the Earth and Venus, it doesn’t have much of an atmosphere. It does have a thin atmosphere, but nothing as reflective as the clouds around both Venus and the Earth. Mercury looks even brighter than Mars, despite its much smaller size and it’s lack of an atmosphere at all! Mercury is a lot closer to the Sun than Mars, so the intensity of the light hitting it is just a little higher. The paler grey colour of Mercury may also reflect a bit more light than the dull rusty colour of Mars. Mars can also get a lot further from Jupiter than Mercury can. It can get closer as well, but when it’s on the same side of the Sun as Jupiter it is the nighttime side of Mars that we would see, which is of course much darker. Mercury does occasionally get further away from Jupiter than Mars, but never as far away as Mars can get at its furthest. On average Mercury is the closest planet to every other planet, as Mercury can only ever be around the Sun and the furthest it will get away from any other planet is the distance to the Sun plus Mercury’s orbit, where as the furthest all the other planets can get is the distance to the Sun plus their orbits, which are always much bigger than Mercury’s.

Although the inner planets are quite far away and a little tricky to see, the view from Jupiter does give us a great view of its moons. The Galilean Moons in particular loom huge in the sky, but we normally only see a few at a time, and often one or more will be on the other side of the planet. However, we can take a look at Jupiter from a different perspective, which will allow us to see all of the larger moons. We will of course take a look at Saturn from the same perspective in the video as well. I mentioned in the previous video that Saturn has over 100 moons at the last count, it’s gone up to 146 as the time of writing. Jupiter has slightly fewer, just 95, but this number may increase. The bigger planets in the solar system have more gravity, they have a stronger gravitational pull, or a deeper wider gravity well around them, giving plenty of room for moons. This includes many very small moons. Small moons close to the planet will often be caught as people observe the planet itself, but small moons that orbit the planet distantly, outside the larger moons, are far trickier to spot and are still being discovered.

Looking down on Jupiter, with the orbits of the moons visible, we can see many orbits tracing roughly concentric rings around the planet. If we only observe the major moons and moons closer to the planet, the orbits will look especially regular, and getting so close together that some moons virtually share the same orbit. Further out, many of the orbits become eccentric, looking a lot more like the orbits of comets and often overlapping. Some of these orbits are also very inclined, practically going from pole to pole rather than around the equator. We see the same pattern in the solar system, with the small and often distant comets as well asteroids in the Kuiper Belt and Oort Cloud. This is not the only similarity between the moons orbiting gas giants and the planets orbiting the Sun. Just like the planets, the moons closer to the planet orbit more quickly, while the further out ones orbit a bit slower. Even just looking at the four Galilean Moons, we can see that the closest one to Jupiter, Io orbits the fastest, followed by Europa, followed by Ganymede, followed by Calisto on the outside.

One difference of course is that many of these moons can pass into the shadow of their parent planet, causing them to fall into darkness, making night and day a little bit more complicated. They also usually orbit their parents a bit faster, with many orbiting faster than our moon orbits us. Some of the more distant moons may take a bit longer, a few years, but comparably distant solar system objects can take centuries. The systems of moons around Saturn and Jupiter, the Saturnian and Jovian systems respectively, are a little bit like shrunken, sped-up solar systems, and this gives us a chance to see interesting orbits, as they do not replicate our solar system exactly. Some of the moons have highly irregular orbits, including truly retrograde orbits. Some of the distant moons can have highly eccentric or inclined orbits, as mentioned above.

Moving on to Saturn, we can see some more interesting features. Saturn’s largest moon Titan is certainly the most visible, while Iapetus is smaller and outside it, but still much larger than the smaller distant moons, and Hyperion is also smaller and just inside Titan’s orbit. Closer to Saturn again we find the second largest moon Rhea. Rather than the largest moons being next to each other, like Jupiter’s and the largest planets in the solar system, these ones are separated by smaller intervening moons. Saturn’s ring and the high amount of dust around Saturn in general also add some distinct features, helping the Saturnian system to show us something both like and very different to our solar system.

One thing we can see is moons sharing the same orbit. Tethys and Telesto are one example, but rather than the gravity of other moons keeping them in the same orbit, as I originally said in the video, it is down to Tethys and it’s Lagrangian Point 4. I will deal with Lagrange points in a future video, but we don’t see a relationship quite like this in the solar system, with a such a comparatively large object in the Lagrangian point of a planet, although Jupiter does have a cloud of asteroids in some of its Lagrange points. Tethys also has a little moon, Calypso, trailing behind it in a similar way. Two more of Saturn’s moons, Janus and Epimetheus, orbit each other in such a strange way it almost deserves its own video. If we look close enough at the rings, we’ll even see some moons within the rings! There are also some small moons zipping around just outside the rings, and together these are often called Shepherd Moons because they shepherd the rings into shape. In the video, you can see that Pan is inside the ring, within a little gap between the rings of Saturn called the Encke gap. This is one of the small gaps close to the outer edge of the rings we can see. This gap, along with other small gaps are kept clean by little moons like this. However, the larger gaps closer in to the planet are caused by the larger outer moons, like Titan. Dust and rock in certain positions will be in orbital resonance with Titan, it will want to orbit a certain amount of times for every time Titan orbits. This stable orbit will pull dust and rock out of less stable orbits nearby, leading to gaps.

With so many interesting orbital phenomenon, there will of course be plenty more to say when we take a look at each of these planets in a dedicated video in the coming months. We’ll be returning to the solid ground of the Earth in the next video to take a look at our own, more familiar sky, so I hope you’ll return then.