The title of "largest moon in the solar system" is held by Ganymede, the icy satellite of Jupiter that was discovered by Galileo in 1610 as one of three moons to orbit the fiery planet. With a diameter of 3273 miles (5268 km) it is larger than Mercury and three quarters the size of Mars.
While little was previously known about Ganymede, space missions to the natural satellite initiated in the 1970s, including Pioneer 10 in 1972 followed by Voyager and Galileo missions over the next 25 years. Facts about Ganymede soon unfolded, and a look into its unique composition, atmosphere, and underground ocean was made possible.
Its composition mostly consists of iron, silicate, and water in the form of ice. Its solid iron core is surrounded by a thin layer of silicate rock. A solid rock shell - with bits of iron, silicate, and ice - then surrounds that. The outer crust is made almost entirely of water ice, although bits of rock are also found within the ice. Beneath the ice is a layer of melted saltwater ice, approximately 200 km from the surface layer crust.
This melted "outer mantle" is considered to be a "hidden ocean" of Ganymede and was first discovered by the Galileo spacecraft in 2000. Its formation was most likely due to neural activity and the magnetosphere within the interior of the moon. Faults in the ice have been witnessed, allowing water to seep through to a layer that is held between two thick sheets of ice and continually enlarge because of this.
The theory of a subterranean saltwater ocean is also connected to Europa, another moon of Satellite that is believed to be able to support life. The ability to support life is not likely on Ganymede, however, because of the large depth of the water layer. If this water was ever to be mined and retrieved, potential damage to Ganymede's outer crust could ensue.
The moon's large magnetosphere (made possible because of the iron core; Ganymede is the only moon in the solar system to have a magnetic field) does create a high electrical conductivity (thermal heat) within the ocean and would probably protect life against harmful radiation, but the atmosphere, albeit containing oxygen, is too thin to support the necessities of life.
For years it was unknown whether or not Ganymede had an atmosphere. Differing reports from 1972-1995 were unable to provide a definite answer. Astronomers at the Bosscha Observatory in Indonesia first detected a thin atmosphere in 1972, but subsequent Voyager missions failed to bring back results, and if anything, they concluded that the moon did not have an atmosphere (there was no evidence to support that there was one). Ganymede's atmosphere was discovered by the Hubble Space Telescope in 1995, and it was found that the atmosphere contained high levels of oxygen molecules.
The oxygen is a result of the breakage of water bonds via radiation in the magnetosphere; this theory was backed up by traces of oxygen found on the icy crust. While the hydrogen molecules are too small to dissipate into the air (although some do, most are simply lost during radiation), the oxygen molecules have essentially created the atmosphere. Once individualized they become ionized by the electrons in the magnetosphere, and because of Ganymede's magnetic field and the gravitation pull that it emanates, the particles are unable to break away. Whether or not there was a noticeable atmosphere in 1972 is still unknown.
The atmosphere also contains traces of elemental oxygen and ozone, and although the particles are ionized, an ionosphere is not known to exist at this time.
Ganymede's "underground" oceanic environment and oxygen-based thin atmosphere add interest and character to a moon that has been the subject of space missions and research for many years. In addition, a combination of old, cratered terrain and new, light-ridged terrain give reason to believe that the moon's formation occurred over a long span of time, from various different events and happenings. As new information is discovered, the peculiarity and distinction of Ganymede become increasingly ingrained in the field of astronomy.