Scientists working with NASA's Cassini spacecraft have found the strongest evidence yet for an extensive liquid-water ocean beneath the frozen surface of Enceladus, a small, icy moon of Saturn. The moon is believed to have a rocky core, so the ocean would sit atop that core, centred under the south pole.
"It is as large as, or larger, than Lake Superior," said David Stevenson, a professor of planetary science at the California Institute of Technology and a member of the team that made the discovery.
Because of the importance of water for living organisms, the result further bolsters the case for Enceladus as a prime destination in the search for life elsewhere in the solar system.
Enceladus has been an object of intense interest since NASA's Cassini mission arrived at Saturn a decade ago and began providing detailed images of the moon.
After one close pass, the spacecraft spotted a fine spray of water vapour emanating from the south polar region of Enceladus. During subsequent encounters, the spacecraft flew through the plume of ejected material that is constantly streaming off from Enceladus and it detected traces of minerals thought to be associated with salt water.
This is enough to reveal that Enceladus has internal heat and that somewhere below there's enough energy to turn ice into vapour. The question until now has been whether there's liquid water too – and how much of it. The minerals suggest the jets may be connected to a source of water with dissolved salts (which would help keep it from freezing), but they don't indicate the amount of water present.
THE NEW EVIDENCE
Scientists carefully measured the motion of Cassini during several close passes of Enceladus to see how the spacecraft is affected by the moon's gravitational pull.
Hidden variations in the internal structure of Enceladus can cause the spacecraft to accelerate and decelerate as it flies over different regions. Those changes can be read as subtle shifts in the frequency of Cassini's radio signal – a phenomenon known as the Doppler effect.
When all the shifts are put together, the result is a gravity map of Enceladus. The map shows pronounced regions of lower gravity near the south pole. The gravity is lower because there is a surface depression in that location, so there is less mass to pull on the spacecraft as it passes overhead. But the the size of the depression suggests the gravity should be even lower than Cassini measured.
To explain this discrepancy, something more dense than ice must be sitting under the surface and pulling on the spacecraft. The researchers say the most plausible explanation is a layer of water, more than 30 kilometres down and up to 10 kilometres deep.
The result appears to confirm the existence of a large reservoir that could be feeding the vapour jets through a network of cracks.
By all rights, Enceladus should be a cold, dead world.
It's only 500 kilometres across and its surface temperature hovers around –200. It should long ago have radiated away any heat that was created when it formed.
So where is the heat coming from that can keep a small ocean from freezing?
One explanation is that Enceladus is caught up in an elaborate gravitational dance with a larger moon called Dione. Because Enceladus orbits around Saturn exactly twice every time Dione orbits once, the smaller moon feels a periodic tug from the larger one. This tugging makes the orbit of Enceladus less circular and more like an ellipse. As a result, Enceladus flexes and bulges as it repeatedly leans in and then backs away from Saturn. Like a squash ball that is vigorously squeezed to warm it up, this flexing heats Enceladus from the inside. In theory, the process could deliver enough energy to keep the interior of Enceladus partially liquid.
Water is a key condition for life, so the discovery is exciting but comes with some qualifiers.
"The presence of water alone is not enough to ensure an environment habitable for life as we know it," said Alexandra Pontefract, a post-doctoral researcher in astrobiology at the University of Western Ontario.
In general, three conditions are needed to satisfy the requirements for all life. Water is one. Nutrients – the basic chemicals that fuel metabolism – is another. The third is an energy gradient – a pathway for the chemicals to react and drive living processes.
The latest find shows there's water on Enceladus in abundance. It can be assumed the nutrients are there too, because the water is in contact with a rocky core that likely has all the chemical ingredients life would need. The final condition involves the energy. That's where the story gets murky because it's not clear how much energy there is under the ice. However, researchers say it's a promising sign that there appears to be enough heat available to drive water up to the surface and power the vapour jets.
"I think you can say objectively that it has all the conditions you need to be habitable," said Sean Hsu, a planetary scientist at the University of Colorado who was not involved in the finding.
Another caveat is that the conditions needed to support life may not be enough for life to evolve in the first place.
Also, the viability of Enceladus as an astrobiological enclave may depend on how long the water has been present. Enceladus could, for example, go through repeating episodes of freezing and melting tied to long-term cycles in its orbit around Saturn.
What can be said is that if the right kind of bacteria from Earth were deposited on Enceladus today, they would almost certainly be able to eke out a living.
"There are terrestrial organisms that would be perfectly comfortable in that environment," said Jonathan Lunine, a team member and planetary scientist at Cornell University.
Scientists agree it will take further exploration to determine whether Enceladus is actually a haven for life or merely a sterile moon waiting for life to arrive. Finding out will be an expensive and long-term proposition given that the Saturn system is about a billion and a half kilometres from Earth. The Cassini mission, a joint U.S.-European effort that launched in 1997, cost more than $3-billion.
The presence of the jets adds to the attractiveness of sending a mission, since evidence for life might already have been sprayed out onto the surface where it can be found without having to explore below the surface.
But the development of such a mission will also fuel debate over how best to allocate resources in space. Both NASA and the European Space Agency are heavily invested in missions to Mars, which is closer and easier to get to. But on Mars the science has largely focused on the search for past life. Many researchers say Enceladus is potentially far more exciting.
One other contender is Europa, a moon of Jupiter that is believed to have a more extensive subsurface ocean. But the high radiation environment around Jupiter poses a significant challenge for any spacecraft trying to land on Europa.
However long it takes, scientists keen to search for life on Enceladus will also have an additional hurdle to clear because of the possibility of "inoculating" it with microbes carried from Earth, said Lyle Whyte, a microbiologist at McGill University.
"What it really means is that you'd have to sterilize the spacecraft before you send it," he said, adding another $100-million or so to the cost of the mission.
But given the latest data, it's clear that an ocean of possibility awaits.