Ganymede’s Subsurface Ocean is Like a Club Sandwich

by Nancy Atkinson on May 1, 2014

This artist's concept of Jupiter's moon Ganymede, the largest moon in the solar system, illustrates the club sandwich model of its interior oceans. Credit: NASA/JPL

This artist’s concept of Jupiter’s moon Ganymede, the largest moon in the solar system, illustrates the club sandwich model of its interior oceans. Credit: NASA/JPL

Moons with subsurface oceans are all the rage these days. There’s Europa, Titan, and just recently Enceladus joined the short list of moons that likely harbor large amounts of subsurface water.

Jupiter’s moon Ganymede – the largest moon in the solar system — has long been a member of this club. The idea of this moon having water deep within its surface first “surfaced” back in 1970, and in 2000 after the Galileo mission flew by Ganymede, data confirmed the moon’s ocean, and showing it extends to depths of hundreds of miles, with additional evidence of salty seas.

Now, a new study says that the configuration of this moon’s interior might be more like a club sandwich, according to Steve Vance of NASA’s Jet Propulsion Laboratory, who led the research.

“Ganymede’s ocean might be organized like a Dagwood sandwich,” Vance said in a NASA press release, with ice and oceans stacked up in several layers, as in the graphic above.

The results also support the idea that primitive life may have possibly arisen on this icy moon.

Ganymede is about to hide behind Jupiter.  Credit: NASA, ESA, and E. Karkoschka (University of Arizona)

Ganymede is about to hide behind Jupiter. Credit: NASA, ESA, and E. Karkoschka (University of Arizona)

This layered look was actually proposed last year by Vance and his team, and this latest research is based on theoretical computer modeling, where areas previously thought to be layers and lumps of just rocks and ice in Ganymede’s interior are actually layers of ice, water and rock.

Usually, places where water and rock interact are ripe for the development of life, scientists say. For example, its possible life began on Earth in bubbling vents on our sea floor.

The model computed by Vance and his team gets complicated when the different forms of ice are taken into account, which can cause varying amounts of pressure. This can change the whole dynamics of the moon’s interior.

If the lightest ice is on top, then the saltiest liquid is heavy enough to sink to the bottom. As the oceans churn and cold plumes snake around, ice in the uppermost ocean layer form in the seawater. When ice forms, salts precipitate out. The heavier salts would thus fall downward, and the lighter ice, or snow, would float upward. This snow melts again before reaching the top of the ocean, possibly leaving slush in the middle of the moon sandwich.

And if the first layer on top of the rocky core is salty water, that’s even better.

“This is good news for Ganymede,” said Vance. “Its ocean is huge, with enormous pressures, so it was thought that dense ice had to form at the bottom of the ocean. When we added salts to our models, we came up with liquids dense enough to sink to the sea floor.”

You can read more about their modeling here, and the research appears in the journal Planetary and Space Science. We’ll add a link to the paper when it becomes available.

About 

Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

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