I have long been interested in the topic of subsurface oceans on our solar systems moons and dwarf planets, and the consensus for pluto's moon charon is that charon had a subsurface ocean early in it's history that then froze and resurfaced the moon, though that got me wondering, the average temperature of charon is only 30 degrees below the average temperature of enceladus, plus it orbit much closer to its companion and is around 600 miles in diameter, could/would/does the tidal heating from its orbit provide enough thermal energy to keep at least the very bottom layers of a briny ocean in a liquid state? Or would it simply have frozen solid eons ago as most articles state?
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3$\begingroup$ :/ They're tidally locked. $\endgroup$– MithoronCommented Jul 23 at 12:42
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5$\begingroup$ @Mithoron Enceladus is tidally locked to Saturn too. That doesn't preclude tidal heating. The orbit's eccentricity can cause tidal heating, not only rotation. $\endgroup$– Darth PseudonymCommented Jul 23 at 16:15
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Tidal heating is complex, but the friction can come from either rotation (the tidal bulge moves around, stretching the body in different directions over time) or from orbital eccentricity (the bulge becomes larger at the nearest approach to the primary and smaller as it moves away). However, because the tidal heating is stealing energy from the body's movement, over millions of years it should stop. Rotation will slow as the bodies tidally lock, and the orbit will circularize, unless it's somehow being prevented.
Enceladus is tidally locked to Saturn, but has an eccentricity of 0.0047, which, given Saturn's great mass, is enough to generate the heat necessary to liquify the subsurface water. Normally, the heating would sap energy from Enceladus's orbit, slowly circularizing it over time, but its resonance with Dione causes it to stay in that slightly oblong orbit.
Unfortunately, Charon isn't going to benefit from that sort of interaction. It has an eccentricity of only 0.000161, 1/30th of Enceladus's, and Pluto's tides would be much smaller to begin with due to its tiny mass, even with Charon being much closer. We should not expect Charon to have any sub-surface water.
If Charon did have any internal heat, it would almost certainly be radioactive rather than tidal, and it's hard to imagine such a small object holding onto heat well enough to reach the melting point. But, well, nobody expected Pluto to have evidence of sub-surface water either, so it's not impossible.
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$\begingroup$ And I assume that due to charons small mass, any heat produced radioactive decay would have ceased long ago? $\endgroup$ Commented Jul 23 at 17:54
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1$\begingroup$ Radioactive heating would last as long as the radioactives held out, but the low mass would make retaining that energy difficult -- so, no, there probably isn't any meaningful radioactive heating going on out there. $\endgroup$ Commented Jul 23 at 18:06