Frost line (astrophysics)

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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:

  • 170 K at 2.7 AU (Hayashi, 1981)[1]
  • 143 K at 3.2 AU to 150 K at 3 AU (Podolak and Zucker, 2010)[2]
  • 3.1 AU (Martin and Livio, 2012)[3]

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.[4] 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,[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 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 Rsnow = 2.7 AU from the Sun.[3]

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.[5][6]

Each volatile substance has its own snow line, e.g. carbon monoxide[7] and nitrogen,[8] 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.[9] 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.[10] 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.[11]

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

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References[edit]

  1. ^ "Structure of the Solar Nebula, Growth and Decay of Magnetic Fields and Effects of Magnetic and Turbulent Viscosities on the Nebula by Chushiro Hayashi". 
  2. ^ "A note on the snow line in protostellar accretion disks by M. PODOLAK and S. ZUCKER, 2010". 
  3. ^ a b "On the Evolution of the Snow Line in Protoplanetary Discs by Rebecca G. Martin, Mario Livio (STScI)". arXiv. 
  4. ^ Jewitt, D; Chizmadia, L.; Grimm, R.; Prialnik, D (2007). "Water in the Small Bodies of the Solar System". In Reipurth, B.; Jewitt, D.; Keil, K. Protostars and Planets V (PDF). University of Arizona Press. pp. 863–878. ISBN 0-8165-2654-0. 
  5. ^ McCord, T. B.; Sotin, C. (2005-05-21). "Ceres: Evolution and current state". Journal of Geophysical Research: Planets 110 (E5): E05009. doi:10.1029/2004JE002244. Retrieved 2015-03-07. 
  6. ^ O'Brien, D. P.; Travis, B. J.; Feldman, W. C.; Sykes, M. V.; Schenk, P. M.; Marchi, S.; Russell, C. T.; Raymond, C. A. (March 2015). "The Potential for Volcanism on Ceres due to Crustal Thickening and Pressurization of a Subsurface Ocean" (PDF). 46th Lunar and Planetary Science Conference. p. 2831. Retrieved 1 March 2015. 
  7. ^ "Imaging of the CO Snow Line in a Solar Nebula Analog by Chunhua Qi, Karin I. Oberg, et al". arXiv. 
  8. ^ "UltraCarbonaceous Antarctic micrometeorites, probing the Solar System beyond the nitrogen snow-line by E. Dartois, et al". ScienceDirect. 
  9. ^ Kaufmann, William J. (1987). Discovering the Universe. W.H. Freeman and Company. p. 94. ISBN 0-7167-1784-0. 
  10. ^ Chambers, John (2007-07-01). "Planet Formation with Type I and Type II Migration". 38. AAS/Division of Dynamical Astronomy Meeting. Bibcode 2007DDA....38.0604C.
  11. ^ "Asteroid Belts of Just the Right Size are Friendly to Life". Nasa. 1 November 2012. Retrieved 3 November 2012. 

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