This is an illustration of the interior of Jupiter's largest moon, Ganymede. It is based on theoretical models, in-situ observations by NASA's Galileo orbiter, and Hubble Space Telescope observations of the moon's aurorae, which allows for a probe of the moon's interior. The cake-layering of the moon shows that ice and a saline ocean dominate the outer layers. A denser rock mantle lies deeper in the moon and an iron core beneath that. (NASA, ESA and A. Feild)

NASA announced evidence on 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, according to new Hubble observations.

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.

While scientists have speculated since the 1970s about the presence of an ocean on Ganymede -- the largest moon in our solar system -- 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.

“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.”

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.

[Hubble catches a rare three-moon-parade in front of Jupiter]

This animation, based on images taken by NASA's Galileo orbiter, shows what Jupiter's moon, Ganymede, looks like, with each color indicating a different type of terrain. "Brown regions are those that are heavily cratered and much older than the light shaded regions that are smoother with few craters. These lighter shaded regions are believed to be formed by flooding of the surface with water coming from faults or even cryo-volcanos that have taken place over billions of years. Perhaps even tectonic processes are at work with some crustal ice sheets being forced downward by the emergence of newer icy material." (NASA via YouTube)

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

[From the Hubble, a new image of a glittering cosmic wonderland with stars]

"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," executive vice president of the Association of Universities for Research in Astronomy Heidi Hammel 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.

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CORRECTION: A previous version of this post attributed the discovery to "gravitational" analysis of Ganymede. In fact, the researchers measured parts of the moon's magnetisphere, or outer magnetic field. A quote was Hubble senior project scientist Jennifer Hubble that was in fact said by Heidi Hammel, executive vice president of the Association for Universities for Research in Astronomy.