P-type asteroid
Encyclopedia
P-type asteroids have low albedo
and a featureless reddish electromagnetic spectrum
. It has been suggested that they have a composition of organic rich silicate
s, carbon
and anhydrous
silicates, possibly with water ice
in their interior. P-type asteroids are found in the outer asteroid belt
and beyond.
. 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").
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 asteroid
s 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
.
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
and a featureless reddish electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
. It has been suggested that they have a composition of organic rich silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and anhydrous
Anhydrous
As a general term, a substance is said to be anhydrous if it contains no water. The way of achieving the anhydrous form differs from one substance to another...
silicates, possibly with water ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
in their interior. P-type asteroids are found in the outer asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
and beyond.
Taxonomy
An early system of asteroid taxonomy was established in 1975 from the doctoral thesis work of David J. TholenDavid J. Tholen
David James Tholen is an American astronomer at the Institute for Astronomy of the University of Hawaii, who specializes in planetary and solar system astronomy.-Professional life:...
. 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
C-type asteroid
C-type asteroids are carbonaceous asteroids. They are the most common variety, forming around 75% of known asteroids, and an even higher percentage in the outer part of the asteroid belt beyond 2.7 AU, which is dominated by this asteroid type...
and S-type asteroid
S-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
classifications. In 1976, some of these U-type asteroids with unusual moderate albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
levels were labeled as M-type
M-type asteroid
M-type asteroids are asteroids of partially known composition; they are moderately bright . Some, but not all, are made of nickel-iron, either pure or mixed with small amounts of stone. These are thought to be pieces of the metallic core of differentiated asteroids that were fragmented by impacts,...
.
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").
Properties
The P-type asteroids are some of the darkest objects in the Solar SystemSolar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
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 asteroid
S-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
s and they do not show spectral features. The red coloration may be caused by organic compounds related to kerogen
Kerogen
Kerogen is a mixture of organic chemical compounds that make up a portion of the organic matter in sedimentary rocks. It is insoluble in normal organic solvents because of the huge molecular weight of its component compounds. The soluble portion is known as bitumen. When heated to the right...
. The reflectance spectra of P-type asteroids can be reproduced through a combination of 31% CI and 49% CM groups of carbonaceous chondrite
Carbonaceous chondrite
Carbonaceous chondrites or C chondrites are a class of chondritic meteorites comprising at least 7 known groups and many ungrouped meteorites. They include some of the most primitive known meteorites...
meteorites, plus 20% Tagish lake meteorites, after undergoing thermal metamorphism and space weathering
Space weathering
Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...
.
The outer part of the main asteroid belt beyond 2.6 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
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
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 awarded the honour of naming it to William Henry Smyth,...
, 65 Cybele
65 Cybele
65 Cybele is one of the largest asteroids in the Solar System and is located in the outer asteroid belt. It gives its name to the Cybele family of asteroids that orbit outward from the Sun from the 2:1 orbital resonance with Jupiter. Cybele is a C-type asteroid, meaning that it is dark in color...
, 76 Freia
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 Heinrich d'Arrest on October 21, 1862, in Copenhagen,...
, 87 Sylvia
87 Sylvia
87 Sylvia is one of the largest main-belt asteroids. It is a member of the Cybele group located beyond the core of the belt . Sylvia is remarkable for being the first asteroid known to possess more than one moon....
, 153 Hilda
153 Hilda
153 Hilda is a large asteroid in the outer main belt, with a diameter of 170 km. Because it is composed of primitive carbonaceous materials, it has a very dark surface. It was discovered by Johann Palisa on November 2, 1875 from the Austrian Naval Observatory at Pula...
and 476 Hedwig
476 Hedwig
476 Hedwig is a main-belt asteroid discovered on August 17, 1901, by Luigi Carnera at Heidelberg.-External links:* -See also:*List of Solar System objects by size...
.