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When Earth’s oceans evaporate, one of two feedback situations could occur:

  • The "moist greenhouse" where water vapour dominates the troposphere while water vapour starts to accumulate in the stratosphere, which then rapidly escapes into space through photodissociation (Earth will look like the first picture);
  • The "runaway greenhouse" where water vapour becomes a dominant component of the atmosphere (Earth will look like the second picture, but under a very thick steamy atmosphere).

I want my own terrestrial earth-like planet to go through the latter situation, where photodissociation of the water vapour is slow, allowing for a stage in which the planet obtains an extremely thick water vapour atmosphere (with supercritical water near the surface) and temperatures hot enough to melt the crust.

What are the main astronomical and climatic factors that will determine which feedback situation my planet will have?

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  • $\begingroup$ You can't boil off the oceans without heating up the crust. It's the ground getting hot that's the problem, not the atmosphere. So long as the ground remains cool water continues to condense. If you want #2, you need to heat the ground without heating the atmosphere so that the water continues to evaporate but doesn't blow off into space. At least that's my Holliday Inn Express opinion based on years of helping people get plants to grow in their gardens. $\endgroup$
    – JBH
    Commented Jul 15 at 23:31

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Not likely because water does not remain water when the planet gets that hot

Your molten surface means you have to achieve a surface temperature in excess of 1200°C. In isolation, water doesn't break down until it reaches at least 2000°C so it seems like you should be able to achieve this goal, but unfortunately, there are a number of natural substances that make up large portions of the crust (mostly sulfates) that will catalyze the breakdown of water into separate Oxygen and Hydrogen components at temperatures of just 900°C. So your biggest obstacle is not the water being blown away by solar wind nearly so much as your water simply becoming no longer water.

When you water decomposes, the Hydrogen will quickly escape into space, and the Oxygen is not a greenhouse gas; so, the planet will quickly begin to lose greenhouse gases and cool keeping it from exceeding 900°C and melting the planet's surface.

So the ultimate problem here is that you as you pass 900°C the Greenhouse Effect will self-stabilize. If you heat the planet up slowly, then you reach equilibrium without passing it up, but if you heat the planet up quickly (like with a nearby gamma ray burst or something fun like that), then the surface water will decompose faster than the water deep in the oceans can be heated up so your water vapor will be destroyed as it is made. So, there's not really a speed or mechanism by which you can heat a planet to get your desired effect.

... unless your planet is very flat and has a high gravity

The problem with an Earth like world is that you have a lot of exposed land mass at the same time as you have deep oceans. But the catalyzing effect can only happen if there is exposed land mass. If instead of being truly Earth like world with district Oceans and Continents, if the whole planet were just a giant swath of shallow marsh lands with too much gravity to let tall land formations build up and to prevent the dense water vapor from escaping, then the whole surface of the planet would boil with very little dry land to be able to make contact with the supercritical water vapor, and this process would continue allowing the temperature of the air to rise up to a possible 2000°C before all of the water beds dry up all at once allowing the intense heat to start melting the surface.

As soon as waterbeds start to dry up exposing the supercritical water to the sulfates below, the water will start to break down very quickly; so, this condition will not last a very long time, but you should have a window of time where this happens as the planet is cooling down.

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  • $\begingroup$ Couldn’t an equilibrium be reached through high enough pressure though? I could imagine a steamy supercritical water atmosphere which gets dissociated slowly enough that the crust becomes similar to the outer mantle (hot and sludgy)? Of course the water atmosphere wouldn’t last over astronomical time scales but wouldn’t it be able to last at least a few thousand years? Water by itself only starts to breakdown at temperatures >2000-3000°C because it starts to act like a plasma. $\endgroup$
    – casualworldbuilder
    Commented Jul 17 at 23:13
  • $\begingroup$ @casualworldbuilder Water, in isolation breaks down at 2000-3000°C, yes, but it's the whole chemistry of a planet you need to consider. Sulfur is one of the most abundant elements in the universe, and there are many naturally occurring sulfates that can be used to catalyze the breakdown of water at much lower temperatures. As soon as the planet surface hits about 900°C, the water vapor will begin to react with the ground and increased pressure will only speed this reaction up. $\endgroup$
    – Nosajimiki
    Commented Jul 18 at 16:38
  • $\begingroup$ Your planet will likely never get much past 900°C from the Greenhouse Effect alone. This is probably contributed to why Venus stabilized at the exact temperature that it did and has such negligible amounts of water left. Granted... so few people understand this that your setting would still be believable to a general audience, but I'd expect a few skeptical chemists to pop up in a comments section or two. $\endgroup$
    – Nosajimiki
    Commented Jul 18 at 16:39
  • $\begingroup$ My main source for this is youtu.be/HqEd_VpHKLc?feature=shared which describes what might happen to Earth when it has a runaway greenhouse. Tbh I am really just going for a believable dramatic affect. I have made previous questions dealing with a global resurfacing event leading to a doom-of-Valyria runaway cataclysm of a formerly-habitable planet. The idea the planet ending up with supercritical water and a glowing crust is a bit of icing on top really. $\endgroup$
    – casualworldbuilder
    Commented Jul 18 at 17:43
  • $\begingroup$ @casualworldbuilder I did think of an exception that should help you. See revised answer. $\endgroup$
    – Nosajimiki
    Commented Jul 18 at 20:49
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The two main factors of your planet are:

  • Gravity
  • Magnetosphere

I would consider the greatest threat to your water to be solar winds. If your planet has a strong magnetosphere, then that problem is mitigated. After that, you just need enough gravity to ensure your superheated water cannot escape.

Note that the energy for the runaway phenomenon will at some point reach homeostasis. The energy absorbed by the planet will equal the energy radiated. You can decide that point by modifying your distance to the main star or composition of your atmosphere.

Additionally, your crust melting phenomenon must come from the core of the planet, not from radiated energy from the main start. Solar winds will wipe your atmosphere clean way before it starts to melt the crust. Some mechanism of heating from the mantle must be taking place (nuclear fission is common) at the same time if the surface is molten or close to it. The planet you're describing is similar to Venus, if we replace the sulfuric acid for water.

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