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Another planetary thought experiment (edit: sorry, credit where it's due: excerpt from The Algebraist by Iain M. Banks): I was born in a water moon. Some peopl
by morsch 15y ago
Another planetary thought experiment (edit: sorry, credit where it's due: excerpt from The Algebraist by Iain M. Banks):
I was born in a water moon. Some people, especially its inhabitants, called it a planet, but as it was only a little over two hundred kilometres in diameter, 'moon' seems the more accurate term. The moon was made entirely of water, by which I mean it was a globe that not only had no land, but no rock either, a sphere with no solid core at all, just liquid water, all the way down to the very centre of the globe.
If it had been much bigger the moon would have had a core of ice, for water, though supposedly incompressible, is not entirely so, and will change under extremes of pressure to become ice. (If you are used to living on a planet where ice floats on the surface of water, this seems odd and even wrong, but nevertheless it is the case.) The moon was not quite of a size for an ice core to form, and therefore one could, if one was sufficiently hardy, and adequately proof against the water pressure, make one's way down, through the increasing weight of water above, to the very centre of the moon.
Where a strange thing happened.
For here, at the very centre of this watery globe, there seemed to be no gravity. There was colossal pressure, certainly, pressing in from every side, but one was in effect weightless (on the outside of a planet, moon or other body, watery or not, one is always being pulled towards its centre; once at its centre one is being pulled equally in all directions), and indeed the pressure around one was, for the same reason, not quite as great as one might have expected it to be, given the mass of water that the moon was made up from.
This was, of course,—
- jcurbo 15y agoHe's got another novel that deals with a unique world - Matter (https://secure.wikimedia.org/wikipedia/en/wiki/Matter_%28novel%29 https://secure.wikimedia.org/wikipedia/en/wiki/Matter_%28nov...) takes place on/in a shellworld.
- idlewords 15y agoIt's a bait-and-switch, though. Most of the book is awful dragons-and-swords fantasy.
- LukeShu 15y agoI used to think that the ice core would work that way, but water is most dense at 3.7C (or something like that). So, under enough pressure, it would remain liquid.
- a-priori 15y agoWater has a very complicated phase diagram. I'm not sure how high a pressure water at a core of a planetoid of that size would be, but if it's above 100GPa then it would certainly be solid (Ice X or XI). Below that pressure, it would depend on the temperature. http://upload.wikimedia.org/wikipedia/commons/0/08/Phase_diagram_of_water.svg http://upload.wikimedia.org/wikipedia/commons/0/08/Phase_dia...
- dalke 15y agopressure = Integral[h=x to infinity] of rho g(h) dh where x is the current distance from the center. "h" here means "height." Since we're at the center, x=0. Since rho(vacuum) = 0, this reduces to Integral[h=0:200km] rho g(h) dh. I simplify and say that water is incompressible, with density of rho = 1000 kg/mmm. (At 1 giga pascals the density is 1.2 that of its normal density.) g(r) = G * M(r) / r * r where M(r) is the mass inside a sphere of radius r. That's G (rho 4/3 pi r * r * r) / r * r or 4/3 rho G pi r. Therefore, pressure = Integral[h=0:200km] rho * (4/3 rho G pi h) dh. This is 2/3 rho * rho G pi h * h, giving 5.6 mega pascals. Looking at the phase diagram you pointed out, 6MPa is well within the liquid range. (Presumably the temperature would be around 4C). Even with a higher density it's at most about 7MPa, and you need to get to 1 GPa for water to always be a solid. In other words, unless I did my math wrong or left something out of the calculations, this water sphere isn't close to one of the ice phases of water. BTW, for more fun, since there's no heat source for the water (radioactive decay, heat from the phase change to ice, etc), then how does it stay liquid? If exposed to vacuum it would turn to vapor until it cooled down enough (about -60C) to freeze. There isn't enough mass to hold down much atmosphere, so I presume it's covered. How also does it get enough energy to stay liquid? Most of the energy will be dumped into the top layer, so there will be a warm zone on top of a thermocline, like with Earth's oceans. If the energy comes from the sun, then do the poles freeze? If it doesn't rotate fast enough then the backside will freeze. If ice does form, it reflects more heat than water so that region will stay ice.