Two Earths tidally locked

[rainy]

If there were two earths, tidally locked in a set up like Pluto and Charon…

<insert massive hand-waving>

…then the tidal bulges created on both planets would mean that no sea would exist on either world at their closest points? Is that correct? My understanding of this is that the tidal pull would deform the crust of the planet, creating a “high spot.” So even if this point came to rest, as it were in the Pacific Ocean, then the crust would rise up (bulge) in that area and the sea would flow away to lower ground.

Have I got that even roughly right?

Thanks,

-rainy

[DrFidelius]

I doubt it, although I have not run any numbers, and I am sure that seperation is a factor.

Given that the hydrosphere is more fluid than the lithosphere, I would think that any tidal forces would work more visibly on the oceans than on the crust.

Given that, I would expect to see permanent oceans at the tidal bulges.

[CalMeacham]

Robert L. Forward, in his Rocheworld series, has two planets tidally locked and so close that, not only do the oceans bulge there, but you sometimes get a “waterfall” from one planet to the next.

I haven’t worked the numbers, but presumably Forward had. He was physics R&D at Hughes before he started writing

[rainy]

Excellent, thanks for setting me straight.

[standingwave]

@rainy

…then the tidal bulges created on both planets would mean that no sea would exist on either world at their closest points?

Reminds me of Jinx from Larry Niven’s “Known Space” universe:

Human colony on satellite moon of a gas giant named Binary in the Sirius A system. One of the more unusual planets, it has a distinct horizontal oval or egg shape resulting in 5 very distinct environmental zones. At the center, or “waist”, is a vertical band of high-pressure atmosphere. On either side are habitable zones known as the East Band, a civilized area, and the West Band, still a developing frontier. At the ends are the East End and West End which literally stick out of the planets atmosphere, making it excellent for vacuum industries. The East End always points to Binary. It has a period of rotation of 4 Earth days long.

http://upload.wikimedia.org/wikipedia/en/9/98/Analog_jan_1975.jpg

Encyclopedia of Known Space

[EdwardLost]

The rock and the water would respond in the same way to the tidal effects. Sea level would follow the contour of the bulge. So there could be an ocean on the inward side.

[cornflakes]

Wouldn’t the more dense rock deform, or “sink” into the sea, making an island at the point closest to the other planet?

[Chronos]

The fact that rock is denser than water just means that you’ll tend to have rock below and water above, just like on our planet. You might end up with an island right below the other planet, but if so, it’d be for the same reasons we end up with islands here on Earth. Locally, nothing would be any different.

[LSLGuy]

@cornflakes

Wouldn’t the more dense rock deform, or “sink” into the sea, making an island at the point closest to the other planet?

I’m baffled. If the rock sinks, why would that produce an island? An island is a place where the rock hasn’t sunk. If the rock *did *sink, I’d expect a lake or ocean to form there. IOW, the exact opposite of an island.

As **Chronos **said, the physics doesn’t work that way. I’m struggling with your sentence & logic.

[md2000]

The tidal bulge - rock or water - is a compromise or balance between tidal forces and the gravitational forces of the planet’s mass. If the planet were all water (or some similar shape) my gut instincts about physics tell me it would be an ovoid.

The question is - how pronounced? Significantly egg-shaped, or only say, a kilometer off in a diameter of 12,000km? Because with a more distinctive oval shape, gravity will be significantly stronger in the equatorial band, being closer to the center of mass (hence the poles in vaccuum of Jinx).

If gravity varies, then my inclination would be to suggest that in fact the poles would be water, since the densest material is pulled closest to the center of mass. (On earth, the core is mostly iron, while the floating crust mostly is that greebly crud of silicon, carbon, H2O and other lighter scum.) You could end up with a world that very quickly allowed all the water to run up to the poles and then evaporate out into space.

If the ovoidity was not too pronounced, I suspect the actual geography of the planet would depend on plate techtonics; again, the impetus of plate techtonics would probably be upwelling in the poles (lighter stuff pushed out to the poles) and subduction in the area of highest gravity, the equator. Just sayin’

Also, the limit to tidal forces is IIRC called “Roche’s limit” about 2.7 times the diameter of the larger body? Too close and the smaller body becomes an ornamental ring of debris - gravity is not enough to prevent tidal forces from disassembling it. Forward may have worked out the physics, but I have trouble finding his world credible or long-term stable.

Tidal force -
imagine a trio of satellites A-B-C evenly spaced on a connecting 20-mile long rod. They are orbiting a large planet, which we will call “earth”. The satellite in the middle (B) orbits at the proper speed to stay in a circular orbit. A is 10 miles closer so it needs to orbit faster to stay in that circular orbit - but is orbiting at the same speed as B. What happens to something that should orbit at speed X, but is actually going slower? It wants to fall inward. Similarly C is going too fast for it’s orbit, and wants to fly outward due to centrifugal force. Only the 20-mile-long rod, our analog to gravity of the second world, stops the pieces from flying off in other directions. The stable result is where the rod continously points to the center of gravity of earth.

[EdwardLost]

@md2000

The tidal bulge - rock or water - is a compromise or balance between tidal forces and the gravitational forces of the planet’s mass. If the planet were all water (or some similar shape) my gut instincts about physics tell me it would be an ovoid.

The question is - how pronounced? Significantly egg-shaped, or only say, a kilometer off in a diameter of 12,000km? Because with a more distinctive oval shape, gravity will be significantly stronger in the equatorial band, being closer to the center of mass (hence the poles in vaccuum of Jinx).

If gravity varies, then my inclination would be to suggest that in fact the poles would be water, since the densest material is pulled closest to the center of mass. (On earth, the core is mostly iron, while the floating crust mostly is that greebly crud of silicon, carbon, H2O and other lighter scum.) You could end up with a world that very quickly allowed all the water to run up to the poles and then evaporate out into space.

If the ovoidity was not too pronounced, I suspect the actual geography of the planet would depend on plate techtonics; again, the impetus of plate techtonics would probably be upwelling in the poles (lighter stuff pushed out to the poles) and subduction in the area of highest gravity, the equator. Just sayin’

Again, the rock and the water would respond equally to the gravitational field plus tidal effects. If the planet were all rock or all water it would settle into the same ovoid (or whatever - I’m also not sure of the exact shape). Absent any tectonic or other geologic processes, the rock bulk of the planet would be a smooth ovoid and the ocean would be a similar ovoid around it. The ocean would be a little deeper at the ends facing at and away from the other planet and shallower along the midline.

The planet Jinx is not a relevant model because it’s distortion is not caused by tidal forces; the atmosphere gathers around the midline because the distortion in the planet’s shape causes a gravitational gradient and the air just flows “down”. I suppose Jinx was originally face-locked with other planet which was sucked away by a worm hole shortly before humans arrived. Gravity will eventually pull Jinx into a sphere.

[cornflakes]

@LSLGuy

I’m baffled.

I’m overthinking the concept (and a little embarrased besides.) If the planets were close enough that the rock in one would rise out towards the other, then the two planets would have become one.

[The_Controvert]

@CalMeacham

Robert L. Forward, in his Rocheworld series, has two planets tidally locked and so close that, not only do the oceans bulge there, but you sometimes get a “waterfall” from one planet to the next.

I haven’t worked the numbers, but presumably Forward had. He was physics R&D at Hughes before he started writing

If that were the case, wouldn’t all the water end up in the point of center of gravity between the two planets?

[Cerowyn]

@The_Controvert

@CalMeacham

Robert L. Forward, in his Rocheworld series, has two planets tidally locked and so close that, not only do the oceans bulge there, but you sometimes get a “waterfall” from one planet to the next.

I haven’t worked the numbers, but presumably Forward had. He was physics R&D at Hughes before he started writing

If that were the case, wouldn’t all the water end up in the point of center of gravity between the two planets?

I’m no physicist, but how could tidal forces be so great as to siphon water off the surface of a planet, and yet one or both not be inside the Roche limit? A little back-of-the-envelope modeling suggests to me that the water can’t achieve escape velocity for any world massive enough not to break up under such conditions. Of course, I could be totally missing something.

[Chronos]

I’m no physicist, but how could tidal forces be so great as to siphon water off the surface of a planet, and yet one or both not be inside the Roche limit? A little back-of-the-envelope modeling suggests to me that the water can’t achieve escape velocity for any world massive enough not to break up under such conditions. Of course, I could be totally missing something.

It’d be a difficult balancing act, but if you positioned the planets just right, and the oceans were deep enough, you could get siphoning of water without siphoning of rock. It wouldn’t last long, though: All that water sloshing around would dissipate significant amounts of orbital energy, so pretty soon, you’d start siphoning the rock, too.

For the shape, if the distortion is small, it’ll be close to symmetric, with the far bulge being the same shape as the near bulge. As the distortion gets larger, though (increasing the mass of the distorting object, or decreasing the distance), the near bulge gets pointier, until eventually you’re entirely filling the Roche lobe and have a cone-shaped cusp there.

And Niven did state somewhere in the Known Space books (though I don’t remember where) that Jinx was originally closer to its primary and was shaped by tidal forces, but then got moved out to a more distant orbit somehow after that.