Inastronomy orplanetary science, thefrost line, also known as thesnow line orice line, is the minimum distance from the centralprotostar of asolar nebula where thetemperature is low enough forvolatile compounds such aswater,ammonia,methane,carbon dioxide andcarbon monoxide tocondense intosolid grains, which will allow theiraccretion intoplanetesimals. Beyond the line, otherwisegaseous compounds (which are much more abundant) can be quite easily condensed to allow formation ofgas giants andice giants; while within it, only heavier compounds can be accreted to form the typically much smallerrocky planets.
The term itself isborrowed from the notion of "frost line" insoil science, which describes the maximum depth from the surface thatgroundwater can freeze.
Each volatile substance has its own frost line (e.g., carbon monoxide,[1]nitrogen,[2] andargon[3]), so it is important to always specify which material's frost line is referred to, though omission is common, especially for the water frost line. Atracer gas may be used for materials that are otherwise difficult to detect; for examplediazenylium 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 frost line of water ice depend on the physical model used to calculate it and on the theoretical solar nebula model:
The location of the frost line changes over time, potentially reaching a maximum radius of17.4 AU for a solar-mass star before decreasing thereafter.[8]
The radial position of the condensation/evaporation front varies over time, as the nebula evolves. Occasionally, the termfrost line is also used to represent the present distance at which water ice can be stable (even under direct sunlight). Thiscurrent frost line distance is different from theformation frost line distance – which was in effect during the formation of theSolar System, and approximately equals 5 AU.[9]The reason for the difference is that during the formation of the Solar System, the solar nebula was an opaque cloud where temperatures were lower close to the Sun,[citation needed] 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 within5 AU is exposed, e.g., by a crater, then itsublimates relatively quickly. However, out of direct sunlight ice can remain stable on the surface of asteroids (and the Moon and Mercury) if it is located in permanently shadowed polar craters, where temperature may remain very low over the age of the Solar System (e.g.,30–40K on the Moon).
Observations of theasteroid belt, located between Mars and Jupiter, suggest that the water frost line during formation of the 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 frost line was located at around 2.7 AU from the Sun.[6]
For example, thedwarf planet Ceres withsemi-major axis of 2.77 AU lies almost exactly at the lower estimate for the water frost 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.[10][11] Water ice has been detected on the surface of24 Themis that orbits the Sun at an average distance of 3.1 AU.[12]
The lower temperature in the nebula beyond the frost line makes many more solid grains available foraccretion intoplanetesimals and eventuallyplanets. The frost line therefore separates terrestrial planets fromgiant planets in the Solar System.[13]However, giant planets have been found inside the frost line around several other stars (so-calledhot Jupiters). They are thought to have formed outside the frost line, and latermigrated inwards to their current positions.[14][15] 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 anoxygen-rich atmosphere that results fromphotosynthesis whose biochemistry suggests plentiful methane and ammonia at one time, but of courseliquid water andice, which are chemically stable in such an atmosphere, form much of the surface of Earth.
Researchers Rebecca Martin andMario 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.[16] The underlying mechanism may be the thermal instability of frost line on the timescales of 1,000–10,000 years, resulting in periodic deposition of dust material in relatively narrow circumstellar rings.[17]
