Ice worlds

== Surface composition ==
An ice world is a type of planet whose surface is covered primarily by solidified volatileshttps://en.wikipedia.org/wiki/Volatiles. The global cryosphere that spans an ice planet's surface can be composed of water, methane, ammonia, carbon dioxide, carbon monoxide, and other volatiles. In the majority of cases the composition of an ice world's outer layer relates to its surface temperature:
* Methane is the main component if surface temperatures reach below 80K (-193 °C) 
* CO2 and ammonia predominate at temperatures between 80K and 180K ( -93 °C)
* Water ice is the prevailing component at temperatures between 180K and 260K (-13 °C)
== Habitability ==
Because of the low temperatures, the surface conditions of ice planets are hostile to life forms like those living on terrae. In some cases unicellular life can subsist on the surface, as microorganisms exhibit a much higher tolerance for extreme temperatures. Many ice worlds have subsurface oceans, warmed by internal heat or tidal forces from another nearby body. Liquid subsurface water provides habitable conditions for multicellular life. Subsurface plants relying on photosynthesis are extremely uncommon because there is no sunlight in these environments. Microorganisms can produce nutrients using specific chemicals (chemosynthesis) that provide food and energy for other organisms.https://en.wikipedia.org/wiki/Ice_planet
== Morphology of the outer shell ==
The shell of many ice worlds exhibits cracks and streaks. These features are another result of the tidal forces that repeatedly knead the core, the subglacial oceans and the shell.https://en.wikipedia.org/wiki/Europa_(moon) Another typical trait are the penitentes, that are mainly found across the equatorial areas.https://en.wikipedia.org/wiki/Penitente_(snow_formation) These spiky ice blades protruding from the shell originate when direct overhead sunlight melts vertical cracks.
The shell of an ice world is often divided in several plates that converge and diverge in a process similar to plate tectonics. This process generates heat that additionally contributes to the increased temperatures found in the subglacial oceans. The tectonical activity of the surface leads to the gradual transformation of its morphology, which is the reason for the low amount of craters that can be found on most ice worlds.
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