"Long before it's in the papers"
March 17, 2015


Jupiter moon may conceal ocean with more water than Earth’s

March 17, 2015
Courtesy of Hubble News Center
and World Science staff

A moon of Jupiter may conceal under its icy surface an ocean with more wa­ter than all of that on Earth’s sur­face, new find­ings sug­gest.

Sci­en­tists used NASA’s Hub­ble Space Tel­e­scope to in­ves­t­i­gate Gan­y­mede, the So­lar Sys­tem’s larg­est moon—a­bout one-and-a-half times wid­er than ours.

Ganymede as im­aged in 1996 by the Ga­lileo space­craft (cour­tesy NA­SA)

Gan­y­mede is al­so the only moon with its own mag­net­ic field. The mag­net­ic field causes au­ro­rae, which are rib­bons of glow­ing, hot elec­tri­fied gas, in ar­eas cir­cling the north and south poles.

But Gan­y­mede is al­so em­bed­ded in Ju­pi­ter’s mag­net­ic field. When Ju­pi­ter’s mag­net­ic field changes, the au­ro­rae on Gan­y­mede al­so change, “rock­ing” back and forth, sci­en­tists ex­plain.

The thinking behind the study is that if a saltwa­ter ocean were there, Ju­pi­ter’s mag­net­ic field would cre­ate a sec­ond­ary mag­net­ic field in that ocean. This sec­ond­ary field would count­er Ju­pi­ter’s own—and as a re­sult, sup­press the rock­ing of the au­ro­rae. 

This sup­pres­sion indeed hap­pens, and so strongly that it re­duces the rock­ing of the au­ro­rae by some two-thirds, the in­ves­ti­ga­tors cal­cu­lat­ed. As a re­sult, they es­ti­mate the ocean is 60 miles (100 kilo­me­ters) thick — 10 times deeper than Earth’s oceans — and is bur­ied un­der a 95-mile (150-kilometer) crust of mostly ice.

Sci­en­tists led by Jo­a­chim Saur of the Uni­vers­ity of Co­logne in Ger­ma­ny came up with the idea of us­ing Hub­ble to learn more about the in­side of the moon.

“I was al­ways brain­storm­ing how we could use a tel­e­scope in oth­er ways,” said Saur. “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.”

The find­ings were pub­lished on­line in the Jour­nal of Geo­phys­i­cal Re­search: Space Phys­ics on March 12.

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A subterranean ocean on a moon of Jupiter may have more water than all the water on Earth’s surface, new findings suggest. Scientists used NASA’s Hubble Space Telescope to investigate Ganymede, the Solar System’s largest moon—about one-and-a-half times wider than ours. Ganymede is also the only moon with its own magnetic field. The magnetic field causes aurorae, which are ribbons of glowing, hot electrified gas, in areas circling the north and south poles. But Ganymede is also embedded in Jupiter’s magnetic field. When Jupiter’s magnetic field changes, the aurorae on Ganymede also change, “rocking” back and forth, scientists explain. By watching that rocking motion, scientists concluded that huge amounts of salt water underlie Ganymede’s crust, affecting its magnetic field. Scientists led by Joachim Saur of the University of Cologne in Germany came up with the idea of using Hubble to learn more about the inside of the moon. “I was always brainstorming how we could use a telescope in other ways,” said Saur. “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.” If a saltwater ocean were present, Jupiter’s magnetic field would create a secondary magnetic field in that ocean, he explained. And this secondary field would counter Jupiter’s own—as a result, suppressing the rocking of the aurorae. This suppression happens so strongly that it reduces the rocking of the aurorae by some two-thirds, the investigators calculated. As a result, they estimate the ocean is 60 miles (100 kilometers) thick — 10 times deeper than Earth’s oceans — and is buried under a 95-mile (150-kilometer) crust of mostly ice. The findings are to be published online in the Journal of Geophysical Research: Space Physics on March 12.