P-type asteroids are

asteroid An asteroid is a minor planet of the Solar System#Inner solar system, inner Solar System. Sizes and shapes of asteroids vary significantly, ranging from 1-meter rocks to a dwarf planet almost 1000 km in diameter; they are rocky, metallic o ...

s that have low

albedo Albedo (; ) is the measure of the diffuse reflection of solar radiation out of the total solar radiation and measured on a scale from 0, corresponding to a black body that absorbs all incident radiation, to 1, corresponding to a body that refl ...

and a featureless reddish

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of color ...

. It has been suggested that they have a composition of organic rich silicates,

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon makes ...

and

anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achi ...

silicates, possibly with water

ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...

in their interior. P-type asteroids are found in the outer asteroid belt and beyond. There are in the neighborhood of 33 known P-type asteroids, depending on the classification, including

46 Hestia Hestia (minor planet designation: 46 Hestia) is a large, dark main-belt asteroid. It is also the primary body of the Hestia clump, a group of asteroids with similar orbits. Hestia was discovered by N. R. Pogson on August 16, 1857, at the Radcli ...

65 Cybele Cybele, minor planet designation 65 Cybele, is one of the List of notable asteroids#Largest by diameter, largest asteroids in the Solar System. It is located in the outer asteroid belt. It is thought to be a remnant primordial body. It gives its ...

76 Freia Freia ( minor planet designation: 76 Freia) is a very large main-belt asteroid. It orbits in the outer part of the asteroid belt and is classified as a Cybele asteroid. Its composition is very primitive and it is extremely dark in color. Freia w ...

87 Sylvia Sylvia ( minor planet designation: 87 Sylvia) is the one of the largest asteroids (approximately tied for 7th place, to within measurement uncertainties). It is the parent body of the Sylvia family and member of Cybele group located beyond the ...

, 153 Hilda,

476 Hedwig Hedwig (minor planet designation: 476 Hedwig) (1901 GQ) is a main-belt asteroid discovered on August 17, 1901, by Luigi Carnera at Heidelberg Heidelberg (; Palatine German language, Palatine German: ''Heidlberg'') is a city in the States of ...

and, in some classifications, 107 Camilla.

Taxonomy

An early system of asteroid taxonomy was established in 1975 from the

doctoral thesis A thesis ( : theses), or dissertation (abbreviated diss.), is a document submitted in support of candidature for an academic degree or professional qualification presenting the author's research and findings.International Standard ISO 7144 ...

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

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").

Properties

The P-type asteroids are some of the darkest objects in the

Solar System The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...

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 Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 8 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites. The C chondrites represent only a small prop ...

meteorites, plus 20% Tagish lake meteorites, after undergoing thermal metamorphism and space weathering. The density of the only two well-characterized P-type asteroids,

and 107 Camilla P-type asteroids appears to be low, at – lower even than C-type asteroids. It is not clear what this tells us about their compositions. Both Sylvia and Camilla have moons and indications that they have been be disrupted, but they are also quite massive, at over kg, and so are unlikely to have much internal porosity affecting their densities.P. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. ''Astronomy & Astrophysics'' 54, A56 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. There are 33 known P-type asteroids. In addition to this, P-type asteroids are thought to be found in the outer asteroid belt and beyond. The distribution of P-type asteroids peaks at an orbital distance of 4 AU.

Taxonomy

Properties

References

See also