Scientists using NASA’s Hubble Space Telescope have uncovered new evidence for a massive subterranean ocean on Ganymede, the largest moon in the Solar System and one of the four Galilean moons of Jupiter.
This is an artist’s concept of the Jupiter’s moon Ganymede; two auroral ovals can be seen over northern and southern mid-latitudes. Image credit: NASA / ESA / G. Bacon, STScI / J. Saur, University of Cologne.
Ganymede is slightly larger than the planet Mercury and the only moon in the Solar System to have its own magnetic field.
The magnetic field causes aurorae in regions circling the Ganymede’s north and south poles.
Because the moon is close to Jupiter, it is also embedded in the planet’s own magnetic field. When Jupiter’s magnetic field changes, the aurorae on Ganymede also change, rocking back and forth.
If a saltwater ocean were present, the magnetic field of Jupiter would create a secondary magnetic field in the Ganymede’s ocean that would counter Jupiter’s field.
Dr Joachim Saur from the University of Cologne’s Institute of Geophysics and Meteorology in Germany and his colleagues came up with the idea of using NASA’s Hubble Space Telescope to learn more about the inside of the moon.
The new observations were done in UV light and could only be accomplished with a space telescope high above Earth’s atmosphere, which blocks most UV light.
Hubble measured slight shifts in the auroral belts due to the influence of Jupiter’s own immense magnetic field.
The saline ocean under the Ganymede’s crust fights Jupiter’s magnetic field so strongly that it reduces the rocking of the aurorae to 2 degrees, instead of 6 degrees if the ocean were not present.
“This discovery marks a significant milestone, highlighting what only Hubble can accomplish,” said Dr John Grunsfeld of NASA Headquarters, Washington, DC, who was not involved in the study.
The team estimates the ocean is 100 km (60 miles) thick and is buried under a 150-km (95-mile) crust of mostly ice.
The results were published online March 12 in the Journal of Geophysical Research: Space Physics.
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Joachim Saur et al. The search for a subsurface ocean in Ganymede with Hubble Space Telescope observations of its auroral ovals. Journal of Geophysical Research: Space Physics, published online March 12, 2015; doi: 10.1002/2014JA020778
