Ganymede is not only the largest moon of Jupiter, but it’s also, in fact, the largest moon in the Solar System (8% larger than the planet Mercury!) and one of the first to be discovered besides our own Moon, by Galileo in 1610.
Ganymede has a number of interesting characteristics that make it unique among moons that we know of. Here are some:
1: Its surface features are visible with a telescope
Ganymede has a large dark region known as Galileo Regio – named for the famous astronomer. Galileo Regio is composed of organic molecules and clays and is likely older than the surrounding, lighter terrain in addition to being more heavily cratered.
Ganymede’s apparent angular diameter ranges between 1.2 to 1.8 arc seconds. A typical 8” telescope has a resolution of about 0.6 arc seconds and Galileo Regio takes up nearly ⅓ of the moon, so indeed it is possible to observe with a good 8” telescope under very good conditions. However, a 12” or larger telescope is probably best. I have seen Galileo Regio with an 8” Schmidt-Cassegrain once, and with 16” and 12” reflectors on a number of occasions.
What you see will look something like this at best:
No, that’s not an exaggeration. At most, you will see what looks to be a slight brownish patch. Keep in mind that Galileo Regio is on the side opposite of the one facing Jupiter, thus Ganymede will generally be nearly in front of the giant planet when it is optimal to view Galileo Regio.
Jupiter’s other moons Io and Callisto both have albedo features that are detectable with amateur instruments, but far better conditions, equipment, and practice are needed to discern them.
2: It has its own magnetic field & aurorae
Ganymede is the only moon known to possess its own, self-produced magnetic field, which is itself embedded inside the much stronger magnetic field around Jupiter. Ganymede’s metallic core generates this field in a similar manner to Earth’s own magnetic field, working like a dynamo action caused by the movement of conductive material in the moon’s core.
Ganymede’s extremely tenuous (less than 0.001% as thick as Earth’s) oxygen atmosphere combined with the moon’s magnetic field means that it can also host aurorae. However, these aurorae are extremely faint and are only visible in ultraviolet light, meaning you could not possibly see them even if you stood on Ganymede’s surface.
Speaking of standing on Ganymede’s surface… you really can’t. Ganymede receives about 1 rem of radiation a day from Jupiter – which means that in ten days you’d be exceeding the maximum amount of radiation NASA lets astronauts receive over their entire career. Unless you walked around in a lead box, you’d probably die of cancer or radiation sickness very quickly. Jupiter’s more distant moon Callisto is far more hospitable for life than Ganymede since it lies outside the most irradiated parts of the giant planet’s magnetic for
3: It has an underground ocean, which may host life
Like its neighboring moons Europa and Callisto, Ganymede possesses a thick, salty ocean beneath its ice/rock crust. It is possible that this ocean is more than a hundred kilometers thick, which would make it the biggest ocean in the Solar System.
Ganymede is not as suitable for life as Europa for a number of reasons. Europa is subject to more tidal heating from Jupiter and its neighboring moons, which is composed more of H2O than rocks (unlike Ganymede which is just below 50/50), and Ganymede is lacking in organic compounds such as tholins – which are key ingredients for forming life. Ganymede also has less geological activity than Europa, which may also be influential.
4: It has an unusual chain of craters
Crater chains, or catenas, occur in various places throughout the Solar System, but most are very worn away and hard to recognize. Ganymede’s Enki Catena, however, is a fairly recent marker of an unusual impact event. Enki was probably formed when a loosely-bound asteroid or comet broke up due to a close pass by Jupiter and the fragments struck Ganymede in succession. This chain is just over 100 miles long and comprised of 13 craters in all.
5: It will be the first moon other than Earth’s to be orbited by a spacecraft
Ideas for a Ganymede orbiter have been floating around for about twenty years, but it has taken a while for anything to reach past the proposal stage and receive funding. The European Jupiter Icy Moons Explorer spacecraft (JUICE) will launch in 2022 and arrive at Jupiter in 2029. After numerous flybys of Europa, Ganymede, and Callisto, JUICE will enter orbit around Ganymede in 2032 and remain there for at least 18 months, after which it will either deorbit or (more likely) the mission will be extended.