Ganymede moon and Jupiter

500 million miles from the Sun lies Ganymede moon, slightly larger than the planet Mercury, orbiting Jupiter and may contain more water than all of Earth’s oceans.

Above an artist’s concept of the moon Ganymede as it orbits the giant planet Jupiter.  Credit: NASA, ESA, and G. Bacon (STScI).  Science Credit: NASA, ESA, and J. Saur (University of Cologne, Germany)

Temperatures on Ganymede are so cold, though, that water on the surface freezes as hard as rock and the ocean according to evidence from the Hubble Space Telescope, lies roughly 100 miles below the crust.

Nevertheless, where there is water there could be life as we know it. Identifying liquid water on other worlds — big or small — is crucial in the search for habitable planets beyond Earth. Though the presence of an ocean on Ganymede has been long predicted based on theoretical models, NASA’s Hubble Space Telescope found the best evidence for it. Hubble was used to watch aurorae glowing above the moon’s icy surface.

Ganymede moon's ocean
This is an illustration of the interior of Jupiter’s largest moon Ganymede. Credit: NASA, ESA, and A. Feild (STScI)

The aurora are tied to the moon’s magnetic field, which descends right down to the core of Ganymede. A saline ocean would influence the dynamics of the magnetic field as it interacts with Jupiter’s own immense magnetic field, which engulfs Ganymede. Because telescopes can’t look inside planets or moons, tracing the magnetic field through aurorae is a unique way to probe the interior of another world.

source hubblesite