Cold trap (astronomy)

A cold trap is a concept in Planetary Sciences that describes an area cold enough to freeze (trap) volatiles. Cold-traps can exist on the surfaces of airless bodies or in the upper layers of an adiabatic atmosphere. On airless bodies, the ices trapped inside cold-traps can potentially remain there for geologic time periods, allowing us a glimpse into the primordial solar system. In adiabatic atmospheres, cold-traps prevent volatiles (such as water) from escaping the atmosphere into space.

Cold-traps on airless planetary bodies

The obliquity of some airless planetary bodies in our solar system such as Mercury, the Moon and Ceres is very close to zero. Harold Urey first noted that depressions or craters located near the poles of these bodies will cast persistent shadows that can survive for geologic time periods (million-billion years).[1] The absence of an atmosphere prevents mixing by convection, rendering these shadows extremely cold.[2] If molecules of volatiles such as water ice travel into these permanent shadows, they will become trapped for geologic time periods.[3]

Studying cold-traps on airless bodies

As these shadows receive no insolation, most of the heat they receive is scattered and emitted radiation from the surrounding topography. Usually, horizontal heat conduction from adjacent warmer areas can be neglected due to the high porosity and therefore low thermal conductivity of the uppermost layers of airless bodies. Consequently, the temperatures of these permanent shadows can be modeled using ray casting or ray tracing algorithms coupled with 1D vertical heat conduction models.[4][2] In some cases, such as bowl-shaped craters, it is possible to obtain an expression for the equilibrium temperature of these shadows.[5]

Additionally, the temperatures (and therefore the stability) of cold-traps can be remotely sensed by an orbiter. The temperatures of lunar cold-traps have been extensively studied by the Lunar Reconnaissance Orbiter Diviner radiometer.[6] On Mercury, evidence for ice deposits inside cold-traps has been obtained through radar,[7] reflectance[8][9] and visible imagery.[10] On Ceres, cold-traps have been detected by the Dawn spacecraft.[11]

Atmospheric Cold-traps

In atmospheric science, a cold-trap is a layer of the atmosphere that is substantially colder than both the deeper and higher layers. For example, for Earth's troposphere, the temperature of the air drops with increasing height reaching a low point (at about 20 kilometers height). This region is called a cold-trap, because it traps ascending gases with high melting points, forcing them to drop back into Earth.

For humans, the most important gas to be kept in that way is water vapor. Without the presence of a cold-trap in the atmosphere, the water content would gradually escape into space, making life impossible. The cold trap retains one-tenth of a percent of the water in the atmosphere in the form of a vapor at high altitudes. Earth's cold-trap is also a layer which above ultraviolet intensity is strong, since higher up the amount of water vapor is negligible. Oxygen screens out ultraviolet intensity.

Some astronomers believe that the lack of a cold trap is why the planets Venus and Mars both lost most of their liquid water early in their histories.[12]

Cold traps are thought to function for oxygen on Ganymede.[13]

References
  1. Lucey, P. G. (2009). "The Poles of the Moon". Elements. 5 (1): 41–6. doi:10.2113/gselements.5.1.41.
  2. Rubanenko, Lior; Aharonson, Oded (2017). "Stability of ice on the Moon with rough topography". Icarus. 296: 99–109. Bibcode:2017Icar..296...99R. doi:10.1016/j.icarus.2017.05.028.
  3. Watson, Kenneth; Murray, Bruce C.; Brown, Harrison (1961). "The behavior of volatiles on the lunar surface" (PDF). Journal of Geophysical Research. 66 (9): 3033–45. Bibcode:1961JGR....66.3033W. doi:10.1029/JZ066i009p03033.
  4. Vasavada, A; Paige, David A.; Wood, Stephen E. (1999). "Near-Surface Temperatures on Mercury and the Moon and the Stability of Polar Ice Deposits". Icarus. 141 (2): 179–93. Bibcode:1999Icar..141..179V. doi:10.1006/icar.1999.6175.
  5. Buhl, David; Welch, William J.; Rea, Donald G. (1968). "Reradiation and thermal emission from illuminated craters on the lunar surface". Journal of Geophysical Research. 73 (16): 5281–95. Bibcode:1968JGR....73.5281B. doi:10.1029/JB073i016p05281.
  6. Paige, D. A.; Siegler, M. A.; Zhang, J. A.; Hayne, P. O.; Foote, E. J.; Bennett, K. A.; Vasavada, A. R.; Greenhagen, B. T.; Schofield, J. T.; McCleese, D. J.; Foote, M. C.; Dejong, E.; Bills, B. G.; Hartford, W.; Murray, B. C.; Allen, C. C.; Snook, K.; Soderblom, L. A.; Calcutt, S.; Taylor, F. W.; Bowles, N. E.; Bandfield, J. L.; Elphic, R.; Ghent, R.; Glotch, T. D.; Wyatt, M. B.; Lucey, P. G. (2010). "Diviner Lunar Radiometer Observations of Cold Traps in the Moon's South Polar Region". Science. 330 (6003): 479–82. Bibcode:2010Sci...330..479P. doi:10.1126/science.1187726. PMID 20966246. S2CID 12612315.
  7. Harmon, J; Perillat, P. J.; Slade, M. A. (2001). "High-Resolution Radar Imaging of Mercury's North Pole". Icarus. 149 (1): 1–15. Bibcode:2001Icar..149....1H. doi:10.1006/icar.2000.6544.
  8. Neumann, G. A.; Cavanaugh, J. F.; Sun, X.; Mazarico, E. M.; Smith, D. E.; Zuber, M. T.; Mao, D.; Paige, D. A.; Solomon, S. C.; Ernst, C. M.; Barnouin, O. S. (2012). "Bright and Dark Polar Deposits on Mercury: Evidence for Surface Volatiles". Science. 339 (6117): 296–300. Bibcode:2013Sci...339..296N. doi:10.1126/science.1229764. PMID 23196910. S2CID 206544976.
  9. Rubanenko, L.; Mazarico, E.; Neumann, G. A.; Paige, D. A. (2017). "Evidence for Surface and Subsurface Ice Inside Micro Cold-Traps on Mercury's North Pole". 48th Lunar and Planetary Science Conference. 48 (1964): 1461. Bibcode:2017LPI....48.1461R.
  10. Chabot, N. L.; Ernst, C. M.; Denevi, B. W.; Nair, H.; Deutsch, A. N.; Blewett, D. T.; Murchie, S. L.; Neumann, G. A.; Mazarico, E.; Paige, D. A.; Harmon, J. K.; Head, J. W.; Solomon, S. C. (2014). "Images of surface volatiles in Mercury's polar craters acquired by the MESSENGER spacecraft". Geology. 42 (12): 1051–4. Bibcode:2014Geo....42.1051C. doi:10.1130/G35916.1.
  11. Schorghofer, Norbert; Mazarico, Erwan; Platz, Thomas; Preusker, Frank; Schröder, Stefan E.; Raymond, Carol A.; Russell, Christopher T. (2016). "The permanently shadowed regions of dwarf planet Ceres". Geophysical Research Letters. 43 (13): 6783–9. Bibcode:2016GeoRL..43.6783S. doi:10.1002/2016GL069368.
  12. Strow, Thompson (1977). Astronomy: Fundamentals and Frontiers. Quinn & Boden. p. 425.
  13. Vidal, R. A.; Bahr, D.; Baragiola, R. A.; Peters, M. (1997). "Oxygen on Ganymede: Laboratory Studies". Science. 276 (5320): 1839–42. Bibcode:1997Sci...276.1839V. doi:10.1126/science.276.5320.1839. PMID 9188525.
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