ice

Ice is the solid state of water. The term can also be used to indicate an ice-like substance that is the solid state of some other material that is normally liquid or gas (e.g., dry ice for solid carbon dioxide) and in planetary science, the term is commonly used for frozen volatiles, typically those with a freezing point on the order of 100 K or higher (true of water, methane, and ammonia, but not nitrogen, hydrogen or helium). The chemical is sometimes specified to avoid ambiguity, e.g., water ice, or methane ice.

Ice (water ice) consists of water molecules (two hydrogen atoms covalently bonded to an oxygen atom) with hydrogen bonds (electrostatic bond, a bond between oppositely-charged ions) to the oxygen atom of another molecule. Ice is essentially water molecules stuck to each other (the type of adsorption called physisorption). Ice can be crystalline (showing a crystal structure) or amorphous (more random structure), the latter occurring when the water is brought to a very low temperature very quickly.

Uranus and Neptune are classified as ice giants (unlike Jupiter and Saturn), the term inspired by their inclusion of frozen volatiles. Within the science of planet formation, the term snow line (aka ice line) is used for the circle within a protoplanetary disk beyond which it remains sufficiently cold that some particular volatile remains frozen: a theory of giant planet formation presumes they form beyond this line, from frozen volatiles.

Ices (water and other types) and their detection are of interest in cosmology and in study of cool clouds as some molecules are expected to exist in the form of ice on dust grains in such clouds.