I am not talking about something like the movie Water-World, which was the earth but with so much water there was little or no ‘land’ exposed.
I am asking about a planet that was nothing but water – one enormous drop hanging in space and orbiting a star.
What would the shape be? Generally round or would rotation spin it out into a disk? If more round than disk-like, what would it be like at the center? I assume it would be dark. But what would the effect of pressure be like at the center, given that water is fairly incompressible?
My guess is we’d have a frozen center with a liquid outside, but again that’s just a guess.
There are plenty of planet-like objects that are made mostly of water, but (so far as we know) they’re all far enough from the Sun that they’re frozen (or at least mostly so). I suspect that there’s no mechanism by which a droplet-world could coalesce in the first place, close enough to a star to be liquid.
As for shape, that only depends on the properties of the material at the surface, so a completely-water world would have the same shape as the Earth, assuming the same rotation rate: Very close to spherical, but with a slight equatorial bulge. Heck, at planetary scales, there really isn’t all that much difference between solids and liquids, anyway.
And I’m not at all sure what the core would be like. I don’t know if anyone’s ever studied what water does at those kinds of pressures. It might be that it turns into some other compound, H[sub]4[/sub]O[sub]2[/sub] or something, that isn’t stable at low pressures but is at high.
At high enough pressures water has several different solid states depending on temperature. So your planet-sized ball of water would be covered with liquid water down to about (iirc) 30 miles deep, and then be solid underneath.
If it was entirely or almost entirely water, you’d have an atmosphere that was a mixture of water vapor and oxygen formed by photodissociation. As far as supporting aquatic life, I have no idea if heavier elements would remain in solution or would sink to the core and become unavailable.
Water is something completely different than a mixture of hydrogen and oxygen, and the OP specified water. Besides, I can’t think of any circumstances under which a mixture of hydrogen and oxygen on a planetary scale would be stable: Sooner or later (probably within less than a second), you’d get a spark or something, and the whole planet would go FOOOM, and you’d be left with a planet that is composed of water in some state or another.
In fact, the surface of the planet would likely be fresh water that was pretty close to pure due to evaporation and rain. The fresh rain would sit on top of saltier water. Even on Earth with lots of mixing, you can get a layer of fresh water several inches or even feet deep. (I found this out myself on a school field trip where we went trawling for plankton. It had just rained and our first collection jar had virtually nothing. We had to weight it better to get below the fresh layer.)
Without land (even underwater land) to help mix things up, the fresh layer would be quite stable and essentially devoid of any life or nutrients. Maybe you could have a form of life that soaked up sunlight at the surface for photosynthesis, then descended deeper at night to soak up nutrients.
Here is a phase diagram of water. The center of the earth is at a pressure of about 360 GPa, and a temperature of about 6000K. Unfortunately, the phase diagram only goes up to 1000K, and as far as I can tell no one knows the phase diagram of water much past 1000K. So we may have to assume a planet that is colder than the earth at it’s core (this may be quite reasonable, since the earth’s non-water composition has a lot to do with why it is so hot). So let’s assume the earth is 1000K at it’s core. Then in order to understand what the water planet is like at a depth d we just have to traverse the phase diagram from the surface of the planet (middle left of phase diagram around the boundary between solid and liquid) to the center of the planet (upper right of phase diagram). You can imagine what the surface might be like – think of the surface of the earth – a mixture of liquid water and floating ice. As we descend into the planet we will say mostly liquid water all the way down, until, as some point, the pressure will be high enough to form Ice VII and eventually Ice X. So the planet will have a core made out of a form of crystalline ice.
(OP here) I had not considered, until these responses, that in the right type of solar system the all-water planet would get bombarded by meteors and the like. So it wouldn’t stay “pure” for long.
And…there really is an ICE-9? I wonder if Vonnegut used that or it’s just a coincidence.
Yeah. The general “ice-#” nomenclature, as well as ice-I through ice-VIII were in place before Vonnegut’s Cat’s Cradle. But “ice-IX” (the real-world one) was first discovered and named in 1968, 5 years after Cat’s Cradle. I think it would have been called “ice-IX” regardless of the book.
Just throwing this out there for discussion–maybe water/ice isn’t dense enough to create a high enough gravitational pull to have any atmosphere at all. (Note, I’m a lawyer with a liberal arts background, so this could be totally off.)
You’re correct that the radius of the planet would need to be about 10000 km in order for it’s mass to be the same as earth’s. The gravity at the surface would be about half of earth gravity.
Good point, the water planet would have a lower density and lower surface density, so gas could escape more easily. Also, the Earth’s magnetic field helps prevent the solar wind from stripping the atmosphere away. Pure water is a poor conductor, so water-Earth probably wouldn’t have a strong magnetic field. On the other hand, the entire surface of planet would be volatile, there is plenty of material to form and replenish an atmosphere. I’m not sure what the result would be.
But that was an artificially created all-water planet. Some master race, eons before even the race that currently lives there set up shop, made some giant machine to keep all the water in, oxygenate it, supply it with nutrients, etc…in fact, I think the plot point of that episode is that the giant device was failing and the planet would be no more if it did, so it had to be fixed.
Earthlings tend to have a mistaken notion about how hard it is to maintain an atmosphere, since both ours and that of one of our nearest neighbors are unusually thin. But consider Venus, which is both much hotter and somewhat lighter than Earth, and yet has an atmosphere nearly a hundred times thicker.
The magnetic field might be cause for concern (the lack of magnetic field is the most plausible explanation for why Mars’ atmosphere is so thin), but while pure water is a lousy conductor, it also doesn’t exist outside of laboratory conditions. You hardly need any impurities at all to turn water into a half-decent conductor. It’ll never be as good as iron, but then again, water is also much less viscous than molten iron, so my SWAG is that you could get a good enough dynamo going in a water-world to generate an adequate magnetic field.
Of course, if nothing else, you’d have an atmosphere of water vapor, and even if it is lost to solar wind or whatever, there’s plenty more planet to replenish it from. But the partial pressure of water vapor under Earthly conditions is pretty low, so that wouldn’t be much of an atmosphere, unless the planet is significantly hotter than Earth.