P-type asteroid

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P-type asteroids have low albedo and a featureless reddish electromagnetic reflectance spectrum. It has been suggested that they have a composition of organic rich silicates, carbon and anhydrous silicates, possibly with water ice in their interior. P-type asteroids are found in the outer asteroid belt and beyond. There are 33 P-type asteroids.[1]


An early system of asteroid taxonomy was established in 1975 from the doctoral thesis work of David J. Tholen. This was based upon observations of a group of 110 asteroids. The U-type classification was used as a miscellaneous class for asteroids with unusual spectra that didn't fit into the C and S-type asteroid classifications. In 1976, some of these U-type asteroids with unusual moderate albedo levels were labeled as M-type.[2]

Around 1981, an offshoot of the M-type asteroid branch appeared for minor planets that had a spectra that was indistinguishable from M-type, but they also had low albedo that did not lie within the M-type criteria. These were initially labeled X-type asteroids, then type DM (dark M) or PM (pseudo-M), before acquiring their own unique classification as P-type asteroids (where the P indicates "pseudo-M").[2]


The P-type asteroids are some of the darkest objects in the Solar System with very low albedos (pv<0.1) and appear to be organic-rich, similar to carbonaceous chondrites. Their colors are somewhat redder than S-type asteroids and they do not show spectral features. The red coloration may be caused by organic compounds related to kerogen.[3][4] The reflectance spectra of P-type asteroids can be reproduced through a combination of 31% CI and 49% CM groups of carbonaceous chondrite meteorites, plus 20% Tagish lake meteorites, after undergoing thermal metamorphism and space weathering.[5]

The outer part of the main asteroid belt beyond 2.6 AU from the Sun is dominated by low-albedo C, D and P-type asteroids. These are primitive asteroids that may have had their materials chemically altered by liquid water.[6] The distribution of P-type asteroids peaks at an orbital distance of 4 AU.[7] Asteroids classified as P-type include 46 Hestia, 65 Cybele, 76 Freia, 87 Sylvia, 153 Hilda and 476 Hedwig.[5][8]


  1. ^ "JPL Small-Body Database Search Engine: spec. type = P (Tholen)". JPL Solar System Dynamics. Retrieved 2015-06-17. 
  2. ^ a b Tholen, D. J.; Bell, J. F. (March 1987). "Evolution of Asteroid Taxonomy". Proceedings, 18th Lunar and Planetary Science Conference. Houston, Texas. pp. 1008–1009. Bibcode:1987LPI....18.1008T. 
  3. ^ De Pater, Imke; Lissauer, Jack Jonathan (2001). Planetary Sciences. Cambridge University Press. p. 353. ISBN 0-521-48219-4. 
  4. ^ Ehrenfreund, Pascale (2004). International Space Science Institute, ed. Astrobiology: Future Perspectives. Springer Science & Business. p. 159. ISBN 1-4020-2304-9. 
  5. ^ a b Hiroi, T. et al. (March 15–19, 2004). "What are the P-type Asteroids Made Of?". Proceedings, 35th Lunar and Planetary Science Conference. League City, Texas. Bibcode:2004LPI....35.1616H. 
  6. ^ Lazzarin, M.; Barbieri, C.; Barucci, M. A. (December 1995). "Visible Spectroscopy of Dark, Primitive Asteroids". Astronomical Journal 110: 3058. Bibcode:1995AJ....110.3058L. doi:10.1086/117747. 
  7. ^ McSween, Harry Y. (1999). Meteorites and their parent planets (2nd ed.). Cambridge University Press. p. 101. ISBN 0-521-58751-4. 
  8. ^ Ziffer, J.; Campins, H.; Licandro, J.; Fernandez, Y. R.; Bus, S. (August 2005). "Near-infrared Spectra of Two Asteroids with Low Tisserand Invariant". Bulletin of the American Astronomical Society 37: 644. Bibcode:2005DPS....37.1529Z. 

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