Enceladus: cold crust, soft centre (Image: NASA/JPL/SSI/Corbis)

In the cold of space, a tiny moon is keeping warm with a fluffy heart. Enceladus, the pipsqueak of a world renowned for shooting huge watery plumes into space as it orbits Saturn, has a secret – a core that contradicts everything we thought we knew about the structure of planetary bodies.

Planets and moons usually follow the classic “onion” model: an outer crust or atmosphere over a large mantle, with a sphere of dense material at the core. But new research presented at the Geological Society of America meeting in Vancouver on 19 October suggests Enceladus isn’t following the rules.

The moon’s density suggests it contains some rock. And since its surface is icy and it has a penchant for spewing water and ammonia into space, researchers have concluded it probably has a crust of ice, a watery mantle and a core of solid rock.


As Enceladus orbits Saturn, changes in the planet’s gravitational pull flex the moon, heating it up. But when James Roberts at Johns Hopkins University developed computer models to test just how this works, something didn’t add up.

A stiff rocky core would not flex enough to generate the heat necessary to melt the ice or explain the jets. When Roberts made the core more like a snow cone than a stone, it flexed and created enough heat to fuel its famous water jets.

Cold but not dead

“Enceladus has been surprising us all along,” Roberts says. “You’d think something the size of the North Sea would be cold and dead, but the Cassini spacecraft has been observing activity since it arrived.”

Roberts says several other bodies in the solar system – including Saturn’s moon Mimas, and the dwarf planet Ceres – could have similarly “fluffy” cores.

The finding has intriguing implications for finding life on these small worlds, says Hunter Waite at the Southwest Research Institute in San Antonio, Texas, who has studied Enceladus’s plume eruptions. “You have the ability for water to interact with rock over a vast region,” he says. “You can create hydrothermal systems like those on early Earth where life may have formed.”

However, because of Enceladus’s diminutive size, the chemistry necessary for life may have petered out by now. “It’s been around a long time, and there’s not that much rock,” Waite says. “So after a point you have pretty much taken the free chemical energy out of the rock.”

The only way to find out if the moon is still habitable is to go back on a future mission, he says. “Are we living in the perfect time to observe this? That’s a kind of the wild card here.”