Ganymede harbours layered sandwich of oceans and ice

This artist's concept of Jupiter's moon Ganymede, the largest moon in the solar system, illustrates the layered sandwich model of its interior oceans. 

Credit: NASA /JPL-Caltech

The largest moon in our solar system, a companion to Jupiter named Ganymede, might have ice and oceans stacked up in several layers like a club sandwich, according to new NASA-funded research that models the moon's makeup.

Previously, the moon was thought to harbor a thick ocean sandwiched between just two layers of ice, one on top and one on bottom.

Steve Vance

"Ganymede's ocean might be organized like a layered sandwich," said Steve Vance of NASA's Jet Propulsion Laboratory in Pasadena, Calif., explaining the moon's resemblance to multi-tiered sandwiches.

The study, led by Vance, provides new theoretical evidence for the team's "layered sandwich" model, first proposed last year.

The research appears in the journal Planetary and Space Science.

The results support the idea that primitive life might have possibly arisen on the icy moon.

Scientists say that places where water and rock interact are important for the development of life; for example, it's possible life began on Earth in bubbling vents on our sea floor.

Prior to the new study, Ganymede's rocky sea bottom was thought to be coated with ice, not liquid, a problem for the emergence of life.

The "layered sandwich" findings suggest otherwise: the first layer on top of the rocky core might be salty water.

"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."

NASA scientists first suspected an ocean in Ganymede in the 1970s, based on models of the large moon, which is bigger than Mercury.

In the 1990s, NASA's Galileo mission flew by Ganymede, confirming the moon's ocean, and showing it extends to depths of hundreds of miles.

The spacecraft also found evidence for salty seas, likely containing the salt magnesium sulphate (Epsom salt).

Previous models of Ganymede's oceans assumed that salt didn't change the properties of liquid very much with pressure.

Vance and his team showed, through laboratory experiments, how much salt really increases the density of liquids under the extreme conditions inside Ganymede and similar moons.

It may seem strange that salt can make the ocean denser, but you can see for yourself how this works by adding plain old table salt to a glass of water.

Rather than increasing in volume, the liquid shrinks and becomes denser. This is because the salt ions attract water molecules.

More information: "Ganymede's Internal Structure Including Thermodynamics of Magnesium Sulfate Oceans in Contact with Ice," Steve Vance et al., Planetary and Space Science, 2014, in press, dx.doi.org/10.1016/j.pss.2014.03.011