Jupiter’s third moon Ganymede is the largest satellite in our solar system. It is actually larger in diameter than the planet Mercury but less dense and is only about half its mass. It was named after the Ganymede from Greek mythology who was a young boy carried to Olympus by Zeus (the Greek god corresponding to the Roman god Jupiter) camouflaged as an eagle. Ganymede then became the cupbearer of the gods. The discovery of Ganymede is credited to Galileo who was the first to observe it on January 7, 1610.

Ganymede is composed of roughly equal amounts of silicate rock and water ice; it is a fully differentiated moon that has an iron-rich, liquid core. Ganymede orbits Jupiter at a distance of 1,070,400 km, third among its satellites and completes a revolution every seven days. Like most known moons, Ganymede is tidally locked, which means that one side of the moon always facing toward the planet just like our moon always has the same face showing towards Earth. There are two distinct types of terrain on the surface of Ganymede, a darker region, heavily indented with impact craters covers around a third of the satellite which has been dated to roughly four billion years in age, and a lighter smoother area, scarred by vast grooves and ridges covers the remainder. The reason for the light terrain’s uneven geology is not fully understood, but it is likely to be the result of tectonic activity brought about by the stretching and contracting of the surface caused by tidal heating as the moon is affected by the magnetic pull of Jupiter. It is also believed that a saltwater ocean exists at around 200 km below Ganymede’s surface, sandwiched between layers of ice.

Ganymede is the only satellite in our Solar System which is known to have a magnetosphere, probably created through convection within the liquid iron core. A magnetosphere is created when a stream of charged particles, such as the solar wind, interacts with an astronomical body and is deflected by the magnetic field of a planet.  In 1996, astronomers using the Hubble Space Telescope also found evidence of a thin atmosphere containing oxygen on Ganymede. The atmosphere is far too thin to support life as we know it, but it is a fascinating discovery which boosts the knowledge of the presence of oxygen elsewhere in space.

As time and astronomical advances have progressed, we have been able to study distant satellites more closely. Beginning with Pioneer 10, a robotic probe launched in 1972, spacecraft have been able to examine Ganymede in detail. The Voyager probes launched in 1977 were able to determine measurements of the moon’s size, whereas the Galileo spacecraft which was sent in 1989 with the specific job of studying Jupiter and its moons discovered its underground ocean and magnetic field.

We will have to wait until 2020 for the next mission to Jupiter for the next instalment of knowledge, the Europa Jupiter System Mission is planned to go then and report back information about the satellites and their make-up.