Ganymede is not only the largest of Jupiter's moons, but the largest moon in our solar system Ganymede is actually larger than two of the planets - Mercury and Pluto. Discoveries about its possible oceans and atmosphere have proved of great interest to scientists studying this giant satellite of Jupiter.
Ganymede was discovered in January of 1610, by Galileo Galilei who noted four celestial bodies orbiting Jupiter. At first, he thought they were stars and named them Cosimo Sidera (Cosimo's Stars) after Cosimo de Medici, his former employer from whom he now sought monetary support for his research. The names by which these moons are now commonly known, was given to them by Simon Marius who discovered them at about the same time and named them all for lovers of Zeus - Io, Europa, Callisto and Ganymede.
Evidence of oceans on Ganymede
The surface of Ganymede is covered in ice, but differences in terrain indicate that the age of the surface varies. Darker areas of the terrain may be billions of years old and are marked with craters. However, there are lighter areas which are not cratered, and which may be areas where water has erupted on the surface and then frozen. Images of the surface taken by NASA's Galileo spacecraft show fractures in the surface which could allow for water or partially melted ice to break through to the surface. These areas are highly suggestive of an underground ocean.
Magnetic readings which were taken in May of 2000 by Galileo are also indicative of a subterranean ocean. These readings led scientists to conclude that Ganymede most likely has a salty ocean sandwiched between two layers of ice lying about 120 miles below the surface. Because the ocean is so far from the surface, scientists do not think there is as much hope of finding life on Ganymede as there is on its sister moon, Europa.
Evidence of an atmosphere
The first hint that Ganymede might have an atmosphere came in 1972 when astronomers from the Bosscha Observatory in Indonesia reported that they detected a thin atmosphere when the moon and Jupiter passed in front of a star. Data from the Hubble Space Telescope in 1995 proved the existence of oxygen and ozone on Ganymede. Further information gathered from the Galileo spacecraft detected hydrogen molecules streaming away from the moon's north pole andprovided evidence that Ganymede had not only a magnetosphere, but a very thin atmosphere. The moon's magnetosphere keeps the atmosphere from simply escaping.
Several spacecraft have made flybys of Ganymede since 1973. A joint mission from NASA and the European Space Agency to further explore Jupiter's moons was slated to be launched in 2020, but due to budget cuts may not proceed.