What is "frost line"

The frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of the soil and adjacent materials, and on nearby heat sources. For example, snow cover and asphalt insulate the ground and homes can heat the ground (see also heat island). The line varies by latitude, it is deeper closer to the poles. It ranges in the United States from about zero to six feet. Below that depth, the temperature varies, but is always above .

Alternatively, in Arctic and Antarctic locations the freezing depth is so deep that it becomes year-round permafrost, and the term " thaw depth" is used instead. Finally, in tropical regions, frost line may refer to the vertical geographic elevation below which frost does not occur.

Frost front refers to the varying position of the frost line during seasonal periods of freezing and thawing.

In astronomy or planetary science, the frost line, also known as the snow line or ice line, is the particular distance in the solar nebula from the central protostar where it is cold enough for volatile compounds such as water, ammonia, methane, carbon dioxide, carbon monoxide to condense into solid ice grains. This condensation temperature depends on the volatile substance and the partial pressure of vapor in the protostar nebula. The actual temperature and distance for the snow line of water ice depend on the physical model used to calculate it and on the theoretical solar nebula model:

• 170 K at 2.7 AU (Hayashi, 1981)
• 143 K at 3.2 AU to 150 K at 3 AU (Podolak and Zucker, 2010)
• 3.1 AU (Martin and Livio, 2012)
• ≈150 K for μm-size grains and ≈200 K for km-size bodies (D'Angelo and Podolak, 2015)

The radial position of the condensation/evaporation front varies over time, as the nebula evolves. Occasionally, the term snow line is also used to represent the present distance at which water ice can be stable (even under direct sunlight). This current snow line distance is different from the formation snow line distance during the formation of Solar System, and approximately equals 5 AU. The reason for the difference is that during the formation of Solar System, the solar nebula was an opaque cloud where temperature were lower close to the Sun, and the Sun itself was less energetic. After formation, the ice got buried by infalling dust and it has remained stable a few meters below the surface. If ice within 5 AU is exposed, e.g. by a crater, then it sublimates on short timescales. However, out of direct sunlight ice can remain stable on the surface of asteroids (and the Moon) if it is located in permanently shadowed craters, where temperature may remain very low over the age of the Solar System (e.g. 30–40 K on the Moon).

Observations of the asteroid belt, located between Mars and Jupiter, suggest that the water snow line during formation of Solar System was located within this region. The outer asteroids are icy C-class objects (e.g. Abe et al. 2000; Morbidelli et al. 2000) whereas the inner asteroid belt is largely devoid of water. This implies that when planetesimal formation occurred the snow line was located at around 2.7 AU from the Sun.

For example, the dwarf planet Ceres with semi-major axis of 2.77 AU lies almost exactly on the lower estimation for water snow line during the formation of the Solar System. Ceres appears to have an icy mantle and may even have a water ocean below the surface.

Each volatile substance has its own snow line, e.g. carbon monoxide and nitrogen, so it is important to always specify which material's snow line is meant.

The lower temperature in the nebula beyond the frost line makes many more solid grains available for accretion into planetesimals and eventually planets. The frost line therefore separates terrestrial planets from giant planets in the Solar System. However, giant planets have been found inside the frost line around several other stars (so-called hot Jupiters). They are thought to have formed outside the frost line, and later migrated inwards to their current positions. Earth, which lies less than a quarter of the distance to the frost line but is not a giant planet, has adequate gravitation for keeping methane, ammonia, and water vapor from escaping it. Methane and ammonia are rare in the Earth's atmosphere only because of their instability in an oxygen-rich atmosphere that results from life forms (largely green plants) whose biochemistry suggests plentiful methane and ammonia at one time, but of course liquid water and ice, which are chemically stable in such an atmosphere, form much of the surface of Earth.

Researchers Rebecca Martin and Mario Livio have proposed that asteroid belts may tend to form in the vicinity of the frost line, due to nearby giant planets disrupting planet formation inside their orbit. By analysing the temperature of warm dust found around some 90 stars, they concluded that the dust (and therefore possible asteroid belts) was typically found close to the frost line.

The term is borrowed from the notion of " frost line" in soil science.

In geology, the frost line is the level down to which the soil will normally freeze each winter. By an analogy, the term is introduced in other areas.

• Frost line (astrophysics), a particular distance in the solar nebula from the central protosun where it is cool enough for hydrogen compounds such as water, ammonia, and methane to condense into solid ice grains.
• Frost line (polymers) in polymer film manufacturing, a notion related to physical changes from melt into solid film during extrusion.

The frost line is a term used in plastic film manufacturing by extrusion. It refers to the point beyond the die where the temperature of the molten plastic falls below the softening point and the diameter of the extruded plastic bubble stabilizes. The term was borrowed from the notion of " frost line" in soil science and refers to the frosted appearance of the plastic film above the "frost line".

The distance from the die is called the height of the frost line. It depends on various factors, including the melt temperature, the speed of cooling, the extrusion speed, and the diameter of the bubble. The notion is important, since the higher the frost line, the more difficult to control the uniformity of the film thickness.

For example, a higher frost line due to higher melt temperature and/or lower cooling rate means a longer time to solidify, and a more smooth and transparent film is produced.