Tides on Exomoons

[Interstellar Planet]

I've been thinking about the height of tides on habitable exomoons. For a hypothetical moon around HD 28185 b located at about 15 planetary radii away, I calculated that the tidal forces acting across the hypothetical moon would be almost 7000 times as large as the tidal forces that our moon causes on earth! (I used the formula in this article.) The tidal acceleration at the surface of the hypothetical moon would still only be about 1/1000 of a G, but the tidally acceleration that causes earth's tides is only about 1.1 * 10^-7 G.

How would tidal forces 7000 times stronger than those on earth affect water on an exomoon? I am certainly hoping that the tides would not be 7000 times as high, because that would cause significant problems for life.

 

[fsci123]

I dont think it would effect anything for a very long time. Any gravitational tides will cause the moon to become tidally locked. However you could expect their to be a two bulges of water at the inward and outward side of the moon.

However... If there are any other moons in the system...They would also exert some influence on the tides depending on their mass and proximity.

 

[Interstellar Planet]

How large would the bulges be, though? Even if the moon wasn't rotating, wouldn't the larger gravity difference between the inward and outward sides still cause massive bulges?
I was originally talking about the gravity difference between the two sides being 7000 times more than Earth's.

 

[RisingFury]

There's more to tides than a high tidal force.

Tides on Earth are caused by the Moon and the Sun, sure, but we see the tides go in and come out not because the orbit of the Moon is elliptical, but because the Earth rotates.

It turns out that as a body that has tides rotates, it inevitably slows its rotation to the point when it becomes synchronous. Our Moon has had the time to tidally lock itself to Earth. If there was an ocean on the Moon, the tide would be constantly high where it faces (to and away) the Earth, but the Sun would also provide a bit of tides, which would change over the course of Lunar month...

When it comes to Jupiter and its four large moons - all of them are tidally locked. The reason Io is a ball of molten rock and Europa has a subterranean ocean is because the orbit is slightly elliptical and the moons are getting stretched and squeezed.

Turns out, however, that tidal forces act to circularize the orbit - so a single moon around Jupiter would have a tendency to lock itself and circularize its own orbit, at which point, power from tidal heating would drop off. Io, Europa and Ganymede, however, are locked in a 1:2:4 orbital resonance, which tends to increase eccentricity over time.


So a moon would not have tides like we do on Earth, at least not for long.

But suppose it hasn't yet had the time to circularize its orbit and tidally lock itself - how high would tides be? Well, when Io orbits Jupiter, the difference between the high and low tidal force is so large that the moon is reshaped by some 200 m in diameter. The surface rises by some 100 m on each side.

Keep in mind that's the *difference* between periapsis and apoapsis. This number does not include the already deformed shape of the Moon.

On Earth, the Moon and the Sun can influence the crust to move some 20 cm.

I can imagine that a rotating moon with liquid water (that flows more easily than rock - unrestricted by continents, of course) could have a tide in the 1 km range.

 

[Interstellar Planet]

If there was an ocean on the Moon, the tide would be constantly high where it faces (to and away) the Earth, but the Sun would also provide a bit of tides, which would change over the course of Lunar month.

Sorry, I probably should have worded my question better. I should have said "ocean depth" instead of "tide height". I understand that exomoons would most likely be tidally locked. Even if a moon's orbit was perfectly circular, and the moon was tidally locked, the planet's gravity would still cause the moon to be permanently stretched into an ellipsoid. As you said, there would be "eternal high tides" on both sides of the moon. How large would the difference in ocean depth be? Would all the water be pulled to smaller regions on either side of the moon leaving the rest of the moon exposed? Or would the crust stretch along with the water to cancel the effect?