Titan, whose orbital period is 382 hours, is tidally locked, like all round moons in the Solar System. But Jonathan I. Lunine said of Titan:

One thing that Titan could not have done during its history is to have a liquid layer that then froze over, because during the freezing process, Titan’s rotation rate would have gone way, way up.

This hypothetical escape from tidal locking implies thermal energy lost from a liquid layer would be converted into rotational kinetic energy, and not just for the liquid in the form of currents (in analogy with e.g. the super-rotation of Venus's atmosphere), but of the solid majority of Titan's mass. What mechanism is Lunine alluding to?


1 Answer 1


Here's a theory.
Assuming the liquid layer was in fact rotating with the solid core, and whatever materials are in the liquid are denser when solidified (unlike water), then the freezing process will bring the material closer to the center of Titan. Conservation of angular momentum would then require a faster rotation period.

  • $\begingroup$ I can only see that making a marginal difference. The radius of Moon-plus-liquid would contract at most to the radius of the bare Moon, which would require very little increase in angular velocity. $\endgroup$
    – J.G.
    Mar 30, 2022 at 13:15
  • 1
    $\begingroup$ I think there’s too many qualitative descriptions between everything to say what’s marginal and what’s considered ‘way, way up’, it might be a matter of who you’re asking; if you consider the difference in how fast a neutron spins up from a star then everything probably seems marginal, so maybe it’s enough to be considered a lot by planetary scale standards $\endgroup$
    – Justin T
    Mar 31, 2022 at 1:00
  • $\begingroup$ The quoted description refers to "freezing over", which implies something that expands when it freezes. $\endgroup$ Apr 1, 2022 at 12:51
  • $\begingroup$ @ChristopherJamesHuff No, it does no such thing. $\endgroup$ Apr 1, 2022 at 14:10

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