Suppose there is a tidally locked planet orbiting a star. The planet's surface consists of a global ocean, that is, liquid water. At the inner hemisphere the temperature is so high that the water is constantly boiling, creating the atmosphere of water vapor. But the vapor does not reach the dark side of the planet, and precipitates around the terminator line. The further side has no atmosphere and an icy surface.

Is this setup possible? Can it be such thing that one side of a planet has liquid (and boiling) water surface while the other side has no atmosphere?

Is it possible at all that a part of an ocean was boiling while the other one was icy?


The atmosphere, if present on the daylight part of the surface, will be also present in the night side, providing mechanisms for heat transport from day side to night side. So if daylight side allows liquid water, the night side will be not much colder than cold places on Earth.

Have a look at a paper:

Merlis, T. M., & Schneider, T. (2010). Atmospheric Dynamics of Earth‐Like Tidally Locked Aquaplanets. Journal of Advances in Modeling Earth Systems, 2(12). online pdf

From the abstract:

Free-atmospheric horizontal temperature variations in the slowly rotating atmosphere are generally weaker than in the rapidly rotating atmosphere. Interestingly, the surface temperature on the night side of the planets does not fall below, $240\,\text{K}$ in either the rapidly or slowly rotating atmosphere; that is, heat transport from the day side to the night side of the planets efficiently reduces temperature contrasts in either case.

  • $\begingroup$ It is a non rotating planet in the question above, I think. The differences in temperature between night side and day side will be enormous and any atmosphere going by convection to the dark side will freeze out. $\endgroup$ – anna v Dec 1 '13 at 6:24
  • $\begingroup$ @annav: 'Tidal locking' only means that orbital rotation and angular rotation are synchronous. The speeds of such rotation could still be quite large (note, that for tidal locking to occur, the planet must be close to the star). The point of the paper I cited is exactly to show that no freezing out occurs -- the transport of heat is enough to keep the night side warm. $\endgroup$ – user23660 Dec 1 '13 at 6:38
  • $\begingroup$ This assumes that an atmosphere exists.It all boils down to the boundary conditions of the problem. At 273 ice forms and the planet is not all water, just half of it, with the proviso that that daylight reaches boiling temperatures. $\endgroup$ – anna v Dec 1 '13 at 7:18
  • $\begingroup$ @annav: Sure, atmosphere has to exist. But take for instance Venus. Its temperature is much higher than boiling water and it still has got its atmosphere. And btw, although it is not tidally locked (its solar day is 116 days, which is quite long by my count) the temperature difference between day and night is quite small. $\endgroup$ – user23660 Dec 1 '13 at 8:12
  • $\begingroup$ @user23660 Venus has hyperrotating atmosphere. $\endgroup$ – Anixx Dec 1 '13 at 9:09

This is not long term sustainable system to be called an atmosphere. It might occur if for example a water loaded asteroid fell on the permanent day side, but it will be an unstable system.

The boiling water will escape eventually to space :

1) because of the tails of the kinetic energy in the distributions,

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This always happens in atmospheres but the steam boiled off will have a higher probability of evaporating to space ( higher velocity than the necessary for gravitational binding) .

On the other hand, and happening faster, the rain cascades at the dark edge will rapidly deplete the water in the boiling ocean, so it will end up as a planet with ice on the dark side and no atmosphere on the bright side.

Of course it will depend on the quantity of water , and this would need a calculation: how long would an all water planet last with an atmosphere of sorts in that specific position, for example.

  • $\begingroup$ If the planet is massive enough, how anything can escape into space? This is nonsense. $\endgroup$ – Anixx Dec 1 '13 at 6:10
  • $\begingroup$ There is a tail to the kinetic energy distribution where the atoms in the tail achieve very large energies and can achieve escape velocity. A calculation is needed to see how long a boiling ocean with a steam atmosphere will last. scientificamerican.com/… main problem with your model is that the oceans will be depleted and end up as ice on the dark side, because there is no new source of water. $\endgroup$ – anna v Dec 1 '13 at 6:21
  • $\begingroup$ The ice if it floats over water always drifts, so it migrates to more warm place. It is of course possible that the warm side will become dry due to the flows from the other side evaporate before reaching the center of the warm side. This can happen if there is not enough water on the planet. $\endgroup$ – Anixx Dec 1 '13 at 9:06

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