Could Saturn's moon hold life?

Observations of Enceladus by the Cassini spacecraft suggest that it, like Jupiter’s moon Europa, could have an ocean of water beneath its surface. Helen Maynard-Casely takes a closer look.

Cassini acquired high resolution views of the puzzling ice world of Enceladus. It appears to be a mixture of softened craters and complex, fractured terrains. – NASA/JPL/Space Science Institute

Astronomers have identified another quiet corner of our Solar System where life could be lurking: a subsurface ocean on Enceladus, Saturn’s sixth-largest moon.

The results, published in April in Science by Luciano Iess at Sapienza University of Rome and colleagues, place this ocean close to Enceladus’ southern pole. We briefly noted the discovery on 7 April, but the results were so surprising it is worth taking a closer look.

The moon is tiny – just 500 km from north to south pole – and a body this small shouldn’t contain enough heat energy to keep anything liquid.

This discovery is the latest in a series made by the Cassini spacecraft, which has been circling Saturn and its moons since 2004 and the result of painstaking measurements of the icy moon’s gravitational field.

Data shows gravity varies greatly across Enceladus, suggesting that
under its icy surface it had a wonky internal structure.

On its approach to a fly-by of Enceladus in April 2010, Cassini turned its communications dish back towards Earth, powered down its thruster rockets and coasted towards Enceladus. It came as close as 100 km to the surface during its 26-hour flyby. Over that time Cassini’s speed was constantly measured by the Deep Space Network, a group of large radio telescopes around the world including in Canberra. Working together, the Deep Space Network could detect changes as small as 0.2 millimetres per second by measuring the change in the signal from Cassini – astonishing accuracy given the moon is about 1,200 million kilometres away.

An artist's impression of Cassini's fly-by of Saturn's moon Enceladus. – NASA/JPL

Measuring these tiny speed changes, the Cassini scientists could detect variations in the tug of the moon’s gravity. This data, along with similar results from fly-bys in November 2010 and May 2012, showed gravity varies greatly across Enceladus, suggesting that under its icy surface it had a wonky internal structure with denser material towards the northern hemisphere. For Iess and his colleagues, the most likely interpretation was a regionalised sea sitting under the ice crust close to the south pole.

A subsurface sea is not the only way to interpret the gravitational data, but it does fit with two other strange findings made by the first Cassini fly-bys. The images it sent back in 2005 showed giant cracks on the surface of the southern pole. Dubbed “tiger stripes”, later observations showed them to be active fissures spewing volatile substances, including water, out into space.

A diagram illustrates the possible interior of Saturn's moon Enceladus with an ocean at the southern pole and the plume of water jets gushing from fractures in the crust. – NASA/JPL-Caltech

The other clue that something was happening in the southern hemisphere of Enceladus came from measurements of a massive heat output from this area in 2009; at 15.8 gigawatts, it dwarfs the output of many power stations. Exactly what is going on under the ice to release so much energy is a mystery, but it is certainly enough to keep an ocean of water from freezing.

What’s next? More gravitational data would allow us to rule out, or rule in, whether the ocean is actually global in its extent (as is proposed from magnetic field data for Jupiter’s moon Europa). But there are no current plans to undertake another fly-by. When the spacecraft is turned towards Earth, many of its science instruments cannot be used as they are not pointing to the surface of the moon. And Cassini is working on borrowed time. Its primary mission ended in 2008 and the continuation of funding for the current mission to further investigate Saturn, its ring and moons is in doubt.

In any case, Cassini only has until 2017 when its plutonium fuel runs out.

Helen Maynard-Casely is a physicist and science writer based in Sydney.
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