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WaxRubiks
2006-Jul-19, 08:32 AM
what would a 100% water planet the size of Earth be like? What would the pressures in the center of such a blob of water do to the water?

I don't suppose such a planet exists but I just wondered what it would be like.

Ronald Brak
2006-Jul-19, 08:53 AM
Okay, an ice world forms in the outer region of the solar system. Something, say an ejected gas giant sends it into an earth type orbit. Volia, one earth sized water world. Might be a bit dirty however. And I suppose it would take a long time to thaw out. Or you could just make one for the hell of it out of distilled water if you felt like it and had the resources at your disposal.

I imagine it would be kind of cloudy. Since its density is less than a fifth that of earth its gravity would be low and it would be losing gas to space. The atmophere would consist of water vapour, with a whisp of oxygen and hydrogen. Don't know how long it would last, but the smaller it gets, the quicker it will go.

A naturally occuring water world would have a mixture of other substances in it. It would also collect asteroids. Perhaps life could exist in it, but I doubt there would be anything complex before it boiled away into space leaving a small core of rubble behind.

The water in the core would be under pressure, but not enough to make it anything other than severely squished water I would think.

astromark
2006-Jul-19, 09:13 AM
I can not agree with any of that.
1 I do not imagine one could find one of these. As I assume that the core would heat up because of the water pressure. I would imagine that at its core there might even be fusion. I can not see water being a stable planet.
I do not imagine gravity working to allow a water planet. Hydrogen oxygen ozone, water and what else,?

snarkophilus
2006-Jul-19, 09:14 AM
The water in the core would be under pressure, but not enough to make it anything other than severely squished water I would think.

Oh, but the severe squishing is the most interesting part! Water occurs in all sorts of phases under high pressure, each with different properties. Planetary core pressures? I don't know for sure, but probably a supercritical fluid. But just outside that would be layers of ice with different frictions, densities, and stabilities. They'd dance around in a complex way, like plate tectonics, but only water. You could even have liquid layers sandwiched between solid layers... all sorts of neat stuff. Less dense ice phases would continually break off and rise toward the surface, changing state along the way, leading to very complex currents.

http://www.lsbu.ac.uk/water/phase.html

Ronald Brak
2006-Jul-19, 09:21 AM
Oh, but the severe squishing is the most interesting part! Water occurs in all sorts of phases under high pressure, each with different properties. Planetary core pressures? I don't know for sure, but probably a supercritical fluid. But just outside that would be layers of ice with different frictions, densities, and stabilities. They'd dance around in a complex way, like plate tectonics, but only water. You could even have liquid layers sandwiched between solid layers... all sorts of neat stuff. Less dense ice phases would continually break off and rise toward the surface, changing state along the way, leading to very complex currents.

Yes, you're right. I've hade water do weird things to me under pressure. Imagine what it could get up to with an entire planet to play with.

snarkophilus
2006-Jul-19, 09:30 AM
As I assume that the core would heat up because of the water pressure. I would imagine that at its core there might even be fusion.

Well, it wouldn't be a planet, then... it would be a star, no? Fusion only occurs if you have a lot of mass, and for the sake of this discussion, let's assume that we have less mass than required for fusion. After all, the Earth is really hot at the core, but there's no fusion going on there.



Since its density is less than a fifth that of earth its gravity would be low and it would be losing gas to space.

Well, yes, but not really, as the water gets denser the deeper you go. Problem solved, perhaps. As to the water leaking into space, that will certainly happen a little bit, just like on Earth, but where's it going to go? Right back to the planet, for the most part, much like water on the Earth. A few molecules with very high energy will escape, but the rate will be low.

And of course, if you interpret "the size of Earth" to mean the same mass (as I first did), then there's no problem at all.

I'd think that most of the leakage would occur like this:
1. Lightning strikes.
2. A lot of H2O is turned into H2 and O2.
3. The H2 is light enough to readily escape the planet, and does so.

Thus, a not-too-heavy pure water planet would probably end up with a fair bit of oxygen in its atmosphere (and dissolved in the ocean), too.

Ronald Brak
2006-Jul-19, 10:59 AM
Well, yes, but not really, as the water gets denser the deeper you go. Problem solved, perhaps.

I think you'll soon run into limits as to how squished it can get. The highest density I could find in a quick search for water in any form is 1.16 grams per cubic centimeter. So a water planet with the mass of the earth would be significantly larger, with a diameter roughly 2.4 times that of the earth. This means gravity would be significantly lower in the upper atmosphere, so it would still lose water, but at a drastically reduced rate comparted to a water planet that is merely the volume of the earth.

grant hutchison
2006-Jul-19, 12:16 PM
There's some discussion about water's phase changes under pressure, and a link to a technical paper, over on the Two Wet Neptunes (http://www.bautforum.com/showthread.php?t=44071) thread.

Grant Hutchison

antoniseb
2006-Jul-19, 01:39 PM
We had a thread similar to this somewhere. Someone asked what was the largest sized object that could be pure liquid water. It would be nice if someone could dig up the link. I seem to recall some useful math & physics that could be applied to this question.

Ronald Brak
2006-Jul-19, 02:10 PM
Thanks Grant (again).

So after looking through the two Neptunes thread it looks like the water won't go over a density of 1.16 and the ocean might be 60 miles deep before the pressure turns the water to ice.

Romanus
2006-Jul-19, 02:25 PM
I don't think such a pure water planet is possible. However, as a "what if" game:

1.) As others have mentioned, the overwhelming volume of the planet would be exotic ice.

2.) Its gravity would be much lower than Earth's, and even more so if it water's properties forced it to be larger than our planet.

GOURDHEAD
2006-Jul-19, 02:32 PM
Stay tuned. When the Earth cools sufficiently for vulcanism to stop recycling the surface from the interior and the forces of erosion continue to grind down the mountains, the Earth will become a veneer water world. You may have to stay tuned for quite a while.

Ozzy
2006-Jul-19, 03:01 PM
Would a watery world with an ice core develop an atmosphere through evaporation? (assuming its at the same distance from the sun as earth).

Could a buildup of oxygen in the atmosphere cause fires?

Ronald Brak
2006-Jul-19, 03:40 PM
Would a watery world with an ice core develop an atmosphere through evaporation? (assuming its at the same distance from the sun as earth).

Yes, it would have a water vapour atmosphere. There would also be oxygen and trace hydrogen. I think there would be very little oxygen.


Could a buildup of oxygen in the atmosphere cause fires?

There could be a buildup of oxygen as hydrogen escapes from the planet. But this would be absorbed in the water for a long time which would slow it from building up in the atmosphere. I also think that hydrogen peroxide (H2O2) could build up in the ocean instead of O2 in the atmosphere (but I don't really know). This would tend to sink and so there might be a layer of hydrogen peroxide over the ice core.

There would never be enough hydrogen present in the atmosphere to burn even with oxygen, and my guess is there wouldn't be enough of oxygen in the atmosphere to breath or sustain fires, but I definitely could be wrong on this.

Jeff Root
2006-Jul-19, 03:45 PM
We had a thread similar to this somewhere. Someone asked what was
the largest sized object that could be pure liquid water. It would
be nice if someone could dig up the link. I seem to recall some
useful math & physics that could be applied to this question.
"A Big Ball of Water"
http://www.bautforum.com/showthread.php?t=32649

-- Jeff, in Minneapolis

Dragon Star
2006-Jul-19, 04:12 PM
Stay tuned. When the Earth cools sufficiently for vulcanism to stop recycling the surface from the interior and the forces of erosion continue to grind down the mountains, the Earth will become a veneer water world. You may have to stay tuned for quite a while.

Er..no, the sun will boil all the water and atmosphere off of earth within a billion years. Earth will not cool down for probably another 6 billion, perhaps more.

astromark
2006-Jul-19, 07:43 PM
Oops. . I was wrong. Thats good. Consistency is important.
When I suggested that fusion might be found at the core of a water world. No this is not likely or even possible. Am I right in thinking it might be hot ?
Now I shall look into these links provided., thanks.

snarkophilus
2006-Jul-19, 09:18 PM
I also think that hydrogen peroxide (H2O2) could build up in the ocean instead of O2 in the atmosphere (but I don't really know). This would tend to sink and so there might be a layer of hydrogen peroxide over the ice core.

Oxygen is far more stable than hydrogen peroxide, which tends to decompose into oxygen and hydrogen. So it's more likely that you'd end up with oxygen.

eburacum45
2006-Jul-20, 01:58 AM
For reasons I can't quite follow, Léger et al suggest that the atmosphere of a water planet might have high levels of oxygen, but almost no ozone.

They suggest that this lack of O3 could be used to distinguish spectroscopically between a water planet with an oxygen rich atmosphere and no life, and a more Earth-like planet with life, oxygen and detectable ozone.

Ronald Brak
2006-Jul-20, 02:08 AM
For reasons I can't quite follow, Léger et al suggest that the atmosphere of a water planet might have high levels of oxygen, but almost no ozone.

People have mentioned how it could end up with oxygen. I'm not sure why it wouldn't have much ozone. Perhaps water vapour is effective at blocking UV which leaves less O to combine with O2 to make O3? Or perhaps with all that water vapour around you get H2O2 forming instead of O3?

Basically, I don't have a clue.

snarkophilus
2006-Jul-20, 07:53 AM
For reasons I can't quite follow, Léger et al suggest that the atmosphere of a water planet might have high levels of oxygen, but almost no ozone.

That's quite interesting. The basic ozone production mechanism is via oxygen radicals. What you get is something like this:

O[sub]2 + hv --> O. + O.
O[sub]2 + O. --> O[sub]3[sub]*

The * means that the molecule is in an excited state. Indeed, for the reaction to make stable ozone, there needs to be a collision with another molecule, taking away the internal energy of the ozone. In an atmosphere of mostly water, that means that there's a chance for the collider to be a water molecule, which reacts and picks up that oxygen instead. That's my first guess, anyway.

Where might I find the Léger et al paper?

eburacum45
2006-Jul-20, 12:14 PM
I linked to it in this post recently;
http://www.bautforum.com/showpost.php?p=787616&postcount=3

Ozzy
2006-Jul-20, 01:11 PM
So if a water world would have a high oxygen content in its atmosphere, could the air spontaneously combust?

pghnative
2006-Jul-20, 01:30 PM
Regarding fires and spontaneous combustion, note that there doesn't seem to be any fuel on this "water world", so there would be no fires even in a 90%+ oxygen atmosphere. (I didn't say 100%, since there would be some water vapor around -- the percent water depends on the average temperature)

Ozzy
2006-Jul-20, 02:30 PM
Ive heard that earths atmosphere has experienced fires when the oxygen content reach a high level.

grant hutchison
2006-Jul-20, 04:02 PM
Ive heard that earths atmosphere has experienced fires when the oxygen content reach a high level.That's true, or at least it has been plausibly claimed.
But what is there to burn on a "100% water planet"?

Grant Hutchison

Ozzy
2006-Jul-20, 04:10 PM
Oxygen?!

Kaptain K
2006-Jul-20, 04:17 PM
Oxygen doesn't burn. Burning is rapid oxidation. Something burns when it combines rapidly with oxygen!

mugaliens
2006-Jul-20, 06:13 PM
I can not agree with any of that.
1 I do not imagine one could find one of these. As I assume that the core would heat up because of the water pressure. I would imagine that at its core there might even be fusion. I can not see water being a stable planet.
I do not imagine gravity working to allow a water planet. Hydrogen oxygen ozone, water and what else,?

The bottom of the Marianna's trench is quite cold - near freezing.

Fusion comes from squeezing together H to make He (the simple version). Squeezing water together makes, as was previously mentioned, compressed water (yes, water is compressible).

Gravity would work fine for a water planet, just as it does for gas giants like Jupiter.

The previous poster was right, though - even if you began with pure water, over time, due to all the meteor impacts, it would get dirty, and most of that (except the finest dusts) would settle into the core.

Nerthus
2006-Jul-20, 06:26 PM
Er..no, the sun will boil all the water and atmosphere off of earth within a billion years. Earth will not cool down for probably another 6 billion, perhaps more.

Why will all the Earth's water and atmosphere be boiled away by the sun within the next billion years? I thought that the sun couldn't do that to the Earth until about 5 billion years from now when it reaches the end of its cycle. Have I missed something?

pghnative
2006-Jul-20, 07:12 PM
Why will all the Earth's water and atmosphere be boiled away by the sun within the next billion years?
The oceans won't "boil", but they will eventually evaporate.

The key point is that given enough time, all gases will escape Earth's gravitational pull.

The reason for this is that at any given temperature, individual gas molecules have a range of different energies. If you could pick a million molecules at random, a few would have very little energy, a few would have a lot, and most would be somewhere in between. The ones with the highest energies actually have enough energy to escape Earth.

The other factor is mass. A heavy gas requires more energy to escape Earth's gravitational pull. So at any given temperature, a heavy gas has fewer molecules with enough energy to escape Earth. So heavy gases stick around, light gases eventually escape.

Oxygen (molecular weight of 32) and nitrogen (MW = 28) are relatively heavy gases. They stick around for a long time, and are the major components of Earth's atmosphere. But helium (MW = 4) is a lot lighter, and very little of it exists on earth. (At least, compared to the oxygen and nitrogen).

Water has a MW of 18, so it'll leave earth a little faster than either oxygen or nitrogen. Given enough time, it'll all evaporate away, unless replenished somehow. (The only reason we have any water on Earth is due to prior bombardment by comets)

Dragon Star
2006-Jul-21, 02:37 AM
Yea, sorry about the "boil" business, evaporate was the correct word, my bad.

Ronald Brak
2006-Jul-21, 03:19 AM
Water has a MW of 18, so it'll leave earth a little faster than either oxygen or nitrogen. Given enough time, it'll all evaporate away, unless replenished somehow. (The only reason we have any water on Earth is due to prior bombardment by comets)

Ah, thank-you. Now I see how how oxygen can build up on a water world. I was thinking that water was heavier than oxygen, but oxygen is a diatomic molecule (duh) so it's double the weight of a single oxygen molecule. Sometimes I make the silliest mistakes.

Kaptain K
2006-Jul-21, 03:44 PM
Also, water tends to photo-dissociate into oxygen and hydrogen. The hydrogen, with a MW of 2, tends to escape quickly.

pghnative
2006-Jul-21, 03:59 PM
Yea, sorry about the "boil" business, evaporate was the correct word, my bad.You know, now that I think about it, the word "evaporate" (which I used in my post too) isn't really correct either.

The water will evaporate from the ocean into the atmosphere, and water from the atmosphere will slowly <<insert verb>> from earth. Anybody know what the correct term is? (Where is Roget when you need him...)

Fraser
2006-Jul-22, 04:47 PM
Isn't water vapour one of the most powerful greenhouse gasses? Wouldn't that serve to warm up the surface of the world signficantly?

Comets leave a trail of ice particles, but isn't that because of their low gravity?

This is a really fascinating thought experiment. There are so many different factors going on here. I'd love to write this up as an article for Universe Today if people could come to some kind of conclusion about what would actually happen.

grant hutchison
2006-Jul-22, 05:23 PM
Isn't water vapour one of the most powerful greenhouse gasses? Wouldn't that serve to warm up the surface of the world signficantly?The liquid water oceans would be in equilibrium with a water vapour atmosphere, on average at the saturation vapour pressure of water for the ambient temperature. If the global average temperature was, say, 27 Celsius, then in the absence of any other gases the atmospheric pressure would average 35mb of pure water vapour, and couldn't rise higher without raining out.
Atmosphere would rain out at cold latitudes (just as carbon dioxide condenses out at the Martian winter pole), and would be augmented by evaporation at warmer latitudes.
So the greenhouse effect of water is certainly going to be present, and presumably to a rather larger degree than on Earth, where large volumes of atmosphere are not fully saturated with water vapour (because they lie over, or have just come from, dry land areas).
We're also going to get latitudinal movements of atmosphere and ocean, driven by the evaporation/precipitation cycle. And hurricanes could presumably build in tropical waters, if the water planet is hot enough, and wouldn't die until they moved far enough north or south to hit cold water: no land-masses to rob them of their evaporation power source.

Grant Hutchison

grant hutchison
2006-Jul-22, 05:40 PM
Water has a MW of 18, so it'll leave earth a little faster than either oxygen or nitrogen. Given enough time, it'll all evaporate away, unless replenished somehow.The time constant for water molecules escaping from the Earth's exosphere (temperature around 1500K) is of the order of billions of years. Fiddling with the numbers, it seems that an object with an escape velocity over about ~8km/s (and a similar high exosphere temperature) could hold on to water for geological time scales, in the absence of photodissociation.
IIRC, Earth's water is to some extent protected from photodissociation because there's a cold trap at the stratosphere which largely ensures that water precipitates back into the troposphere, making the upper reaches of Earth's atmosphere very dry.
That's not going to be an option for our water planet in its early stages, since its atmosphere would be water from top to bottom. So maybe there would be initially fierce photodissociaton, with hydrogen escape and oxygen production, followed by the formation of an oxygen-dominated atmosphere containing water vapour at saturation vapour pressure in its lower reaches, and a dryer upper atmosphere capped by a UV filtering ozone layer, thereby curtailing the "photodissociation era".

Grant Hutchison

neilzero
2006-Jul-22, 06:09 PM
Let's assume this "pure" water world is 2.4 times the diameter of Earth. The core would be a bit warmer than the surface, unless the core was still hot from the formation process. My guess is the surface gravity and mass would both be higher than Earth, so oxygen and water vapor would escape at a negligible rate at the top of the atmosphere, but hydrogen would escape. Asteroids, comets, space dust and space ions would reduce the purity to 99.9999999% perhaps in the first year. Most of the free oxygen would disolve in the water. So the atmosphere would be mostly very thin water vapor, unless the average air temperature was above about 10 degrees c = 50 degrees f just above the surface of the water. Life needs lots of trace substances, so there would be no life until the all water planet became quite contaminated. Neil

grant hutchison
2006-Jul-22, 08:16 PM
Density and radius seem to be the variables we'd like to trade off against each other in order for our water planet to retain an atmosphere. Escape velocity from the surface of a spherical body scales with the square root of its density, and directly with its radius.
Just to have a ballpark figure, take the density of the high-pressure ice, Ice VII, as a guesstimate of the overall density of our planet-sized blob of water: 1650 kg.m-3. (Central regions of more compressed Ice VII may be denser, outer regions consisting of Ice VI and water will be less dense.)
That's 0.3 times Earth's density. To keep escape velocity equal to Earth's, we'd need a water object 1.83 times Earth's diameter. It would have a mass 1.83 times Earth's, but its surface gravity would be only 0.55 of Earth's.
(Same escape velocity, lower surface gravity? Yes, because this object's low surface gravity will fall off more slowly as we climb towards space, backed up as it is by 1.83 times the mass.)

Grant Hutchison