P-type (primitive-type)

asteroids An asteroid is a minor planet—an object larger than a meteoroid that is neither a planet nor an identified comet—that orbits within the Solar System#Inner Solar System, inner Solar System or is co-orbital with Jupiter (Trojan asteroids). As ...

have low

albedo Albedo ( ; ) is the fraction of sunlight that is Diffuse reflection, diffusely reflected by a body. It is measured on a scale from 0 (corresponding to a black body that absorbs all incident radiation) to 1 (corresponding to a body that reflects ...

and a featureless reddish

spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...

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

silicate A silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is also used ...

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

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 achie ...

silicates, possibly with water

ice Ice is water that is frozen into a solid state, typically forming at or below temperatures of 0 ° C, 32 ° F, or 273.15 K. It occurs naturally on Earth, on other planets, in Oort cloud objects, and as interstellar ice. As a naturally oc ...

in their interior. P-type asteroids are found in the outer

asteroid belt The asteroid belt is a torus-shaped region in the Solar System, centered on the Sun and roughly spanning the space between the orbits of the planets Jupiter and Mars. It contains a great many solid, irregularly shaped bodies called asteroids ...

and beyond. There are about 33 known P-type asteroids, depending on the classification, including

46 Hestia 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 Radcliffe Observatory, Oxford. Pogson ...

65 Cybele 65 Cybele is one of the 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 name to the Cybele group of asteroids that orbit outward from the Sun from the ...

76 Freia 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 was discovered by the astronomer Hei ...

87 Sylvia 87 Sylvia is 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 main asteroid belt (see minor-planet ...

153 Hilda 153 Hilda is a large asteroid in the outer main belt, with a diameter of 170 km. The spectrum matches that of a P-type asteroid. It was discovered by Johann Palisa on 2 November 1875, from the Austrian Naval Observatory at Pula, now Croati ...

476 Hedwig 476 Hedwig (1901 GQ) is a main-belt asteroid discovered on 17 August 1901 by Luigi Carnera at Heidelberg. Named in honour of the wife of Swedish-Danish astronomer Elis Strömgren. See also * List of Solar System objects by size This articl ...

and, in some classifications,

107 Camilla 107 Camilla is one of the largest asteroids from the outermost edge of the asteroid belt, approximately in diameter. It is a member of the Sylvia family and located within the Cybele group. It was discovered on 17 November 1868, by English as ...

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: D ...

work of

David J. Tholen David James Tholen (born 1955) is an American astronomer at the Institute for Astronomy of the University of Hawaiʻi. He holds a 1978 B.S. from the University of Kansas, a 1984 PhD from the University of Arizona, and specializes in planetary ...

. 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 did not fit into the C and

S-type asteroid S-type (stony-type or silicaceous-type) asteroids are asteroids with a spectral type that is indicative of a siliceous (i.e. stony) mineralogical composition, hence the name. They have relatively high density. Approximately 17% of asteroids are ...

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 SystemCapitalization 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 "Sola ...

with very low albedos (pv<0.1) and appear to be organic-rich, similar to carbonaceous chondrites. Their colors are somewhat redder than

s and they do not show spectral features. The red coloration may be caused by organic compounds related to

kerogen Kerogen is solid, insoluble organic matter in sedimentary rocks. It consists of a variety of organic materials, including dead plants, algae, and other microorganisms, that have been compressed and heated by geological processes. All the kero ...

. 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 propo ...

meteorites, plus 20% Tagish lake meteorites, after undergoing thermal metamorphism and

space weathering Space weathering is the type of weathering that occurs to any object exposed to the harsh environment of outer space. Bodies without atmospheres (including the Moon, Mercury, the asteroids, comets, and most of the moons of other planets) take ...

. The density of the only two well-characterized P-type asteroids,

and

P-type asteroids appears to be low, at – lower even than

C-type asteroid C-type (carbonaceous ) asteroids are the most common variety, forming around 75% of known asteroids. They are volatile-rich and distinguished by a very low albedo because their composition includes a large amount of carbon, in addition to rocks ...

s. 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 The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

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