Alien life may be hiding beneath a "club sandwich" layer of ice and water on Ganymede, according to a new study. 

The largest moon in the solar system, Ganymede, may feature liquid oceans layered between vast sheets of ice. Astronomers previously believed the surface of the planet was composed of a single layer of ocean between two layers of ice, like a simple sandwich. This formation would make life on the giant moon unlikely, as liquid water would be kept under high pressure, and few nutrients would dissolve into the ocean from the rocky layer beneath the ice floor. 

The new study suggests the situation may be more complex than that basic  idea. 

"Ganymede's ocean might be organized like a Dagwood sandwich," Steve Vance of NASA's Jet Propulsion Laboratory said. In the Blondie comic strip, published since 1930, the husband Dagwood often makes sandwiches stacked impossibly high with far too many fillings. 

At standard pressures on Earth, ice is less sense than water, and floats. At greater pressures, water molecules in the material become more compact, and sink. Within these layers of liquid and frozen waters, marine life may have found a habitat in which it could thrive. 

Europa, another moon of Jupiter, as well as Enceladus, orbiting Saturn, are also considered to be likely locations for alien life, according to astrobiologists. 

"The large icy moons of Jupiter contain vast quantities of liquid water, a key ingredient for life. Ganymede and Callisto are weaker candidates for habitability than Europa... high-pressure ice layers cover their seafloors and prevent significant water-rock interaction. Water-rock interactions may occur, however, if heating at the rock-ice interface melts the high pressure ice," researchers wrote in an article detailing the results of their study. 

Vance and his team concluded oceans on Ganymede may contain salt, allowing liquid water to run through the bottom layer of ice. Life may have first originated on Earth around hydrothermal vents, which feed waters with the building blocks of genetic material. 

Oceans have been discovered under the frozen surfaces of Ganymede, Europa and Callisto by the Galileo mission, orbiting Jupiter. 

The Jupiter Icy Moons Explorer (JUICE), a European space mission to explore the three moons, will launch in 2022. This mission should be able to reveal secrets of the alien oceans. 

Study of the club sandwich nature of Ganymede's oceans was published in the journal Planetary and Space Science.

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