Hycean Worlds

Hot Water Worlds with a hydrogen atmosphere

Image from Steve Bowers
Thrasyllus, a hycean world in the former Conver Ambi volume

Hycean worlds are a subtype of large ocean world, covered in a deep water ocean and a relatively thick atmosphere consisting mainly of hydrogen, generally with helium and nitrogen present in lesser amounts. Hycean worlds are superterrestrial in size, and have sufficiently high escape velocity to retain hydrogen. Although these worlds are rich in oxygen, all of the oxygen present in the atmosphere and ocean is found in combination with hydrogen. For this reason, the atmosphere is anoxic.

Many such worlds have a moderately high surface temperature, reaching at least 100° C in some or all locations on the planet, although some are cooler. For this reason, hycean worlds are sometimes referred to as pyrohydrothallasic worlds. Hycean worlds worlds are classified among the aquatic worlds in the EWoCS classification system, since the dominant fluid is water.

They occupy a middle ground between Panthalassic (smaller, cool-water ocean worlds with oxygen-rich atmospheres) and pyrothalassic (very hot magma-ocean worlds, where water only exists as steam).

The water layer on a Hycean world may be hundreds of kilometres deep, and on many such worlds the water is deep enough to compress the water into high-pressure ice, forming an ice mantle which covers the rocky layers below. The depth at which this ice mantle forms depends on the temperature of the ocean, which depends in turn on the heat received from the local star(s), tidal heating and radioisotope decay in the core.

Pyrohydrothallasic green
Image from LordOther
Kabosu, a green-tinged pyrohydrothallassic world, with a thin acidian haze
The water oceans on these worlds remain liquid because of high atmospheric pressure; in many cases this pressure and temperature is sufficient to cause the formation of a supercritical fluid layer between the gaseous atmosphere and the liquid ocean. The supercritical fluid layer acts like both water and steam, dissolving solids without forming an easily definable liquid surface. Heavier-than-water vessels cannot float on the surface of such a layer, so explorers must use powered flight or balloons for support.

Protein-based life can only survive on the coolest pyrohydrothalassic worlds. Some tidally-locked (vesperian) hot-water-worlds have a habitable zone on the dark side, where the temperature is cool enough to permit protein stability. A few tidally-locked hot-water worlds are cool enough to form surface ice on the dark side, but in most cases the dense atmosphere of such planets transports heat quite efficiently to all locations on the surface.

If the temperature of the planet exceeds 200° C everywhere on its surface, any colonisation effort must use artificial life-like processes, often derived from thiogen biochemistry. Due to volcanic activity within the ice mantle layers the oceans of these worlds often contain a rich mix of dissolved ions and gases, and colonies on such worlds generally base their economy on mineral extraction technology.

White Pyrohydrothalassic
Image from LordOther
Meggio, a hycean world in the Vangelis system

Other water-rich world types

Worlds with shallow water oceans and regions of continental crust are known as Gaian worlds; these come in a wide range of subtypes, and a small but significant portion of these are life-bearing worlds.
Venus-like wet-greenhouse worlds with liquid water seas or oceans are known as PelaCytherean Subtype worlds; worlds of this kind which have macroscopic biospheres are given the name To'ul'hian worlds.
Worlds with a layer of ice overlaying a liquid water ocean are known as Europan type worlds; tidally-locked ice worlds with a small region of liquid water at or near the substellar point are known as Polyphemian subtype worlds.

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Development Notes
Text by Steve Bowers
Initially published on 22 February 2012.