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 鈥� a core that contradicts everything we thought we knew about the structure of planetary bodies.
Planets and moons usually follow the classic 鈥渙nion鈥� 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鈥檛 following the rules.
The moon鈥檚 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.
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As Enceladus orbits Saturn, changes in the planet鈥檚 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鈥檛 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
鈥淓nceladus has been surprising us all along,鈥� Roberts says. 鈥淵ou鈥檇 think something the size of the North Sea would be cold and dead, but the has been observing activity since it arrived.鈥�
Roberts says several other bodies in the solar system 鈥� including Saturn鈥檚 moon Mimas, and the dwarf planet Ceres 鈥� could have similarly 鈥渇luffy鈥� cores.
The finding has intriguing implications for finding life on these small worlds, says at the Southwest Research Institute in San Antonio, Texas, who has studied Enceladus鈥檚 plume eruptions. 鈥淵ou have the ability for water to interact with rock over a vast region,鈥� he says. 鈥淵ou can create hydrothermal systems like those on early Earth where life may have formed.鈥�
However, because of Enceladus鈥檚 diminutive size, the chemistry necessary for life may have petered out by now. 鈥淚t鈥檚 been around a long time, and there鈥檚 not that much rock,鈥� Waite says. 鈥淪o after a point you have pretty much taken the free chemical energy out of the rock.鈥�
The only way to find out if the moon is still habitable is to go back on a future mission, he says. 鈥淎re we living in the perfect time to observe this? That鈥檚 a kind of the wild card here.鈥�
