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.
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.
"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.
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.