page_title: Ganymede (moon)
section_title: Subsurface oceans
hierarchical_section_title: Ganymede (moon) / Physical characteristics / Internal structure / Subsurface oceans
context_page_description: Ganymede, a satellite of Jupiter, is the largest and most massive of the Solar System's moons. The ninth-largest object in the Solar System, it is the largest without a substantial atmosphere. It has a diameter of 5,268 km, making it 26% larger than the planet Mercury by volume, although it is only 45% as massive. Possessing a metallic core, it has the lowest moment of inertia factor of any solid body in the Solar System and is the only moon known to have a magnetic field. Outward from Jupiter, it is the seventh satellite and the third of the Galilean moons, the first group of objects discovered orbiting another planet. Ganymede orbits Jupiter in roughly seven days and is in a 1:2:4 orbital resonance with the moons Europa and Io, respectively.
  Ganymede is composed of approximately equal amounts of silicate rock and water. It is a fully differentiated body with an iron-rich, liquid core, and an internal ocean that may contain more water than all of Earth's oceans combined. 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.
context_section_description: In the 1970s, NASA scientists first suspected that Ganymede has a thick ocean between two layers of ice, one on the surface and one beneath a liquid ocean and atop the rocky mantle. In the 1990s, NASA's Galileo mission flew by Ganymede, and found indications of such a subsurface ocean. An analysis published in 2014, taking into account the realistic thermodynamics for water and effects of salt, suggests that Ganymede might have a stack of several ocean layers separated by different phases of ice, with the lowest liquid layer adjacent to the rocky mantle. Water–rock contact may be an important factor in the origin of life. The analysis also notes that the extreme depths involved (~800 km to the rocky "seafloor") mean that temperatures at the bottom of a convective (adiabatic) ocean can be up to 40 K higher than those at the ice–water interface.
  In March 2015, scientists reported that measurements with the Hubble Space Telescope of how the aurorae moved confirmed that Ganymede has a subsurface ocean. A large salt-water ocean affects Ganymede's magnetic field, and consequently, its aurora. The evidence suggests that Ganymede's oceans might be the largest in the entire Solar System.
  There is some speculation on the potential habitability of Ganymede's ocean.
caption: Artist's cut-away representation of the internal structure of Ganymede. Layers drawn to scale.