Can tides still be on tidal-locked bodies?

[SiberianTiger]

This topic is inspired by the line from the recently published "The Shoals of The Kraken Mare":

Hey o hey hear the tug of Titan's tide,

My first thought that once Titan is tidally locked, there could be no tide on Titan. But then I tried to calculate the tidal pull caused by Saturn on Titan's surface and it occurred to me that it's very different in Titan's periapsis and apoapsis (by magnitude it's 90 and 76 times stronger than Earth's tidal acceleration exerted by the Moon in its perigee).

So my question is: can the difference in the tidal force caused by Titan's orbit eccentricity really cause movement of liquid on its surface?

 

[Urwumpe]

Yes. But it should be much weaker. A high eccentricity actually results in the rotation period approaching a resonance, like 3 rotations of the planet per 2 orbits.

 

[agentgonzo]

Yes, slightly. As the orbit is highly eccentric, it's orbital speed will change. As its rotation cannot change, the difference between its orbital speed (which is constantly changing) and its rotational speed (which is constant) will produce slight tides.

Also, as it approaches periapsis, the tidal effects will grow stronger, essentially making the tides greater (than an apoapsis) - the high tide will be higher, and the low tide will be lower. This will cause movement of liquid on its surface away from the low-tide towards the hight-tide.

As it approaches apoapsis, the tidal effects will grow weaker, making the high-tide lower and the low-tide higher, resulting in a movement of water on the surface away from the high-tide towards the low tide.

 

[RisingFury]

Yes. But it should be much weaker. A high eccentricity actually results in the rotation period approaching a resonance, like 3 rotations of the planet per 2 orbits.

A high eccentricity cannot be sustained for a long time, actually.



Though the answer to your question, SiberianTiger, is yes, there can be tides on a tidally locked body.

The eccentricity of the orbit does not need to be high, but because a tidally locked body tends to circularize its orbit, you need another moon or body that sustains the orbital eccentricity.

So what happens then is that as the body comes closer to the primary, it gets stretched more due to gravitational gradient and when it orbits further away, it reshapes itself to more closely resemble a sphere.

Io's crust raises and drops about 100 meters as it orbits around Jupiter. Its eccentricity is sustained by a 1:2:4 resonance with Europa and Callisto and is tidally locked with Jupiter.

If you think of tides as rising and falling surface, then yes, a tidally locked body can have tides...

 

[Linguofreak]

A high eccentricity cannot be sustained for a long time, actually.

It can if the orbit falls into a resonance with the body's spin or the orbit of another body. See Mercury and Pluto. Mercury is in exactly the kind of eccentric orbit and spin-orbit resonance that Urwumpe described.