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.
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 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").
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.
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