Can a water world be turned into a dryer habitable planet by an early runaway greenhouse effect?

Question

Consider an Earth clone with oceans several hundred kilometers deep, potentially deep enough to solidify into exotic ices. Such a planet would lose its initial hydrogen and helium atmosphere when the sun ignites its fusion. Water, carbon dioxide, ammonia and methane would make up the atmosphere. Our earth eventually lost this steam atmosphere when the surface solidified, oceans formed and carbon was sequestrated via weathering.

This world wouldn't be able to get rid of its greenhouse gas rich atmosphere and would warm rapidly as the sun gets brighter. Eventually a runaway greenhouse effect would occur, resulting in the rapid loss of the oceans to space.

I wonder if this would inevitably result in a Venus like planet. Could weathering, as the escape of the oceans comes to an end and land emerges for the first time, sequestrate the surplus CO2 with the help of the remaining water? Could weathering, which is temperature dependent, be strong enough to save the remaining water during the last phase of the runaway greenhouse effect? Additionally, would it be more likely that such a planet would retain more or less water? So are we more likely to get a desiccated desert with a few inland seas or an archipelago world with only some land. Especially when one considers that there would be a lot of water in the crust that could be released via subsequent out gassing.

Answer 1

The dialectic of planetary creation is dominated by two main forces: the loss of lighter elements of the primary atmosphere due to atmospheric escape, and the "ice line" within which water is less common.

For a habitable world it seems to be preferable for plate tectonics renew the atmosphere, which requires a large core, which requires relatively reducing conditions (lack of oxygen/water) so that iron interacts less in the mantle and sinks to the center.

For this reason, a planet near the sun starting with a large amount of steam and CO2 in the atmosphere sounds a bit off. Not tremendously unlikely, since planets migrate, impacts happen, and none of these models come with a money back guarantee.

If the planet gets to a runaway greenhouse, it seems in dire trouble. Hydrogen will definitely be lost in preference to carbon and oxygen, so the planet will steadily dry out. By the time you get to the "last phase" (I assume you mean something like Venus) the planet could probably calcine its own lime, at least underground. Plate tectonics are unlikely (they need water). There may be an ocean of ringwoodite deep underground - even on Venus there may be ringwoodite and some circulation of the mantle, despite what I said - but how to get at it?

Still, there are options. Planets keep migrating in and out. Collisions happen, even massive ones. The planet might lose much of its atmosphere because it's a little smaller than Venus, then it begins outgassing again. Perhaps it becomes tidally locked over time and the dark side is able to accumulate water.

My favorite for today is to suppose that the first two papers I quoted are right, and a planet in the position of Mars usually becomes depleted of oxygen in its atmosphere by the abundant iron and lack of plate tectonics, and develops dust storms like Mars, and sprays out a zodiacal light all over its star system like Mars. And in this light, there are primitive organisms that evolved during the first flush of its youth -- or life landed on it from the dust of other Mars clones caught in comets that had been in the Oort clouds of passing stars. Just so, life is available to all these worlds in rudimentary form, and on this one, it gets especially lucky in the comfortable outer layers of the atmosphere. These life forms do what you then want - they terraform your version of Venus. They manage to extract carbon into some kind of fireproof diatomaceous earth that rains down on the planet until it is cooler. They pull water out of sulfuric acid or find it deep underground and continue until the plate tectonics starts renewing the crust. Just so the Fravashis who play in our game of stellar conquest have ensured that even "acid worlds" will have some representation in the events that follow.