Ganymede is the third of the Galilean moons, orbiting Jupiter. It is the largest Moon in the solar system with a diameter of 3,280 miles (5,262 km). Ganymede´s structure is composed principally of silicate rocks and water ice. A salty ocean is believed to lie beneath its surface. It is the only Moon in the entire solar system known to possess a magnetosphere. Ganymede has been visited by the voyager probes in 1979 and the Galileo probe in the 1990s, both of which returned data about this satellite.
Ganymede is composed primarily of silicate rock and water ice. A saltwater ocean is believed to exist 200 km beneath its surface. Close approaches of the Galileo probe on Ganymede on May 2000 suggested a thick layer of melted, salty water. Minerals found on the surface of Ganymede indicate that water may have emerged and melted at the surface. Images taken by Galileo probe show how water may have surfaced through the fractured crust, resembling the linear features seen in Europa (second Galilean moon) which is believed to have a water ocean beneath its ice crust.
Ganymede´s surface consist of two main types of terrain. One third is covered by dark regions of highly cratered terrain. The remainder is covered by lighter regions of grooved terrain and ridges. It is believed that the lighter region disrupted geology could be due to tectonic activity caused by tidal heating. Scientists discovered Irregular lumps beneath the icy surface which may be rock formations supported by Ganymede icy crust. Ganymede is the only satellite known to possess a magnetosphere, which is thought to stem from the convection process created within the liquid iron core.
The oceans and atmosphere of Ganymede:
The Ganymedean Ocean is similar in many respects to that of Callisto, and different to that of Europa. Europa´s Ocean is much closer to its surface and in more direct contact with hydrothermal processes on the seafloor. Ganymede´s Ocean is deeper and inserted between layers of ice, making it less likely to support life; however, magmatic activity might still generate pockets of water melt, supplying the ocean with nutrients to sustain a biosphere. Natural radioactivity should provide heat enough to maintain a stable layer of liquid water between layers of ice 150-200 km (90-120 miles) below Ganymede´s surface.
In 1995, Hubble Space Telescope (HST) observed a tiny airglow of atomic oxygen in the atmosphere of Ganymede suggesting a neutral atmosphere composed primarily of O2 molecules. Additional evidence of an oxygen atmosphere on Ganymede comes from the spectral detection of gases found in the ice crust on Ganymede’s surface. Oxygen is believed to be produced when water ice is split into hydrogen and oxygen by the process of radiation. Ganymede´s atmosphere consists of oxygen, O, O2 and possibly ozone (O3) Atomic hydrogen might exist in trace amounts.
The Galileo mission discovered that Ganymede had not only a magnetosphere like the one found on Earth, but a very thin atmosphere as well. The atmosphere is produced by fast moving molecules from the magnetosphere, colliding with the surface and knocking out a water molecule. Galileo instruments detected a stream of hydrogen molecules moving away from Ganymede over its north pole. Another instrument detected the existence of an ionosphere, suggesting that there must be an active process creating Ganymede’s atmosphere.
Ganymede was discovered by Galileo Galilei in 1610. It was the first satellite discovered orbiting other planet than the Earth. In Greek mythology, Ganymede was a beautiful young boy who was taken to Olympus by Zeus disguised as an eagle. Ganymede became the cup bearer of the Olympian Gods. Exploration of Ganymede began with Pioneer 10 and Pioneer 11. Voyager took measurements of its size, and Galileo spacecraft discovered an underground ocean and magnetic field. Europa Jupiter System Mission (EJSM) is a new mission to Jupiter´s moons planned to be launch in 2020.
