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
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.