Scientists using the Hubble Space Telescope have found the best evidence yet of a salty ocean of liquid water lurking beneath the surface of Jupiter’s largest moon Ganymede, ripening the icy world’s allure as mission planners design probes for more detailed exploration.
Long thought to possess an underground ocean, Ganymede is the largest moon in the solar system and measures bigger than Mercury.
Researchers studied the oscillation of auroral belts near Ganymede’s north and south poles with Hubble to conclude a substantial reservoir of liquid water must exist beneath the moon’s icy crust.
“Our new HST observations provide the best evidence to date for the existence of an ocean on Ganymede,” said Joachim Saur, professor of geophysics at the University of Cologne in Germany.
Ganymede is the only only moon in the solar system with its own magnetic field, triggering belts of polar aurorae similar to the northern and southern lights on Earth.
“If someone could be standing on Ganymede looking up into the night sky, it would appear as red aurora to you, and it would be visible even with the naked eye,” Saur said Thursday in a teleconference with reporters.
The magnetic field of Ganymede also interacts with Jupiter’s powerful magnetism, causing the belts to shift in latitude. Saur’s science team won prized observing time with Hubble to measure how much the auroral belts oscillate over time.
“This is a really great example of using a remote sensing technique — using a telescope in orbit around the Earth — to study a moon that’s in orbit around Jupiter and yet be able to make inferences about the interior of that moon just by looking at it from the outside,” said Heidi Hammel, executive vice president of the Association of Universities for Research in Astronomy. “We aren’t at Jupiter. Hubble is at the Earth, and yet it can probe the internal structure of this moon remotely. That’s a really powerful tool.”
It turns out the volume of water persisting beneath Ganymede’s surface has an influence on the aurorae, counteracting the tug from Jupiter that would cause the belts to “rock” up to 6 degrees back and forth every 10 hours.
“However, when there is a salty, and thus electrically-conducting ocean present, this ocean counterbalances Jupiter’s magnetic field influence and reduces the rocking of the aurora to only 2 degrees,” Saur said.