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What would happen to an earth sized body of water?

  1. Oct 27, 2013 #1
    Assuming no other compounds are initially present, what would happen to a body of earth's mass, orbiting the sun at earth's distance and speed, which is entirely composed of water? Assume that it just blinks into existence, and initially all the water molecules are arranged in a liquid sphere, and it is 50 degrees celsius at all points.

    Would it form a sphere? What would the density at the centre of the planet do to the water? Would hydrogen and oxygen somehow form at the surface, or would the atmosphere be solely composed of steam? Perhaps the surface would freeze over?

    What properties would the planet possess once it has stabilised? How much is it possible to theorise about the chemical composition at this point, as the planet contains only hydrogen and oxygen?
  2. jcsd
  3. Oct 27, 2013 #2


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    There are lots of assumptions and impossibilities in your statement. First it would be impossible for it to even happen in the first place; ignoring that, next it would be impossible for all points to be 50C. In order for a liquid planet to be the same mass as Earth, you'd have to have a sphere that is magnitudes larger than Earth.

    The core of the planet at such pressures would likely be solid, and you'd have elliptical planet with a more pronounced bulge at the equator than observed on the Earth. Water would break apart and you'd see ozone, oxygen, hydrogen, ect., forming due to photodissociation.

    Any impurities hitting the planet would sink to the core, and the top layer of the water world would be almost pure, devoid of anything else. The planet would never stabilize; it would constantly be picking up cosmic dust/matter, and would have its atmosphere stripped away due to solar wind and radiation. The planet would never "only contain hydrogen and oxygen."

    There’s no frame of reference for such a thing, this is all just conjecture and not actually accurate. There is no answer to your question.
  4. Oct 28, 2013 #3
    I realise that planets cannot blink into existence, it is a theoretical question meant to facilitate discussion. I am aware of the impossibilities (uniform 50C, full water composition, orbiting our sun, sudden materialization) involved. I apologise if questions are expected to be more focused. If there are any other invalid assumptions or impossibilities please let me know.

    A theoretical discussion full of perhaps's is all I really hoped for with this question. Again, if this is not how a question here is supposed to be posed, please tell me. I am new here.

    I apologise for the phrasing of the title, the "size" was meant to refer to the mass only, it would obviously be much larger in volume.

    This is interesting. Would it have the properties of ice, despite it's temperature? Would the layers of the planet then become, going outwards: solid hot ice, impurities, impure water, mostly pure water, atmosphere (perhaps with some overlap)? Could the impurities make their way into the core?

    I see. Is it possible to know how pronounced the bulge would be, if only in terms of other possibly unspecified factors?

    I was not aware of this effect, it is very interesting! How would one go about creating a list of the compounds which could form from water in the entire planet? How are some of these compounds likely to be distributed?

    I was referring to the elements forming the planet, and did not consider other elements arriving from space.

    I see. But surely the rate of change of the composition of the planet would plateau, and it would 'stabilize' for lack of a better term. I was interested in the properties that the planet might possess after this point.

    For example, can anything be said of the properties its atmosphere could possess?
  5. Oct 28, 2013 #4
    Not "magnitudes". The numbers have been computed. E. g. this:

    Can someone measure the diametre of Earth mass water planet? That logarithmic plot has no pointers, let alone lines, between 1 and 2 Earth diametres.
    That depends on rotation. You can have a planet rotating as slow as Venus, or slower, and that would be a pretty perfect sphere. With a solid core, of course.
    Not all would sink. Some can dissolve in water.
  6. Oct 28, 2013 #5


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    Since one liter of water is one kg at 4C it would take 5.972E24 liters of water to equal the mass of the earth. This gives a volume of 5.972E12 cubic kilometers. Solving for radius you get 11,255 km. A little less then twice the radius of the Earth. So if my math is right, it isn't magnitudes like I previously assumed. However since the density of water is not 1kg at the temperatures and pressures were talking about, it isn't a completely accurate assertion.

    It would all dissolve in water, however, it's likely that do to the fact water would be cycling on the surface constantly that sll dissolved soilds would eventually migrate to the core. However there's a lot of unknowns and assumption about the states of the water at different layers, the currents, the weather, how liquid sitting in an atmosphere of steam reacts. Its almost all conjecture of what would happen. You'd need someone more knowledgeable then me making guesses.
  7. Oct 28, 2013 #6
    Calculated results:
    Initial planet:
    Mass: 1 Earth.
    Diameter: 1.7 Earth.
    Gravity at surface: 3.35 m/s^2, or about 0.34 G.
    Atmospheric composition: Steam.
    Atmospheric pressure: 5 Bars.
    Atmospheric altitude: 60 km scale height near the surface, density about double that of Earth's atmosphere near the surface. Earthlike pressure at ~100 km. Atmosphere is at the "edge of space" at around 550 km, meaning that the planet's radius is >5% atmosphere.
    The temperature would soar to to greenhouse warming, it may exceed 150 C.
    Splashing the ocean would cause vaporization of the surrounding water and condensation of the surrounding steam.
    The ocean's pressure would increase by 1 bar every 30 meters, this would result in a pressure of 30000 atmospheres at a depth of 900 km, which would convert the water to Ice VII.
    The denser forms of ice require more pressure than the planet can generate to form. For example, Ice X require 600,000 atmospheres.
    In the upper atmosphere, considering no magnetic field would be present, water would dissociate into hydrogen and oxygen, Much of the hydrogen, quickly rising to the edge of the atmosphere, would bleed off into space, while the oxygen, quickly forming either O2 and H2O2, would sink until it hit the ocean or the ice below the ocean, respectively.

    It would receive something 1.4E30 energetic photons per second, This roughly correlates to the number of water molecules converted to Oxygen and Hydrogen in the same period. This works out to about 41 tonnes per second of water being converted.
    Assume that all hydrogen forms H2 and flies off into the upper atmosphere and is promptly lost into interplanetary space.
    Assume that all oxygen forms H2O2 and mixes into the ocean.
    The mass of the atmosphere would be something like 40 times the mass of Earth's.

    Mass of water converted to H2O2 in a second: 82 tonnes.
    Mass of H2O2 generated: 78 tonnes.
    One day is 86400 seconds, meaning that the atmosphere would lose 86400*82=7084800 tonnes in a day.
    The atmosphere weighs 2.0E20 Kg, and is losing 7084800000 kg in a day.
    This means that the atmosphere is completely replaced every 77.3 Million years.
    Also, the planet would shrink by about 50 meters due to the higher density of H2O2.
    So by now, the planet would have lost 3 km of its original radius, 1E22 Kg of water, and about 4E20 Kg of mass.

    It would also have a layer of H2O2 6 km thick, or rather, 6km of H2O2 interspersed throughout the oceans, about 1% of the ocean's mass.
    It might be possible that some of the H2O2 would get deeper through settling and remixing of ice VII and water by deep currents, thus allowing a 4000-km-wide H2O2 core to form instead of the H2O2 mixing with the oceans.

    All along, the planet would get slightly higher gravity as the H2O2 built up. But this would be cut short by the sun-expanding and the planet changing drastically, also, as the sun expanded, it would produce fewer UV photons, making the planet no-longer bleeding off water.

    If, by handwaving, the sun stayed constantly as it is today, the planet would take about 2 Trillion years to turn into Hydrogen peroxide completely, and it would, by that time, be 1.44 Earth radii and have about 4.6 m/s^2 gravity.
    Last edited: Oct 28, 2013
  8. Oct 28, 2013 #7


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    Probably the best way to decide if a question is OK to ask here is to look at it this way: if the question basically boils down to "if the laws of physics were suspended, what would the laws of physics say about _______<fill in the blank>", then the question is not a good one to ask here. Your question does, I think, boil down to that.

    The forum is here to discuss actual science, not to speculate on impossible situations. This sometimes annoys people, but read the forum rules.
  9. Oct 28, 2013 #8
    In a turbulently mixed atmosphere, oxygen would be mixed throughout.
    Not a sound assumption!
    The photolysis of water is reversible. The equilibrium is on the side of water. A large part of these energetic photons will be dissociating hydrogen ans H2O2, causing them to react back to water. In absence of hydrogen escape, an equilibrium would have a low concentration of H2 and O2, and even lower concentration of H2O2 and O3.

    The loss of hydrogen does drive the photolysis towards oxygen. That depends on the exact speed of hydrogen escape.
    Improbable in the first place. And H2O2 can and will decay once in the ocean.
    Improbable. Ice does not easily take solid solutions. And as stated, the hydrogen peroxide would have decayed long ago.
  10. Oct 28, 2013 #9
    The Equilibrium is only on the side of 2(H2O)+O2 when at low pressures, At a pressure reaching 30,000 atmospheres, it may be more favorable to have H2O2.

    But good point, the reaction would probably occur in the atmosphere, meaning that H2O2 would not have time to reach the ocean.

    Also, does anyone know when the H2O2 would be favored over O2+H2O in terms of pressure?
    Remember that the O2 SINKS not floats in water beyond a depth of several tens of kilometers.
  11. Oct 28, 2013 #10
    Remember that O2 either dissolves in water or it does not. If it does not dissolve, it stays on surface; if it does then it will mix rather than sink.
  12. Oct 28, 2013 #11

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