A sandy core may have kept Enceladus’ ocean warm

A soft heart keeps Enceladus warm from the inside. Friction within its porous core could help Saturn’s icy moon maintain a liquid ocean for billions of years and explain why it sprays plumes from its south pole, astronomers report November 6 in Nature Astronomy.

Observations in 2015 showed that Enceladus’ icy surface is a shell that’s completely detached from its rocky core, meaning the ocean spans the entire globe (SN: 10/17/15, p. 8). Those measurements also showed that the ice is not thick enough to keep the ocean liquid.

Other icy moons, like Jupiter’s Europa, keep subsurface oceans warm through the energy generated by gravitational flexing of the ice itself. But if that were Enceladus’ only heat source, its ocean would have frozen within 30 million years, a fraction of the age of the solar system, which formed roughly 4.6 billion years ago.

Planetary scientist Gaël Choblet of the University of Nantes in France and his colleagues tested whether friction in the sand and gravel thought to make up Enceladus’ core could heat things up.

The team made computer simulations of water circulating through the spongy core using data from the Cassini spacecraft and geoengineering experiments with sand and gravel on Earth. They found that, depending on the core’s makeup, the ocean should get enough heat to stay liquid for tens of millions to billions of years.

The simulations also showed that certain hot spots in the core, including at the poles, correspond to regions where the ice shell is thinner.

“That was quite cool,” Choblet says. “It explains the internal structure and the way things are organized and the dynamics interior to Enceladus.”

And that could explain why the moon spews plumes of water from its south pole: More heat from the core at that spot could melt the ice and let water out. It doesn’t explain why the north pole is plume-free, though.