Our spacecraft Juno (been at Jupiter for five years now) will finally begin flying by the moons. first one: Ganymede, on 7jun at 11am EDT. very short flyby but we haven't been there for 20 years so any new observations can be fruitful.
Ganymede is one of the 4 large Jovian moons, all very fascinating and different (Io has active volcanoes! Europa has an ocean covering ice shell that has been reworked recently, and possibly plumes to chemically analyse, Callisto is the furthest from Jupiter and geologically quiet)
The moons form a progression from closest to Jupiter to farthest, closest have more rock than ice and are more heated by tidal forces, so Io has no water at all, and Europa has the most active ocean/ice shell, the other moons have more ice but are cooler and less active.
Ganymede is the largest moon in the solar system, larger than the planet mercury! SOME bodies in the solar system have their own magnetic field and Ganymede is one of them. Ganymede likely has: a molten iron core, a rock mantle, an internal ocean and an ice shell. the shell has complex features and more recent geological activity than Callisto but less recent than Europa.
these were the first new solar system bodies that were discovered by telescope (probly by Galileo, maybe others) and showed that bodies could orbit around other bodies besides the earth (many thought earth was the center of all orbits)
they were also a great surprise when voyager first photographed them back in the 70s as they were all different and complex! They are telling many stories! some of geology, some of chemistry, perhaps even about life! life may exist in their oceans.
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