Jupiter's moon Ganymede is believed to contain an ocean much deeper than oceans on Earth — about 100 kilometres thick and hidden beneath a 150-kilometre crust of mostly ice.
"The solar system is now looking like a pretty soggy place," said Jim Green, director of planetary science at NASA headquarters.
Since the 1970s, scientists have suspected Ganymede may have a watery interior. NASA's Galileo spacecraft flew by Ganymede in the 1990s and beamed back tantalizing signs of an ocean.
The latest evidence comes from the Hubble Space Telescope, which observed the moon's magnetic field to get a glimpse of its interior.
The moon's magnetic field causes auroras — vibrant light shows that are observable from the Hubble telescope. Researchers measured the movements of the auroras, which would be affected by a saltwater ocean underneath Ganymede's surface.
"Jupiter is like a lighthouse whose magnetic field changes with the rotation of the lighthouse. It influences the aurora," said geophysicist Joachim Saur, with the University of Cologne in Germany. "With the ocean, the rocking is significantly reduced."
Ganymede and other moons might contain water
With a diameter of 5,268 km, Ganymede is the largest moon in the solar system, and larger than the smallest planet, Mercury.
It was one of four moons charted in 1610 by astronomer Galileo Galilei, who called it Jupiter III. It was later named after Ganymede, a Trojan prince in Greek mythology. The other three moons are now known as Io, Europa and Callisto. They were the first moons found to orbit a planet other than Earth.
Ganymede is one of a growing number of moons that are thought to have water underground. Its sister moon Europa has long been considered one of the most likely places in the solar system to host extraterrestrial, microbial life. Scientists believe a global ocean may lurk beneath its solid, pearl-like crust of ice.
Unlike Ganymede, however, Europa does not have a magnetic field of its own (though it is affected by the much larger Jupiter's magnetic field), and thus no auroras to observe.
New research also suggests there are hot springs bubbling beneath the icy surface of Enceladus, a much smaller moon orbiting Saturn.
The European Space Agency is planning to launch a mission in 2022 on an eight-year journey toward Jupiter. After circling Jupiter and flying by three of its largest moons, the plan is for the spacecraft to slip into orbit around Ganymede for a close-up look — the first attempt at orbiting an icy moon.
NASA is building radar for the mission that's designed to pierce through ice and peer deep into Jupiter's moons.