Ganymede's complex ocean could host alien life
Sen—Though the giant gasball planets Jupiter and Saturn are too different from Earth to be home to life as we know it, some of their satellites have drawn the interest of astrobiologists because conditions there might be ideal.
Saturn’s moon Enceladus has an undergound sea that constantly spouts geysers of salty water that have been imaged by the international Cassini space probe. Scientists have been pressing for a special mission to go there and to check out one on Saturn’s biggest moon Titan too.
They have also been keen to investigate Europa, one of the four big Galilean moons of Jupiter because it too appears to have a reservoir of liquid water beneath its surface.
A European mission called JUpiter ICy moons Explorer (JUICE) is already being drawn up to start exploring Jupiter’s satellite system in the 2030s. And NASA called last week for proposals for a probe to Europa, having already designed a concept mission called Europa Clipper.
What excited the astrobiologists is that where there is liquid water, there could have developed a simple aquatic form of life. For though Jupiter and Saturn both lie far beyond the Sun’s “habitable zone”, the gravitational pull of the giant planets on their moons provides the energy that keeps the sub-surface water in a warm, liquid state.
An artist's cutaway illustrates the “club sandwich” model of the layers of water beneath the surface of Jupiter's moon Ganymede. Image credit: NASA/JPL-Caltech
But it is not just Europa in the Jovian system that has an underground salty ocean. Since NASA’s Voyager missions of the 1970s, it was suspected that one exists on Ganymede, the largest moon in the Solar System, too.
The existence of Ganymede’s sea was confirmed when NASA’s Galileo mission to Jupiter followed in the 1990s and found it extended to a depth of hundreds of kilometres. It was discovered that Callisto also probably had an ocean beneath its surface.
Latest research suggests Ganymede’s water is structured in many levels, rather like a club sandwich, with multiple layers of ice and water. Studies led by Steve Vance, of NASA’s Jet Propulsion Laboratory in California, produced a computer model that argues Ganymede’s ocean is sandwiched between up to three ice layers as well as a rocky seafloor.
The new results boost the chances that primitive life could have arisen on Ganymede, say the team because interaction between a rocky layer and salty water are important for the development of life. They say it is possible that life began on Earth in bubbling vents on our sea floor.
Previous models of Ganymede’s oceans have assumed that salt did not change the properties of liquid very much under pressure. The experiments by Vance and his team showed how a lot of salt actually increases the density of liquids under the extreme conditions inside Ganymede and similar moons.
Vance said: “This is good news for Ganymede. 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.”
The researchers say their results could be useful in understanding the conditions that might exist on rocky super-Earths that orbit other stars and which might also be home to life.
The research is published in the journal Planetary and Space Science.
NASA’s step towards exploring Europa came with a formal Request for Information (RFI) to science and engineering communities to gain ideas for a mission to find out more about the satellite and the prospects for life.
The mission is intended to cost less than $1 billion, not including the launch vehicle. NASA wants to characterize the extent of Europa’s ocean, plus its ice shell and chemistry, as well as learn how surface features formed, and how the moon interacts with Jupiter’s magnetosphere.