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Astronomers perform 'MRI' to find subsurface ocean on Jupiter's moon, Ganymede

Jupiter's magnetic field and Ganymede's auroras have given astronomers a 'glimpse' into the moon's interior. Credit: NASA/ESA

Scott Sutherland
Meteorologist/Science Writer

Thursday, March 12, 2015, 3:11 PM - Using the Hubble Space Telescope, astronomers have performed what could be thought of as a 'planetary MRI' to confirm that Jupiter's largest moon has a deep subsurface ocean of salty water.

Although Europa has attracted most of our attention regarding subsurface oceans and potential for life, for decades now, astronomers have speculated that Ganymede - Jupiter's largest moon - also possesses an ocean of liquid, salty water under its icy crust. Modeling and spacecraft missions have given some indication that an ocean may be there, but Ganymede has - up until now - kept its secrets safe.

However, scientists here on Earth have come up with a very novel way of learning what Ganymede is hiding. 

"I was always brainstorming how we could use a telescope in other ways," Joachim Saur, of the University of Cologne in Germany, told NASA. "Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon’s interior."

While Ganymede does not actively generate a magnetic field of its own, like Earth does, the moon's iron core is coaxed into producing one as it passes through the intense magnetic field of Jupiter. The result? As charged particles encounter Ganymede's induced magnetic field, it produces aurorae, which Saur and his colleagues observed using the Hubble Space Telescope.

Hubble's ultraviolet look at Ganymede's aurorae. Credit: NASA/ESA

Besides being visually interesting (while Hubble sees them in blue, they would apparently shine red to our eyes), as Saur said, these aurorae can actually tell us about what's going on inside Ganymede.

Similar to how an MRI scanner reads signals that show how the scanner's magnetic fields interact with the tissues of our bodies, the movement of the aurorae - how much they oscillate to the north and south - in response to changes in Jupiter's magnetic field, can reveal what the magnetic fields are interacting with in the moon's interior.

According to NASA:


If a saltwater ocean were present, Jupiter’s magnetic field would create a secondary magnetic field in the ocean that would counter Jupiter’s field. This 'magnetic friction' would suppress the rocking of the aurorae. This ocean fights Jupiter's magnetic field so strongly that it reduces the rocking of the aurorae to 2 degrees, instead of the 6 degrees, if the ocean was not present.


Credit: NASA, ESA, and A. Feild (STScI)

While this evidence is more indirect than what Hubble spotted from Europa back in 2013, it's still pretty remarkable.

"This discovery marks a significant milestone, highlighting what only Hubble can accomplish," John Grunsfeld, associate administrator of NASA’s Science Mission Directorate, 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."

Source: NASA

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