WASHINGTON - NASA announced evidence Thursday that Jupiter's largest moon, Ganymede, has a saltwater ocean under its icy surface. The ocean seems to have more water than all the water on Earth's surface.

New Hubble observations of Ganymede's magnetic field strongly suggest that the moon, which is the largest in our whole solar system, is home to a subsurface ocean. Scientists have already confirmed the existence of an ocean on Europa, another moon orbiting Jupiter, and NASA has announced plans to send an unmanned mission there searching for the life that might come with liquid water.

While scientists have speculated about the presence of an ocean on Ganymede since the 1970s, until now the only observational evidence came from a brief flyby by the Galileo spacecraft, which didn't observe the moon long enough to confirm a liquid ocean.

Scientists estimate that the ocean is 60 miles thick, which is about 10 times deeper than Earth's oceans. But unlike our salty waters, Ganymede's ocean is buried under 95 miles of ice.

"This discovery marks a significant milestone, highlighting what only Hubble can accomplish," John Grunsfeld, assistant administrator of NASA's Science Mission Directorate at NASA Headquarters, said in a statement. "In its 25 years in orbit, Hubble has made many scientific discoveries in our own solar system. A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth."

The Hubble is a telescope that orbits Earth, but because of these impressive gravitational analyses it can be used to study the interior of planets far off in the distance. Using these same principles, Hubble senior project scientist Jennifer Wiseman said during a NASA news conference on Thursday, scientists could theoretically detect oceans on distant exoplanets as well.

"It may require a telescope larger than the Hubble, it may require a new space telescope, but nevertheless it is a tool we have now," Wiseman said.

This news comes just a day after the announcement that one of Saturn's moons might have hydrothermal activity in its subsurface ocean - a phenomenon that could allow it to support life. It seems that water may be relatively common in our solar system, making the search for life all the more exciting and mysterious.