The interaction between liquid water and a rocky seafloor on the largest moon in our solar system may support primitive life, Nasa has revealed.
A study has shown Jupiter's moon, Ganymede, may have a thick ocean stacked between a top and bottom layer of ice, which astronomers have described as a "club sandwich".
The results support the idea that primitive life might have possibly arisen on the icy moon. Scientists say that places where rock and water interact may result in chemical reactions essential to the development of life. For example, the theory that life on Earth began in ancient ocean vents.
Ganymede's rocky sea bottom was thought to be coated with ice, not liquid, which would not support life. Instead, the new study suggests the first layer on top of the rocky core may be salty water.
"Ganymede's ocean might be organized like a Dagwood sandwich," said Steve Vance of Nasa's Jet Propulsion Laboratory.
"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. 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 sulfate.
Previous models of Ganymede's oceans assumed that salt did not change significantly the properties of liquid with pressure. The latest research, however, shows salt increases the density of liquids under the extreme conditions inside Ganymede. This is because the salt ions attract water molecules.
Yet the models get more complicated when the different forms of ice are taken into account. Ice used in drinks is called Ice I, the least dense form of ice. The high pressures of Ganymede's deep oceans compact the ice crystal structures, turning the ice into the denser Ice VI.
The team modelled these processes using computers and came up with an ocean sandwiched between up to three ice layers, in addition to the rocky seafloor. The lightest ice is on top and the heavier saltiest liquid sinks to the bottom.
"We don't know how long the Dagwood-sandwich structure would exist," said Christophe Sotin of JPL told Space.com. "This structure represents a stable state, but various factors could mean the moon doesn't reach this stable state.
According to NBC News, the European Space Agency plans to launch a mission called JUICE (JUpiter ICy moons Explorer) in the early 2020s to study Europa, Callisto and Ganymede.
The research appears in the journal Planetary and Space Science.