Let me introduce you to some of the most exciting objects discovered in our solar system! All were discovered by Galileo hence why they are referred to as Galilean moons. I have listed them in the order they orbit Jupiter.
Io is the most volcanic object in our solar system hosting around 400 volcanoes erupting sulfuric plumes over 500km high. Io is the closest moon orbiting Jupiter and is also very close to the second moon, Europa. It is being pulled strongly from two opposite directions causing tidal heating and therefore lots of eruptions occur. The volcanoes create a very thin atmosphere and we recently discovered that it collapses regularly when the moon is shadowed by its planet.
Europa has an icy crust but underneath it is suspected to be an ocean filled with water that could potentially hold life. From the picture below you can see what looks like scratches on the surface. We believe this is because Europa has a similar geological structure to the tectonic plates we have on Earth. It has a very thin atmosphere consisting mainly of oxygen and because of its icy surface it is highly reflective.
Ganymede is the largest moon in the solar system and is even larger than Mecury. It has a mixed terrain and about 40% of the moon is covered with highly cratered dark regions which are thought to have come from asteroids or comet collisions. It is the only moon that we know of in our solar system to have a magnetosphere. A magnetosphere is a region surrounding an object where charged particles are trapped or deflected. This is usually seen in planets.
Callisto is the most heavily cratered object in the solar system. Although these are old impacts and nothing is thought to have changed since Callisto was originally created. It does not show any signs of any other geological activities. Callisto has a weak atmosphere consisting mainly of carbon dioxide.
All the moons were discovered in the same year. In fact, all the moons were discovered on the same night. Io would have been discovered a night earlier had Galileo been able to distinguish properly between Io and Europa, it wasn’t until the next day he realised they were two separate bodies.
It is said that life could thrive on any of these moons if a ocean is found underneath their crust’s. Halophile’s, which are organisms that thrive on a salty environment, are thought to be the first form of life to evolve if any turn out to be habitable.