It would be a solid due to the pressure, even at relatively high temperatures, and convection would probably rapidly cool down the center* (a lack of radioactive materials would also mean no further heat generation); the chart that Grey posted and the pressure of 4,000 miles of water, about 620,000 atmospheres (using the weight of a 1 foot column x 4,000 miles x 5,280 feet / 14.7 psi); a pressure of about 500,000 atmospheres is enough to solidify water at 1,000 K; further increases in pressure rapidly increase the maximum temperature that solid water can exist, at least until it got so hot that the water underwent thermal decomposition (3,273 K for half the water to dissociate).
*This is why the temperature at the bottom of the oceans is only a few degrees above freezing; although this is in large part because cold water sinks in the polar regions (the maximum density of water is at around the same temperature as the ocean bottoms); a lack of land would mean that water can freely circulate so Earth would likely be ice-free and have a much more uniform temperature than today (average around 15C/59F), probably warmer too (e.g. no high elevation landmasses, one reason why the Greenland ice sheet is able to persist in warmer climates like today; the ice sheet would likely not come back if it were suddenly removed due to the elevation drop unless the climate got considerably colder).
In my opinion, it would be much like the present Earth, without the continents, and with a very different atomsphere and different weather patterns.
Of course, we have to ask whether we are talking about a planet with the same mass, the same diameter, or the same surface gravity. Without any heavy elements, the only thing heating the interior would be gravitational effects. The energy from gravitational collapse would have largely dissipated, but there would be some energy flux due to tidal friction. For the Earth, this is about a million times less than the solar energy flux, but it will still lead to some heating of the planet’s interior, though the core temperature would be significantly less than that of the Earth. The temperature at the bottom of Earth’s ocean is close to 0 centigrade. On a water planet with the same surface gravity as the Earth and a temperature at the bottom of its ocean of almost 0 (since water’s density maximum is just above 0), the bottom will occur at a depth of about 60 miles (pressure of 10 kBar). Thus, almost all the planet will be solid ice. The surface will be like the Earth, except with no continents. Of course the contents of the atmosphere will be much different, since by assumption the planet has no nitrogen and there will be no life to produce oxygen from water.
If water-ball-planet had the mass of earth, it is likely it would be quite a little bit larger than earth – somewhere between twelve and fifteen thousand miles instead of 7100. I think the majority of the inner part of the planet would be ice-vii, which is less dense than water and less dense than most other phases of ice. This would create a strange dynamic, having the heavier (denser) part of the planet pressing down on the less dense part. Not sure if it would be like that or some other way, but it looks like it would be around half the size of Neptune, maybe even larger than that. I think the lower density would affect the profile of the gravity well, so the moon’s orbit would be somewhat different.
assuming that you start off with the planet entirely liquid, what’s keeping it together? wouldn’t the various forces acting on the planet tear it apart?
Hmmm. Would Water-Earth be dense enough at the core to produce metallic hydrogen instead of ice?
It seems to me there’d be a fair amount of oxygen in the atmosphere, due to UV radiation photodissociating water vapor high in the atmosphere into hydrogen and oxygen, the hydrogen escaping the atmosphere, and there being nothing for the oxygen to bind to like there is on Real-Earth.
Gravity keeps it together, just like real-world Earth. Earth isn’t held together by chemical bonds. And there’s actually somewhat less force trying to blow Water-Earth apart due to the interior being cooler.
Assuming this planet was newly created, and had identical properties to Earth, presumably the rotational momentum would have an affect. Earth is already an oblate spheroid, but I think this planet would rotate faster (because water-earth would have less mass than earth-earth), so presumably the effect would be significantly larger than it is at the moment. I’m guessing it wouldn’t be enough to tear the planet apart though
Assuming this newly formed planet didn’t have a solid core of ice (yet), would a purely liquid planet be able to maintain its angular momentum?
I stated that the water vapor would be stripped off based on something I’ve recently heard about Venus. It was some TV special that I unfortunately can’t recall. Wiki seems to echo that info -
Lighter gases, including water vapour, are continuously blown away by the solar wind through the induced magnetotail.[3] It is speculated that the atmosphere of Venus up to around 4 billion years ago was more like that of the Earth with liquid water on the surface.
This paper by Valencia et al might answer some of these questions. Detailed Models of super-Earths: How well can we infer bulk properties?
Fig 2 is interesting; for a water-only planet with the same mass as Earth, the radius is given as approximately 9300 km. This gives a surface gravity of 0.47 gees.
This water ball would lose hydrogen quite easily, because of the lowish gravity; any water that was split by photolysis would turn into hydrogen and oxygen, but the hydrogn would tend to escape, leaving an oxygen atmosphere with water vapour and water-droplet clouds. The water would turn to high-pressure ice at 100km deep or so. Water isn’t very radioactive, so the core probably wouldn’t be very hot - but this depends on how the planet was formed in the first place. Valencia et al say that water-only planets are not realistic, so they don’t consider them in detail.
There’s still plenty of unanswered questions; exactly how warm would the core be, how deep would the high-pressure ice be; how quickly would the planet lose hydrogen (and eventually evaporate altogether) and what kind of magnetic field would it have? All these characteristics effect one another in various ways.
I can’t find good answers to these questions. Since the planet is ‘improbable’ (to say the least) I don’t think there are any good answers.
Comets include a lot of other chemicals. Methane, carbon dioxide and various rocky or metallic inclusions are all common.
But the real problem with this comparison is that the properties of water change at very high pressures and the amount of gravity helps to determine whether there is an atmosphere, and therefore what the surface temperature is. So the mass does matter even if composition is the same.
As an other example of how size matters: Jupiter and the sun are very similar in elemental composition. The difference between the two is primarily an issue of mass. The sun is heavy enough to start fusion and is therefore very hot and composed of a plasma. Jupiter does not have fusion, so is much cooler and mostly composed of metallic hydrogen rather than plasma. (And at the Earth scale: Metallic hydrogen wouldn’t exist because there wouldn’t be enough pressure. An Earth-sized ball of hydrogen wouldn’t even be stable because there wouldn’t be enough gravity to hold together.)
Actually comets have quite a lot of rock in them, and have been referred to as “dirty snowballs”. They are also made of more types of frozen material than just water ice.
