Saturn’s icy moon Enceladus, already known for spitting plumes of water into space, just got even more interesting. New gravity readings suggest it hosts a subsurface sea the size of Lake Superior at its south pole – and that this liquid water is in direct contact with the moon’s core, which is rich in nutrients. Both findings boost hopes that the sea hosts life.
The result comes hot on the heels of the discovery late last year that a second icy moon – Europa, which orbits Jupiter – also spews plumes of water. Both moons are now among the hottest prospects in the solar system for finding alien life – if only a suitable mission could be arranged.
NASA’s Cassini orbiter first spotted spectacular plumes at Enceladus’s south pole in 2005, shortly after arriving at Saturn. The plumes shoot into space at supersonic speeds, feeding one of Saturn’s famous rings, and are known to contain both salts and organic compounds. They make an attractive target for exploration as a craft could potentially fly through them to take samples, much simpler than landing on a moon.
Most astronomers thought that the plumes force their way out through cracks in an icy crust sitting over a sea of liquid water. But until now, no one knew how deep that sea went.
Luciano Iess at the Sapienza University of Rome in Italy and his colleagues used radar on Earth to track Cassini on three separate fly-bys of Enceladus, and watched how the spacecraft was accelerated by the moon’s gravity. This allowed them to map the distribution of mass in the moon’s interior.
Salty ice grains
They found that Enceladus has a rocky core and an icy crust. “Before, we knew almost nothing about the core beyond its likely existence. Now we know roughly how big it is, and also that it has a surprisingly low density,” says team member Francis Nimmo at the University of California, Santa Cruz. “That might be due to open fractures, or low-density hydrated minerals like clays. Either answer suggests that the rock has been in substantial contact with water, for instance allowing minerals to dissolve, and explaining the salty ice grains we see coming out of the surface.”
The team also found that the southern hemisphere has a stronger gravitational pull than its topography would suggest. That could be explained by a localised sea, sitting beneath 35 kilometres of ice and up to 8 kilometres deep. It would contain about as much water as Lake Superior in North America.
Hosting an ocean in contact with rocks boosts the chances that Enceladus hosts life, because the rocks could leach elements like potassium, sulphur and phosphorus, vital for life, into the water.
“One of the alternative models was just little pockets of water driving the jets, and in that model you wouldn’t have much in the way of life because it wouldn’t be in contact with the rock,” says team member Jonathan Lunine at Cornell University in Ithaca, New York. “This gravity map hinting at a much larger ocean is a more favourable model for having some sort of life in Enceladus’s interior.”
If there might be life there, when can we go? Cassini winds down in three years and there are no firm plans for future craft to return to Saturn. However, Cassini team member Carolyn Porco at the Space Science Institute in Boulder, Colorado, has written a paper (soon to appear in the journal Astrobiology) arguing for a mission to collect samples from Enceladus and return them to Earth. She says the new results bode well for such an effort. “The next mission there can immediately get down to the business of searching for signs of life or its precursor chemistry. It’s a big juncture!”
The subsurface-sea idea is just the simplest possible interpretation of the gravity data, cautions William McKinnon at Washington University in St Louis, who was not involved in the work. If the sea exists, there is the question of how long it has been liquid and whether it might eventually freeze – or spray itself away. The former is important as life would require the sustained presence of water to gain a toehold.
As for the possibility of the sea freezing completely, it is true that Enceladus is losing a lot of heat to space, but astronomers suspect that this is an unusual episode. “We are looking at Enceladus at a wonderful special time, where it’s very active and there’s a lot of heat,” McKinnon says.
Could the plumes deplete the sea completely? Probably not. Even if they continue at the current rate, the moon would only have lost 30 per cent of its water by mass when the sun becomes a red giant in 6 billion years. “A lot of things can happen in 6 billion years, and it may shut off long before then, although the idea of this thing blowing all of its ice away and becoming a little rocky moon is kind of nice,” Lunine says. “Some future extraterrestrials visiting our solar system will be able to look at the naked rocky core of what was once an ice moon.”