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Radiation spectrum of planet

x axis is wave length in the unit of nm y axis is intensity of the astronomic object in an arbitrary unit. Temperature of planet used for calculation were mean value.

Surface conditions and "atmosphere" (exosphere)Edit

The mean surface temperature of Mercury is 442.5 K,[1] but it ranges from 100 K to 700 K,[2] due to the absence of an atmosphere. On the dark side of the planet, temperatures average 110 K.[3] The intensity of sunlight on Mercury’s surface ranges between 4.59 and 10.61 times the solar constant (1,370Wm−2).[4]

Merc fig2sm

Radar image of Mercury's north pole

Despite the generally extremely high temperature of its surface, observations strongly suggest that ice exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K; far lower than the global average.[5] Water ice strongly reflects radar, and observations by the 70m Goldstone telescope and the VLA in the early 1990s revealed that there are patches of very high radar reflection near the poles.[6] While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.[7]

The icy regions are believed to contain about 1014–1015 kg of ice,[8] and may be covered by a layer of regolith that inhibits sublimation.[9] By comparison, the Antarctic ice sheet on Earth has a mass of about 4×1018 kg, and Mars' south polar cap contains about 1016 kg of water.[8] The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by impacts of comets.[8]

Terrestrial planet size comparisons

Size comparison of terrestrial planets (left to right): Mercury, Venus, Earth, and Mars

Mercury is too small for its gravity to retain any significant atmosphere over long periods of time; however, it does have a "tenuous surface-bounded exosphere"[10] containing hydrogen, helium, oxygen, sodium, calcium and potassium. This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen and helium atoms probably come from the solar wind, diffusing into Mercury’s magnetosphere before later escaping back into space. Radioactive decay of elements within Mercury’s crust is another source of helium, as well as sodium and potassium. MESSENGER found high proportions of calcium, helium, hydroxide, magnesium, oxygen, potassium, silicon and sodium. Water vapor is present, being released by a combination of processes such as: comets striking its surface, sputtering creating water out of hydrogen from the solar wind and oxygen from rock, and sublimation from reservoirs of water ice in the permanently shadowed polar craters. The detection of high amounts of water-related ions like O+, OH-, and H2O+ was a surprise.[11][12] Because of the quantities of these ions that were detected in Mercury's space environment, scientists surmise that these molecules were blasted from the surface or exosphere by the solar wind.[13][14]

Sodium and potassium were discovered in the atmosphere during the 1980s, and are believed to result primarily from the vaporization of surface rock struck by micrometeorite impacts. Due to the ability of these materials to diffuse sunlight, Earth-based observers can readily detect their composition in the atmosphere. Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic dipoles. This would indicate an interaction between the magnetosphere and the planet's surface.[15]

See alsoEdit

ReferencesEdit

  1. Cite error: Invalid <ref> tag; no text was provided for refs named nssdcMercury
  2. Prockter, Louise (2005). Ice in the Solar System. Volume 26, Johns Hopkins APL Technical Digest, http://adsabs.harvard.edu/abs/2007ASPC..370..265C. Retrieved on 18 May 2009. 
  3. Murdock, T. L.; Ney, E. P. (1970). "Mercury: The Dark-Side Temperature". Science 170 (3957): 535–537. doi:10.1126/science.170.3957.535. PMID 17799708, http://www.sciencemag.org/cgi/content/abstract/170/3957/535. Retrieved on 9 April 2008. 
  4. Lewis, John S. (2004). Physics and Chemistry of the Solar System, Academic Press. p. 461, http://books.google.co.uk/books?id=ERpMjmR1ErYC&pg=RA1-PA461&lpg=RA1-PA461&dq=solar-constant+mercury+-wikipedia+-wiki+-encyclopedia&source=web&ots=5jprP6dXYk&sig=iJEN0OU01yxgxnZhPcG17z-exYw&hl=en#PRA1-PA461,M1. Retrieved on 3 June 2008. 
  5. Ingersoll, Andrew P.; Svitek, Tomas; Murray, Bruce C. (November 1992). "Stability of polar frosts in spherical bowl-shaped craters on the moon, Mercury, and Mars". Icarus 100 (1): 40–47. doi:10.1016/0019-1035(92)90016-Z. Bibcode1992Icar..100...40I. 
  6. Slade, M. A.; Butler, B. J.; Muhleman, D. O. (1992). "Mercury radar imaging — Evidence for polar ice". Science 258 (5082): 635–640. doi:10.1126/science.258.5082.635. PMID 17748898. 
  7. Williams, David R. (June 2, 2005). "Ice on Mercury". NASA Goddard Space Flight Center. Retrieved on 2008-05-23.
  8. 8.0 8.1 8.2 Rawlins, K; Moses, J. I.; Zahnle, K.J. (1995). "Exogenic Sources of Water for Mercury's Polar Ice". Bulletin of the American Astronomical Society 27: 1117. Bibcode1995DPS....27.2112R. 
  9. Harmon, J. K.; Perillat, P. J.; Slade, M. A. (January 2001). "High-Resolution Radar Imaging of Mercury's North Pole". Icarus 149 (1): 1–15. doi:10.1006/icar.2000.6544. 
  10. Domingue, Deborah L. et al. (August 2009). "Mercury's Atmosphere: A Surface-Bounded Exosphere". Space Science Reviews 131 (1–4): 161–186. doi:10.1007/s11214-007-9260-9, http://adsabs.harvard.edu/abs/2007SSRv..131..161D. 
  11. Hunten, D. M.; Shemansky, D. E.; Morgan, T. H. (1988). [www.uapress.arizona.edu/onlinebks/Mercury/MercuryCh17.pdf "The Mercury atmosphere"]. Mercury, University of Arizona Press. ISBN 0-8165-1085-7. 
  12. Lakdawalla, Emily (July 3, 2008). "MESSENGER Scientists 'Astonished' to Find Water in Mercury's Thin Atmosphere". Retrieved on 18 May 2009. Archived from the original on 7 July 2008. 
  13. Zurbuchen, Thomas H. et al. (July 2008). "MESSENGER Observations of the Composition of Mercury’s Ionized Exosphere and Plasma Environment". Science 321 (5885): 90–92. doi:10.1126/science.1159314. PMID 18599777. 
  14. "Instrument Shows What Planet Mercury Is Made Of", University of Michigan (June 30, 2008). Retrieved on 18 May 2009. 
  15. Cite error: Invalid <ref> tag; no text was provided for refs named chaikin1

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