On Earth, the heat that drives geology is partly leftover from the planet's formation and partly the result of radioactive decay. For the smaller bodies of our Solar System, neither of these should be big factors. Yet many of them are geologically active, thanks to heat generated by gravitational interactions. Uneven gravitational forces throughout a moon's orbit leads to internal flexing, generating enough heat to power geysers and volcanoes.
Or we think. In the case of Enceladus, Saturn's geyser-riddled moon, calculations suggest that the heat generated by orbital torques would only be enough to keep the moon's internal ocean liquid for about 30 million years. And, once its sub-surface ocean freezes, the moon's ability to flex goes down, which means less internal friction to warm it back up again. So why does Enceladus have an ocean at all, billions of years after it formed?
According to new research published in Nature Astronomy, that ocean survives because the core of the moon isn't a solid sphere of rock and metal; instead, it's a porous, loosely aggregated hunk of rock. Its sponge-like nature allows tidal heating to warm up its water to roughly 90 degrees Celsius.
There's some reason to think that Enceladus might have a porous core. Based on the moon's total mass and the knowledge that much of it is water, we can make some informed guesses about what its core looks like. And they're all on the low side of what you'd expect from a mixture of rock and metal. That can be explained by internal channels that allow some of the sub-surface ocean into the core. If we assume that the core has a composition similar to that of common asteroids, then its density indicates that about 20 to 30 percent of the core is water.
People have attempted to model what that would mean for the physical processes that go on inside Enceladus, but the models have been pretty simple up until now. But recently, a small international team built a fully 3D model of the entire moon, including its core, ocean, and icy surface. The model accounted for gravitational friction in a core that was "highly granular/fragmented," and it tracked the heat as it was distributed through the rest of the moon.
The models varied based on details like the porosity used and how much friction was generated. But all of the conditions produced a significant amount of power, anywhere from 10 to 30 Gigawatts. Water ended up heated to 363 Kelvin, which is just shy of the boiling point on Earth.
Warmer water is less dense, and so it should rise toward the surface. This sets off a process a bit like the mantle plumes seen on Earth, where hot upwellings drive material upward, displacing cooler material that sinks toward the core. Any seafloor hotspot that generates power over 5GW is able to create a plume. Given a small enough core and high enough energy, these combine to create a web of upwellings across Enceladus' interior. Heat also ends up concentrated under the poles, which may help explain why liquid water is closest to the surface there, powering its geysers.
As long as the power generated is above 15GW, it's enough to keep Enceladus' global ocean liquid for billions of years. No other sources of heat other than the gravity-driven tidal effects are required.
While it may be hard to imagine the core of an entire moon being fractured enough for any of this to work, Enceladus is relatively small and might not have formed in a way that generated enough heat to completely rework its core. Its radius, for example, is only half that of the dwarf planet Ceres in the asteroid belt. So there's a chance that the core is something akin to a "rubble pile" that is seen on smaller asteroids.
Nature Astronomy, 2017. DOI: 10.1038/s41550-017-0289-8 (About DOIs).
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