Largest Moon in the Solar System - Ganymede (7th Moon outwards from Jupiter)

Discovery

Ganymede's discovery is credited to Galileo Galilei, who was the first to observe it on January 7, 1610. The satellite's name was soon suggested by astronomer Simon Marius, for the mythological Ganymede, cupbearer of the Greek gods and Zeus's lover.

Together with Io, Europa and Callisto, they are collectively known as Galilean satellites after the discover.





Stats

Ganymede is the largest moon in the Solar System and it is even 8% larger than that the planet Mercury. It also has the highest mass of all planetary satellites, with 2.02 times the mass of the Earth's moon.

Diameter: 5,262 km

Semi-major axis: 1,070,412 km

Orbital Period: 7.15 days

Ganymede completes an orbit in roughly seven days and participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.

Ganymede takes as long to rotate on its axis as it does to make one orbit of Jupiter; and therefore always keeps the same hemisphere pointed to Jupiter.

Composition

Ganymede is composed of approximately equal amounts of silicate rock and water ice. It is a fully differentiated body with an iron-rich, liquid core. A saltwater ocean is believed to exist nearly 200 km below Ganymede's surface, sandwiched between layers of ice.

Its surface is composed of two main types of terrain. Dark regions, saturated with impact craters and dated to four billion years ago, cover about a third of the satellite. Lighter regions, crosscut by extensive grooves and ridges and only slightly less ancient, cover the remainder. The cause of the light terrain's disrupted geology is not fully known, but was likely the result of tectonic activity brought about by tidal heating.

Ganymede is the only satellite in the Solar System known to possess a magnetosphere, likely created through convection within the liquid iron core. The meager magnetosphere is buried within Jupiter's much larger magnetic field and connected to it through open field lines.

Atmosphere

Ganymede also has a thin oxygen atmosphere that includes O, O2, and possibly O3 (ozone). Atomic hydrogen is a minor atmospheric constituent. Whether the satellite has an ionosphere associated with its atmosphere is unresolved.

Life?

There is a distinct possibility that Ganymede's saltwater ocean could harbour life. If there are thermal vents on the floor of this saltwater ocean as there are on Earth, it is remotely possible that similar organisms to those which live around the vents on Earth could also survive there.

But because of the depth of the saltwater ocean (possibly 200 km) from the surface of Ganymede, we may never know whether life exist there.