NASA: Evidence of ocean beneath solar system's biggest moon

LOS ANGELES— The largest moon in the solar system harbors a salty ocean beneath its icy shell, the latest member to join a growing club of watery moons, NASA said Thursday.

"The solar system is now looking like a pretty soggy place," said Jim Green, director of planetary science at NASA headquarters.

The ocean on Jupiter's Ganymede is estimated to be much deeper than the oceans on Earth — about 60 miles (100 kilometers) thick and buried under 95 miles (150 kilometers) of ice. The moon's ocean is believed to have more water than all of Earth.

The quest to find potentially habitable places where microbes can thrive has centered on objects in the solar system with hidden water, a key ingredient for life.

Ganymede joins a cadre of solar system moons where evidence of underground oceans has been discovered in recent years. The list includes Jupiter's Europa moon and the Saturn moons Titan and Enceladus. A separate team of scientists reported this week that hot springs may be bubbling beneath the chilly surface of tiny Enceladus.

The latest evidence comes from the Hubble Space Telescope, which observed Ganymede's magnetic field for a glimpse of its interior. The workhorse telescope studied changes in colorful auroras in the moon's polar regions that are produced by its magnetic field to determine the existence of an ocean lurking underneath.

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.

Hubble's observations are the most convincing yet, said Joachim Saur, professor of geophysics at Germany's University of Cologne, who led the study published in the Journal of Geophysical Research.

Ganymede is one of about five dozen moons circling Jupiter. It's the largest moon around the giant planet and the biggest moon in the solar system. With a diameter of 3,270 miles (5,260 kilometers), it's slightly larger than the smallest planet, Mercury.

Ganymede is among several big Jupiter moons discovered in 1610 by Galileo, who called it Jupiter III. It was later named after Ganymede, a Trojan prince in Greek mythology.

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 spacecraft plans 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.