Panthalassic Type

Giant Superterrestrial Waterworlds

Image from Steve Bowers
Panthalassa, a giant waterworld

Superterrestrial Subtype: 2.5 to 10 earth masses
Terrestrial Subtype: 0.05-2.5 earth masses

Panthalassic worlds have deep oceans and rocky cores. Many have a thick mantle of high-pressure ice beneath the ocean; the depth of the ocean depends on the gravity and temperature of the world. The atmosphere of this kind of world generally has a high water vapour content; if enough ultraviolet light is received from the local star(s) the water vapour will be split into hydrogen and oxygen, and on all but the largest panthallasic worlds the hydrogen will escape, leaving an oxygen-rich atmosphere behind. Other gases which may be present are nitogen, carbon dioxide (which may be largely dissolved into the ocean), argon, neon and (on the largest superterrestrial examples) helium.

During the early formative period, swift-growing gas giants often migrate inward through the protoplanetary disk. However, past a certain point, called the snowline, abundant icy materials used for gas giant growth become unavailable, and instead only rocky material of a lesser amount is used for planetary development. These worlds thus largely stop growing when they migrate inward, but remain composed primarily of icy materials. In the warmer region of a solar system, they then develop tremendously deep oceans and thick atmospheres.

Example: Panthalassa

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Development Notes
Text by John M. Dollan and Steve Bowers

Initially published on 20 November 2008.