It is getting cold and sunny here after a cloudy few weeks, which means the nights are clear for astronomy. It can be difficult to use a telescope or camera outside in the cold because lenses fog up easily, but I experimented with my camera some to get this picture. This was with my 75-300 mm telephoto lens. This is really a situation where the camera was just better than our eyes, because even at that relatively low zoom level, I could pick up the four commonly visible moons of Jupiter: Io, Europa, Ganymede and Callisto.
The Wikipedia pages I linked there are pretty interesting to read through in their entirety, and I would definitely recommend reading through them some more if you are interested, but I noticed each moon seems to have some unique feature, so I will talk about them a bit below!
This is Ganymede. The largest moon in the Solar System, it has a slightly larger diameter than Mercury but with less than half its mass. It is composed of silicate rock and water ice, and is the only moon in the Solar System known to possess a magnetosphere, created by its liquid iron core.
Next up is Callisto. This moon is the second largest in the Jovian system. It is also extremely close to the diameter of Mercury, 99%. It has only a third of Mercury’s overall mass though. Its surface is the oldest and most heavily cratered in the Solar System, with no signs of volcanism. It has some very impressive impact craters though.
This is Europa, a really fascinating one. Europa is a bit smaller than our Moon, and is mostly composed of silicate rock, but its surface is mostly frozen water. There are few impact craters, but the major surface structures are cracks. In fact, it has the smoothest surface of any solid object in the Solar System. There is a hypothesis that there is a ocean of liquid water underneath its surface, which might be able to support life. And, in 2014, NASA found evidence for plate tectonics in its ice shell, which has not been seen on any other body than Earth.
And, last of all, we have Io, the innermost moon and the fourth largest. It is the driest object in the Solar System, but has more than 400 active volcanoes, making it the most geologically active object in the Solar System. The tidal interactions between Io, Jupiter, and the other Jovian moons lead to a lot of heat in its interior. This also leads to many mountains, some of which are taller than Mount Everest. Check out the Wikipedia article I linked to earlier on about Io to see one of its volcanoes erupting (under Volcanism), they are truly massive and unique.
These facts are all taken from the Wikipedia articles on each moon. If you are interested, I would recommend starting with those articles and reading some more.
Oh, and if you have a pair of binoculars, try looking at Jupiter sometime! It is visible in the sky right now if you live in Minnesota, one of the brightest objects rising in the East. Venus is brighter but sets in the West with the Sun. Later in Winter they will be closer together, but you can tell them apart through binoculars or a telescope because Venus just looks like a very bright spot, almost like a star. With a pair of binoculars, you can probably see Jupiter’s moons for yourself, while there are no such moons around Venus. Otherwise, look up when you can see Jupiter for wherever you live.
Thanks for reading!