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An Underground Ocean on Jupiter’s Largest Moon

By David Reneke

An underground ocean has been discovered on Jupiter’s largest moon.

NASA’s Hubble Space Telescope has the best evidence yet for an underground saltwater ocean on Ganymede, Jupiter’s largest moon. The subterranean ocean is thought to have more water than all of the water on the Earth’s surface.

“This discovery marks a significant milestone, highlighting what only Hubble can accomplish,” said John Grunsfeld of NASA’s Science Mission Directorate. “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.”

Ganymede is the largest moon in our solar system and the only moon with its own magnetic field. The magnetic field causes aurorae, which are ribbons of glowing, hot, electrified gas in regions circling the north and south poles of the moon. Because Ganymede is close to Jupiter, it is also embedded in Jupiter’s magnetic field. When Jupiter’s magnetic field changes, the aurorae on Ganymede also change, rocking back and forth.

By watching the rocking motion of the two aurorae, scientists were able to determine that a large amount of saltwater exists beneath Ganymede’s crust, affecting its magnetic field.

If a saltwater ocean is indeed present, Jupiter’s magnetic field would create a secondary magnetic field in the ocean that would counter it. This “magnetic friction” would suppress the rocking of the aurorae.

In fact, this ocean fights Jupiter’s magnetic field so strongly that it reduces the rocking of the aurorae to 2° instead of 6° if the ocean was not present. Scientists estimate the ocean is 100 km deep, or ten times deeper than the Earth’s oceans, and is buried under a 150 km crust of mostly ice.

Scientists first suspected an ocean in Ganymede in the 1970s. NASA’s Galileo mission measured its magnetic field in 2002, providing the first evidence supporting those suspicions.

Galaxies that “Live Fast, Die Young”

Galaxies can die early because the gas they need to make new stars is suddenly ejected, new research suggests. Most galaxies age slowly as they run out of the raw materials needed for growth over billions of years.

But a pilot study looking at galaxies that die young has found that some might shoot out this gas early on, causing them to redden and kick the bucket prematurely.

Astrophysicist Ivy Wong, from the University of Western Australia node of the International Centre for Radio Astronomy Research, said there are two main types of galaxies: “blue” galaxies that are still actively making new stars and “red” galaxies that have stopped growing.

Most galaxies transition from blue to “red and dead” slowly after two billion years or more, but some transition suddenly after less than a billion years .That’s young in cosmic terms.

Wong and her colleagues looked for the first time at four galaxies on the cusp of their star formation shutting down, each at a different stage in the transition. The researchers found that the galaxies approaching the end of their star formation phase had expelled most of their gas.

Wong said it was initially hard to get time on telescopes to do the research because other astronomers did not believe that the dying galaxies would have any gas left to see. The exciting result means the scientists will be able to use powerful telescopes to conduct a larger survey and discover the cause of this sudden shutdown in star formation.

Wong said it is unclear why the gas was being expelled. “One possibility is that it could be blown out by the galaxy’s supermassive black hole,” she said. “Another possibility is that the gas could be ripped out by a neighbouring galaxy, although the galaxies in the pilot project are all isolated and don’t appear to have others nearby.”

Swiss Federal Institute of Technology Professor Kevin Schawinski said the researchers predicted that the galaxies had to rapidly lose their gas to explain their fast deaths. “We selected four galaxies right at the time where this gas ejection should be occurring,” he said. “It was amazing to see that this is exactly what happens!”

David Reneke is an astronomy lecturer and teacher, a feature writer for major Australian newspapers and magazines, and a science correspondent for ABC and commercial radio. Subscribe to David’s free Astro-Space newsletter at