In astronomy, the snow line is the annulus within a protostellar disk beyond which water molecules can efficiently condense to form ice. Planets which form interior to the snow line in a young solar system will probably be composed of rocky and metallic material, while planets beyond will be composed of more volatile material.

Stars form from dense clouds of hydrogen gas, which (somehow, we don't know exactly why) are induced to collapse under their own gravity. If there is even the slightest rotation of this cloud, the infalling gas will form a disk around the centrally collapsed object. As the cloud shrinks, the rotation gets faster because of conservation of angular momentum (think of a spinning ice skater pulling their arms in to spin faster). Centrifugal acceleration of gas in the plane of rotation keeps it from falling as quickly as gas along the axis of rotation, so the gas collapses into a pancake shape -- hence a disk.

There is a radial temperature gradient within the disk due to heat from the newly-formed star -- the closer to the star you are, the hotter it will be. Since different molecules have different melting and boiling points, different compounds and molecules preferentially condense at different radii in the disk. The snow line is the annulus within the disk which defines the boundary between rocky and metallic planetesimals in the inner solar system and icy, volatile planetesimals in the outer solar system.

Water has the highest condensation temperature of any of the volatiles. At one atmospheric pressure, the freezing point of water is 273 kelvins, but some of the water remains in vapor form. To get most of the water molecules to nucleate, you have to go about 100 kelvins lower, with lower temperatures required at lower pressures. For the solar nebula, this probably occurred at a distance of about 3 AU from the Sun, with ambient pressures of about 10-4 atmospheres.

In the inner disk (inside the snow line), at temperatures higher than a few hundred kelvins, only the metals, metallic oxide and silicate compounds can condense. This is why the inner, terrestrial planets are rich in rocky and metallic material, but (relatively) poor in volatile elements -- the ambient temperature was too high for volatile molecules like water, methane, and ammonia to condense. However, water molecules can be taken into some of the condensing metals and silicates to form hydrated minerals and clays (e.g. serpentine and tremolite).

Beyond the snow line (at temperatures 50-100 kelvins lower), the other volatiles begin to condense, with ammonia and methane clathrates condensing first, followed by molecular ammonia and methane, and then simple gases like argon and nitrogen. At very cold temperatures (10-20 kelvins) molecular hydrogen and neon can start to condense as well.

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