Io, the innermost Galilean satellite, is an active fiery world. Io's diameter is a bit smaller than our Moon's,
and has a rocky surface and a large iron core. It is the most volcanically active and colorful place in the Solar System.
Scientists have seen Io's volcanic eruptions spewing material up to heights of 190 miles.
Most of that material falls back to the surface, changing the surface color.
It also makes rings around the active volcanoes that can be white, yellow or red, due to the content of sulfur dioxide.
Europa is an intriguing icy world, with a surface that resembles a frozen lake containing numerous cracks and ridges,
some of which are hundreds and even thousands kilometers long. While the bluish formations on the surface indicate old icy plains,
the reddish regions along the ridges are young and contain non-ice components.
Scientists think that the reason for this strange appearance is tidal forces,
which stretch the surface, producing cracks. Such tidal forces could cause material from the interior
to be squished out, forming ridges. This tidal heating also is the main reason for the existence of the
subsurface ocean under the icy cap. This ocean makes Europa one of the most viable places for life in the Solar System.
Ganymede, the third Galilean satellite, is completely different. Most of its dark surface is covered with craters,
which are flatter than those on most rocky planets and moons. The reason is that, over time, ice on Ganymede flows
like a glacier.
Beside craters, there are also smooth regions with ridges and grooves that are probably the
result of tectonic activity.
These areas are younger than the cratered regions, but older than the formations on Europa.
Ganymede might also has a subsurface ocean; however, if it does it lies deep under the surface.
Callisto, the most distant of Galilean moons, is almost the same size as Mercury. Although it is comparable in structure to Ganymede,
it has a completely different appearance. Its crust is a mixture of ice and rocks.
The old dark surface is covered with impact craters and a thick dusty layer. The craters differ in age and size.
The larger craters are surrounded with multiple rings. There are no signs of tectonic activity or of a subsurface ocean.
The strong gravitational force of Jupiter combined with the tidal resonance between
Io, Europa, and Callisto causes geological activity. As a result the moons are alternatively stretched and squeezed.
Such stretching and squeezing generates so much heat that the interior melts.
This drives Io's volcanoes and forms the subsurface oceans on Europa and Ganymede.
Io, Europa, and Ganymede are in tidal resonance. That means that every time Ganymede completes one rotation around Jupiter,
Europa makes two and Io makes four.
All the ice on Europa contains more water than all of Earth's oceans put together. If this ice ever melts (when the Sun became a red giant, for example,)
water will cover the entire surface in one giant ocean.