Could Ganymede harbour alien life? Jupiter's moon has a 'club sandwich' of oceans and ice under its crust
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If you thought Mars was the best place in the solar system to find signs of life, you might need to think again.
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.
And the Nasa-funded research suggests that this may have once been a suitable environment for primitive life.
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Steve Vance and his team showed that ice and water under the surface has different densities. At the top is the lightest 'Ice I' that is similar to ice cubes in your drink, ranging to the heaviest and densest 'Ice VI' that would sink to the bottom of the sea
Previously, the moon was thought to harbour a thick ocean sandwiched between just two layers of ice, one on top and one on bottom, but now it seems it has multiple layers.
The study, led by Steve Vance of Nasa's Jet Propulsion Laboratory in Pasadena, California, provides new theoretical evidence for the team's 'club sandwich' model, first proposed last year.
The Nasa-funded research modelling the moon's makeup supports 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 'club 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.'
Ganymede is one of Jupiter's four largest satellites known as the Galilean moons. From left to right in this artist's mosaic they are Io, Europa, Ganymede and Callisto. They are pictured above the Great Red Spot, a storm on Jupiter that has been raging for over 400 years
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 sulfate.
Previous models of Ganymede's oceans assumed that salt didn't change the properties of liquid very much with pressure.
THE SOLAR SYSTEM'S ICY MOONS
Ganymede is one of five moons in our solar system thought to support vast oceans beneath icy crusts.
The other moons are Jupiter's Europa and Callisto and Saturn's Titan and Enceladus.
The European Space Agency (Esa) is developing a space mission, called Jupiter Icy Moons Explorer (Juice), pictured above, to visit Europa, Callisto and Ganymede in the 2030s.
Nasa and JPL are contributing to three instruments on the mission, which is scheduled to launch in 2022.
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.
The lightest ice is on top, and the saltiest liquid is heavy enough to sink to the bottom.
What's more, the results demonstrate a possible bizarre phenomenon that causes the oceans to 'snow upwards.'
As the oceans churn and cold plumes snake around, ice in the uppermost ocean layer, called 'Ice III,' could form in the seawater.
When ice forms, salts precipitate out. The heavier salts would fall downwards, and the lighter ice, or 'snow,' would float upwards.
This 'snow' melts again before reaching the top of the ocean, possibly leaving slush in the middle of the moon sandwich.
Sotin and Vance are both members of the Icy Worlds team at JPL, part of the multi-institutional Nasa Astrobiology Institute based at the Ames Research Center in Moffett Field, California.
The results can be applied to exoplanets too, planets that circle stars beyond our sun.
Some super-Earths, rocky planets more massive than Earth, have been proposed as 'water worlds' covered in oceans.
Vance and his team think laboratory experiments and more detailed modeling of exotic oceans might help find answers as to whether they could have life.
Jupiter's Ganymede, artist's illustration of the interior pictured, is the largest moon in the solar system. Although it was previously thought to have an underground ocean, new research from Nasa suggests it may actually have layers of ice of different density surrounding its subsurface seas that may have hosted life
HOW GANYMEDE'S ICY INTERIOR WORKS
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.
The models get more complicated when the different forms of ice are taken into account.
The ice that floats in your drinks is called 'Ice I.' It's the least dense form of ice and lighter than water.
But at high pressures, like those in crushingly deep oceans like Ganymede's, the ice crystal structures become more compact.
'It's like finding a better arrangement of shoes in your luggage - the ice molecules become packed together more tightly,' said Vance.
The ice can become so dense that it is heavier than water and falls to the bottom of the sea. The densest and heaviest ice thought to persist in Ganymede is called 'Ice VI.'
By modeling these processes using computers, the team came up with an ocean sandwiched between up to three ice layers, in addition to the rocky seafloor.
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