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NASA: Ganymede joins roster of water moons

This image of Ganymede, the solar system's largest moon, was obtained by the Galileo space probe. Image courtesy NASA.

This image of Ganymede, the solar system’s largest moon, was obtained by the Galileo space probe. Image courtesy NASA.

Ganymede, the largest moon of Jupiter, is a water world.

Researchers using the Hubble Space Telescope have confirmed an ocean beneath Ganymede’s icy crust that are likely ten times deeper than Earth’s oceans.

The moon may have more liquid water beneath its surface than there is on Earth.

To reach this conclusion, scientists measured fluctuations in the moon’s aurorae. Aurorae, which are electrified ribbons of heated gas, are generated when a planetary body’s  liquid metal core produces a magnetic field.

On Ganymede, the aurorae around its north and south poles move, influenced by fluctuations in Jupiter’s huge magnetic field.

Scientists used computer modeling to verify that the only explanation for the extent of the “rocking” of Ganymede’s aurorae is the presence of a saline ocean, which would cause Ganymede’s magnetic field to offset to some extent Jupiter’s magnetic impact.

Several measurements indicated that the aurorae moved by about two degrees, but without the presence of an ocean Jupiter’s influence would have caused movement of about six degrees.

“When there is an electrically conductive ocean present, this counteracts Jupiter’s influence,” Joachim Saur, a professor geophysics at Germany’s University of Cologne and the lead researcher, said.

Saur explained that four independent measurements with HST verified the two-degree movement of Ganymede’s aurorae.

This graphic shows Ganymede's magnetic fields. Graphic courtesy NASA, European Space Agency, (c) A. Feild, Space Telescope Science Institute.

This graphic shows Ganymede’s magnetic fields. Graphic courtesy NASA, European Space Agency, (c) A. Feild, Space Telescope Science Institute.

Scientists had speculated for several decades that Ganymede could be an oceanic moon. The Galileo probe, which flew by the moon several times during its 1995-2003 mission, provided further grist for that supposition when it measured the moon’s magnetic field. But those flybys did not last long enough for scientists to detect the movement of Ganymede’s aurorae.

“The flybys lasted only 20 minutes each,” Saur said. “In the new Hubble observations we have seven hours of data, so we do not have the ambiguity anymore.”

Ganymede’s sea may also be impacting the surface of the moon. Mapping of the satellite by the U.S. Geological Survey reveals areas of the surface that are smoother and less cratered than other areas, which could indicate the presence of tectonic forces that permit movement of sea water from beneath the icy crust.

This illustration of Ganymede's interior shows a layered structure. It is based on observations by the Galileo spacecraft and the Hubble Space Telescope, as well as theoretical models.  Graphic courtesy NASA, ESA, (c) A. Feild, STSci.

This illustration of Ganymede’s interior shows a layered structure. It is based on observations by the Galileo spacecraft and the Hubble Space Telescope, as well as theoretical models. Graphic courtesy NASA, ESA, (c) A. Feild, STSci.

“These lighter shaded regions are believed to be formed by flooding on the surface by water coming to the surface by faults or even cryovolcanoes,” Jim Green, NASA’s director of planetary science, said.

Ganymede is Jupiter’s largest moon, with more mass than Mercury. It joins fellow Galilean moon Europa and Saturn’s moon Enceladus on the list of moons known to have water oceans.

Another of Jupiter’s four largest moons, Callisto, is also thought likely to have liquid water.

“Every observation that we make, every mission that we send to various places in the solar system, is taking us one step closer to finding that truly habitable environment, a water-rich environment in our solar system,” Heidi Hammel, executive vice president of the Association of Universities for Research in Astronomy, said. “Everywhere we look there’s water.”

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