Astrobiology Top 10: Ganymede's Auroras Point To Ocean Below Its Icy Surface

As 2015 comes to a close, Astrobiology Magazine is counting down our ‘Top 10’ stories from the past year. At number 7: The Hubble Space Telescope picks up clues from Ganymede’s thin atmosphere. This story was originally published on March 16, 2015. 


Artist's conception of aurora on Ganymede (foreground) with Jupiter in the distance. Credit: NASA, ESA, and G. Bacon (STScI)

Artist’s conception of aurora on Ganymede (foreground) with Jupiter in the distance. Credit: NASA, ESA, and G. Bacon (STScI)

The Hubble Space Telescope has revealed that Jupiter’s largest moon, Ganymede, has a vast ocean below its icy surface that’s larger than all of Earth’s surface water.

The telescope picked up these clues from lights high in Ganymede’s thin atmosphere that shimmer with an aurora that’s common on Earth and other worlds in the Solar System. The aurora is created when particles from the Sun strike molecules in the moon’s atmosphere.

Ganymede‘s auroras are influenced by a magnetic field that the moon generates and also the intense magnetic environment of its parent gas giant planet, Jupiter.

The two magnetic fields cause the auroras to slightly rock back and forth across the surface of the moon, and was the clue to the ocean below.

“We know [the ocean] is water because gravity measurements by the Galileo orbiter suggest the first thousand kilometers of Ganymede has a mass density of one. Only water has that mass,” said lead investigator Joachim Saur of the University of Cologne in Germany.

Saur spoke in a Google Hangout hosted by the Hubble team on Thursday (March 12).

It’s unclear if the ocean is habitable, but this places Ganymede high on the list of candidates. Water is believed to be one of the crucial ingredients of life as we know it, along with a source of energy, which Ganymede likely has in spades, given the heat generated from its interior and radiation from Jupiter’s environment.

Salty water

Scientists have suspected an ocean beneath Ganymede’s surface since the 1970s, Saur said in the Hangout. This isn’t an unusual hypothesis, as there are many moons that have similar properties to Ganymede. This gives scientists high hopes that life, even microbial life, could be present in other locations in the Solar System.

Orbiting Saturn, Enceladus has been observed erupting geysers of water from a presumed internal ocean. Europa, a Jovian moon, is also believed to have an ocean, and Hubble spotted what was believed to be a plume of water emanating from its surface in results reported in 2013, although that result has not yet been duplicated.

Potential future missions could see a spacecraft burrow beneath Europa’s ice to explore the ocean, although funding for such pursuit has not been secured.

Artist's conception of the interior of Ganymede. Credit: NASA, ESA, and A. Feild (STScI)

Artist’s conception of the interior of Ganymede. Credit: NASA, ESA, and A. Feild (STScI)

Suspicions of an ocean below Ganymede were fueled by NASA’s Galileo spacecraft, which visited Jupiter and its moons in the 1990s and early 2000s. Galileo flew by Ganymede six times and made brief observations of the magnetic field, 20 minutes at a time. The results were useful, but didn’t catch the cycle that Hubble did.

Hubble made two sets of observations across seven hours each, long enough to catch the rocking. Jupiter’s magnetic field rotates every 10 hours, just as fast as the rotation of the planet itself. The observations took place over more than half of a Jupiter rotation, and that’s when the rocking became clear, the team said.

Through modeling, the team estimated that the ocean is saltwater, not quite as salty as on Earth, but strong enough to battle Jupiter’s magnetic field. The ocean is believed to suppress the rocking of the aurora to only two degrees, instead of six degrees if there was no ocean at all. The measurements are the strongest evidence so far that Ganymede has an under-ice ocean, which could increase its prospects for habitability.

Deep ocean

The depth of the ocean is likely quite extensive, about 10 times deeper than what you would find on Earth. (Ganymede’s circumference is about 2.5 times smaller than Earth.) Modeling of the interior suggested that the ocean is about 60 miles (100 kilometers) thick and is found underneath a huge crust of ice about 95 miles (150 kilometers) thick.

Beneath the ocean is another icy layer formed due to the pressure of all that water. Because Ganymede can generate its own magnetic field, this suggests a rocky core and iron mantle surrounding it, similar to what is believed to be in Earth’s interior.

Artist's conception of the magnetic environment around Ganymede. Credit: NASA, ESA, and A. Feild (STScI)

Artist’s conception of the magnetic environment around Ganymede. Credit: NASA, ESA, and A. Feild (STScI)

Follow-up observations will be difficult because the auroras were only visible in ultraviolet light, which is seen only in telescopes above Earth’s atmosphere because our atmosphere blocks most UV rays. Further, there is no spacecraft orbiting Jupiter right now, although there are two on the way.

NASA’s Juno spacecraft is en route to the planet and will arrive there in 2016. While most of its observations will be focused on Jupiter, from time to time it will be able to observe the planet’s moons. That said, the spacecraft is spinning and won’t be able to do long observations of the moons in the same way that Hubble can, investigators said in the Hangout.

The next opportunity for observation will be 2030, when the the European Space Agency’s JUICE (JUpiter ICy moons Explorer) will arrive at Jupiter. Ganymede will be among the major targets of the mission, along with Europa and Callisto.