This article's lead section contains information that is not included elsewhere in the article. (May 2019) (Learn how and when to remove this template message)
P-type asteroids are asteroids have low albedo and a featureless reddish 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 known P-type asteroids.
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.
Around 1981, an offshoot of the M-type asteroid branch appeared for minor planets that have spectra that are indistinguishable from M-type, but that also have low albedo not consistent with the M type. 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").
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. 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.
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. The distribution of P-type asteroids peaks at an orbital distance of 4 AU. Asteroids classified as P-type include 46 Hestia, 65 Cybele, 76 Freia, 87 Sylvia, 153 Hilda and 476 Hedwig.
- "JPL Small-Body Database Search Engine: spec. type = P (Tholen)". JPL Solar System Dynamics. Retrieved 2015-06-17.
- 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.
- De Pater, Imke; Lissauer, Jack Jonathan (2001). Planetary Sciences. Cambridge University Press. p. 353. ISBN 0-521-48219-4.
- Ehrenfreund, Pascale (2004). International Space Science Institute (eds.). Astrobiology: Future Perspectives. Springer Science & Business. p. 159. ISBN 1-4020-2304-9.CS1 maint: Uses editors parameter (link)
- 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.
- 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.
- McSween, Harry Y. (1999). Meteorites and their parent planets (2nd ed.). Cambridge University Press. p. 101. ISBN 0-521-58751-4.
- 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.