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Universe Sandbox tracks three [https://en.wikipedia.org/wiki/Phase_(matter) phases] of a surface material (either water or CO<sub>2</sub>) across the surface of the object. At every [[
Time Step|time step]], at every point on the surface map, Universe Sandbox calculates how much of this material will exist as a solid, liquid, or gas. The temperatures at which the material changes between these phases (e.g., the [[Boiling Point]] or [[Freezing Point]]) depend on several properties of the object, including its [[Surface Pressure]].
The amount of material that exists as a solid at a given point on the surface is represented by the Ice Thickness, the amount of liquid by the Water/Liquid CO<sub>2</sub> Depth, and the amount of gas by the Vapor Mass. Liquid and gas material can also move across the surface (see below).