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...Such planets have already been found, but these objects
even have not yet received a special name. “Ocean planet” is, after all, any
planet whose surface is completely covered with water. But in order to flood
the mountains, you don’t need a lot of water. Most likely, any geologically
active super-earth will be able to acquire a sufficiently deep, but at the same
time quite Earth-like hydrosphere. It’s another matter if a planet the size
of Earth or larger consists of three-quarters water, like TRAPPIST-1e, GJ1214b
and probably Gliese 581c. And we are talking only about planets located in the
habitable zone, at a moderate distance from the star. Ice worlds didn’t count.
The first question to be asked is: where does so much water come from? If we consider the Solar system, then bodies consisting mainly of it will immediately be found. These are the moons of giant planets. Water is a common compound, and there is more snow in the protoplanetary nebula than mineral dust. Another matter is that during the formation of a planetary system, the inner part of the disk heats up, snow turns into steam, and steam is displaced by the radiation pressure of the flaring star into the outer rings – far beyond the borders of the habitable zone. Therefore, the inner planets of the Solar System are very poor in water. The outer ones are rich, but only in frozen one. Subglacial oceans don’t count.
The answer is that all the ocean planets are found in red dwarf systems. The above-described mechanisms of “drying” planets do not work there, since the gas-dust disk heats up less due to the low gravity of the star. In the ring from which a life-suitable planet is formed, water remains in the form of snow. And in order to “blow away” a snowflake, but not a single molecule, the radiation pressure of a dwarf is not enough. Everything is simple.
...It’s simple. Planetesimals stuck together from mineral dust with ice frozen on it run around in a circle, continuing to accumulate dust and snow. They merge, at first quietly, but then, when their masses increase, mutual absorption becomes catastrophic. Water evaporates during impacts, but simply returns to the ring, where it freezes again to become part of one of the primary bodies, over and over again. This cycle can be observed by the example of the interaction of Saturn’s moon Enceladus with the E ring. After several unsuccessful attempts to find a permanent home, snow falls on the already fully formed stone core. Snowfall creates a planet.
If the planet is large enough (and under the conditions of the problem, it is the Earth-sized one), under the influence of impact, gravitational and radiation heat, melting and final differentiation of the subsurface into a metallic core and a liquid stone mantle will occur. Of course, the ice will also melt, but not every kind of it. In the conditions of colossal pressures, exotic forms of ice are formed, including its cubic, high-temperature modification – ice VII with a density of 1.65 g/cm3. The stone mantle will hide under a shell of overheated but solid water having a thickness of 4000 kilometers. And only on top of it will a liquid ocean with a depth of about 200 kilometers be able to exist. It is quite possible that it, like the ocean of Ganymede, will be divided by one, two or even three layers of exotic ice of younger, less resistant modifications, suspended in the vertical current. However, the excess of exotics in the depths is not combined with the presence of open water on the surface. Rather, in the entire thickness of the ocean – to the bottom – the temperature will be too high for crystallization.
Thus, the ocean will have a bottom – of ice having a temperature of hundreds of degrees and forming a relief consisting of domes. A reduced, indoor, pocket-sized version of it can now be seen on the surface of Triton. In the ice, even if it is solid, vertical flows will arise, transferring heat from the stone mantle to the ocean. The cooled masses from the surface will sink, carrying into the bowels the cosmic dust falling on the surface of the planet and sinking in the water.
Due to active convection, the bottom will always remain clean, lack of deposits. But this does not mean that the ocean will consist of distilled water. Not at all. It will be very salty – at least in the depths. After all, at high pressure and temperature, water turns into a solvent of monstrous efficiency. The rising ice flows will enrich the ocean with metals and salts removed from the core. Powerful currents arising from the melting of ice on the tops of domes and its freezing in the lower areas of the bottom will contribute to the removal of dissolved compounds into the upper layers of the ocean.
Will there be life in such an ocean? In the depths it is hardly probable, even taking into account the fact that life originated in the vents of underwater geysers. But the “black smokers” are not the same as the overheated exotic ice… These are quite different things. In the near-surface layers – maybe, it is possible.
Maybe, but there is a nuance. The intensity of organic synthesis in such an ocean will be very limited, even regardless of the specific conditions of the red dwarf system, because the cycle of chemicals will be partially open. Phosphorus and other chemical elements necessary for life, carried into the depths together with dead organic matter, can be returned to the photosynthesis zone only by abiogenic processes. There is a similar problem in Earth waters, but on the ocean planet it will acquire a more significant scale. Therefore, in a giant planetary sea, one does not have to expect the appearance of giant creatures. There is simply not enough food for large animals there.
Translated by Pavel Volkov, 2021
The original Russian article is here