Concept
In astronomy or planetary science, the frost line, also known as the snow line or ice line, is the minimum distance from the central protostar of a solar nebula where the temperature is low enough for volatile compounds such as water, ammonia, methane, carbon dioxide and carbon monoxide to condense into solid grains, which will allow their accretion into planetesimals. Beyond the line, otherwise gaseous compounds (which are much more abundant) can be quite easily condensed to allow formation of gas and ice giants; while within it, only heavier compounds can be accreted to form the typically much smaller rocky planets. The term itself is borrowed from the notion of "frost line" in soil science, which describes the maximum depth from the surface that groundwater can freeze. Each volatile substance has its own frost line (e.g. carbon monoxide, nitrogen, and argon), so it is important to always specify which material's frost line is referred. A tracer gas may be used for materials that are otherwise difficult to detect; for example diazenylium for carbon monoxide. Different volatile compounds have different condensation temperatures at different partial pressures (thus different densities) in the protostar nebula, so their respective frost lines will differ. 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 the Solar System, and approximately equals 5 AU.