Cecil's "Tube through the earth" column, a 2nd question

In regards to: “What if you fell through a tube in the Earth?” column…

What would happen if this tube were opened so that both ends opened into the oceans letting the seas drain in and fill the tube?
clarification: I am inquiring about the effects this would have on a) the oceans themselves b)the center of the earth and c) the earth overall

I’m going to ignore any possible question of the change in sea level, since that would require knowing the shape and size of the tube, as well as math. I’m just not up to it.

But it’s interesting to think of a bottomless sea channel. The pressure would increase as you went down, but only to a certain point. At some point, the attraction from the mass of earth and water above you would cancel out some of the gravity acting on you and the water, and the pressure would start to decrease. In the center, the pressure would be zero, I believe.

What would happen then if you blew a big bubble? I think it would stay in the center of the earth, offering a dry place for travellers through the tube.

Caveat: This is all off the top of my speculating head. I fully expect it to be clarified, if not totally demolished, by those with the stamina to check the math.

Interesting, but I think there would be pressure at the center. It’s true that the gravity at the center would pull you equally in all directions, and thus cancel out. But all the water extending up the tube (the tube would be going up in opposite directions) is attracted to the center.

The pressure at the center would be the integral of the mass of the water times the gravity at the given radius, from 0 to the radius of the Earth. That gravity function would be a little weird. It would increase with r^2, but decrease with r^3 (the amount of mass still attracting you to the center, all of the Earth’s mass outside your current radius perfectly cancels).

Think of it this way. Starting at the surface and sinking toward the center, the pressure will increase. For a given depth that you sink, the pressure will increase a certain amount. That amount will shrink as you get closer to the center. The pressure increase from r=2000 meters to r=1000 meters will be tiny, but there is never some opposing force that will cause the pressure to decrease as you sink farther.

[ At some point, the attraction from the mass of earth and water above you would cancel out some of the gravity acting on you and the water, and the pressure would start to decrease. In the center, the pressure would be zero, I believe. ]
you are ignoring the pressure caused by gravity of all the water on top. the pressure would be great. Think of a all water planet. Drop a piece of lead in it, it would sink to the bottom and be suspended by the force of gravity in the center (all G forces cancel) but the pressure is still there, a buble would eventually work its way to a side by curents and start upward. Also in the case of the earth, I think the water would boil for probally tens of thousands of years and act as a underwater gyser.
Everything I state is IMHO, I have never been to a planet made entirely of water, nor have I drilled a hole through the earth ocean to ocean.

Can a water/air planet exist? (no land) or would it deform under gravity of star/moons enough to rip it apart?

If the tube was full of air and the water entered from each end at the same time and under the same pressure, the air trapped in the tunnel would eventually be compressed into it’s components’ liquid state. Which would be at the middle of the earth and would be lighter than the salt water pushing against it. Would the various factors (rotation of the earth at it’s center)(pressure of the salt water), (whatever), cause the unsalted liquid to remain in the center, separating the columns of salt water?

I think gas giants like Jupiter and Saturn are kind of like that - at the very least, they have no land. I’m not a planetary scientist, but from what I remember their interiors deep down become at least vaguely liquid. The pressures and temperatures are incredible, so it’s not quite like anything you or I have actually laid eyes on and handled. :slight_smile:

This is a pretty half-baked answer, and somebody can probably expand on and/or correct it.

Saltire: here’s a simple rule for determining the pressure at any point. Imagine moving everything a differential amount in one direction, leaving a vacuum. The amount of work need to do this is equal to the pressure in that direction times the volume of the vacuum left. So the center of the Earth would ideed have a very large pressure, since moving water away from the center of the Earth would take a lot of work.

tcburnett: you would have to fill the tunnel very quickly to have the air liquify before it bubble out. Mere gravity would not be enough; you’d need some powerful explosives.

jupiter is a gas planet. toward the center of jupiter hydrogen exists as a liquid metal, not unlike mercury. chunks of rocky material are at the center of this core of molten metalic hydrogen. scientists have no way of knowing how large this rocky mass is, but some have speculated that it might be the size of the earth. It stands to reason that a similar rocky mass would exist at the center of other gas giants and possibly the sun as well.

I always thought that the sun was so hot that it would vapourise and even plasmorise (is that a word, and if it is, is it spelt right?) absolutely anything that even approached the centre. Arthur C. Clarke in 2061: Oddesy three had the pressures of Jupiter compress carbon at the centre of the planet to form the largest diamond in the solar system? This seems pretty far out, I admit, but hey, this guy proposed satelites and geostationary orbit. So I wouldn’t be inclined to dismiss it entirely out of hand. Does diamond count as rock?
I also don’t think that it’s possible for an all-water planet to exist, especially one with an aptmosphere - maybe a solid ball of ice, or a world entirely covered by water, but not 100% liquid water. I doubt that it would be dense enough to attract an aptmosphere, or even to hold itself together. We all know how the moon affects water on earth (tides), so it would be likely that any large mass nearby would tear it apart (including the sun - which would be necessary to keep it liquid, so its a kind of catch 22)
Just an interesting sidetrack - what would happen if you drilled a hole through the earth with one end in the ocean and one end on land? would the water fill only to around the centre? would it go halfway back out the other side? or all the way out the other side (assuming that there is enough water to do so).

I kinda suck at science, but I’m fond of astronomy and SF is a minor passion of mine, so I hope that there is some validity to what I say…


*Originally posted by big_yellow_kingswood *

The affect of the moon on water on the earth is extremely small. The tidal effect only distorts the shape of the earth by a couple meters–and the radius of the earth is about 6378136 meters. In order to tear each other apart through tidal effects, two planets, and/or a sun, would have to be very close together.

*Originally posted by big_yellow_kingswood *

All the way out the other side. The surface of the water will move to conform to an equipotential surface. Ignoring, of course, the obvious problems of actually creating such a hole. That’d be a hell of a waterfall.

6376000 or so of which is not water. (given a estimated sea depth of about 2000 metres, which is excessive for most parts of the world, I believe) The question posed by k2dave (and I’m going off the OP, I know) was

and I’m saying that while the moons gravitational effect on earths water is minimal, it is easily measurable. If there was only water, the effect would be a lot bigger as the whole planet would be dynamic enough to be affected, and not just the top 0.1%. Feel free to work out the maths to prove this right or wrong, cause I can’t be bothered.

*Originally posted by big_yellow_kingswood *

I have worked it out and it is wrong, but I’m not going to include the maths here, cause you’ll not bother to read them.

The earth under your feet deforms with the tide as well. It just doesn’t deform as quite as readily as the water–otherwise you wouldn’t even notice the movement of the water, as they would rise and fall together. They both try to fit the equipotential surface. The additional effect, due to the deformation, is much smaller, unless the second planet is very close.