Editing P-type asteroid

{{Short description|Classification of asteroids}}
{{lead extra info|date=May 2019}}
'''P-type''' (primitive-type) [[asteroids]] have low [[albedo]] and a featureless reddish [[Asteroid spectral types|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. There are about 33 known P-type asteroids, depending on the classification,<ref name="SBDB"/> including [[46 Hestia]], [[65 Cybele]], [[76 Freia]], [[87 Sylvia]], [[153 Hilda]], [[476 Hedwig]] and, in some classifications, [[107 Camilla]].<ref name="lpsc35" /><ref>{{cite journal
  | author=Ziffer, J.
  | author2=Campins, H.
  | author3=Licandro, J.
  | author4=Fernandez, Y. R.
  | author5=Bus, S.
  | title=Near-infrared Spectra of Two Asteroids with Low Tisserand Invariant
  | journal=Bulletin of the American Astronomical Society
  | volume=37 | page=644 |date=August 2005
  | bibcode=2005DPS....37.1529Z }}<!--| access-date=2009-09-17 --></ref>

==Taxonomy==
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 did not fit into the [[C-type asteroid|C]] and [[S-type asteroid]] classifications. In 1976, some of these U-type asteroids with unusual moderate [[albedo]] levels were labeled as [[M-type asteroid|M-type]].<ref name="18lpsc">{{cite conference
 | author=Tholen, D. J.
 | author2=Bell, J. F.
 | title=Evolution of Asteroid Taxonomy
 | book-title=Proceedings, 18th Lunar and Planetary Science Conference |date=March 1987
 | location=Houston, Texas | pages=1008–1009
 | bibcode=1987LPI....18.1008T }}</ref>

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").<ref name="18lpsc"/>

==Properties==
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 asteroid]]s and they do not show spectral features. The red coloration may be caused by organic compounds related to [[kerogen]].<ref>{{cite book
  | author=De Pater, Imke|author-link1=Imke de Pater
  | author2=Lissauer, Jack Jonathan
  | title=Planetary Sciences | url=https://archive.org/details/planetaryscience00pate
  | url-access=limited
  | date=2001
  | page=[https://archive.org/details/planetaryscience00pate/page/n394 353] | publisher=[[Cambridge University Press]]
  | isbn=0-521-48219-4
}}</ref><ref>{{cite book
  | first=Pascale | last=Ehrenfreund | date=2004
  |editor1= Ehrenfreund, P. |editor2=Irvine, W.M. |editor3=Owen, T. |editor4=Becker, L. |editor5=Blank, J. |editor6=Brucato, J.R. |editor7=Colangeli, L. |editor8=Derenne, S. |editor9=Dutrey, A. |editor10=Despois, D. |editor11=Lazcano, A. |editor12=Robert, F. |display-editors=3
  | title=Astrobiology: Future Perspectives
  | page=159 | publisher=Springer Science & Business
  | isbn=1-4020-2304-9 }}</ref> 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 (meteorite)|Tagish lake meteorites]], after undergoing thermal metamorphism and [[space weathering]].<ref name="lpsc35">{{cite conference
 | author=Hiroi, T. |display-authors=etal
 | title=What are the P-type Asteroids Made Of?
 | book-title=Proceedings, 35th Lunar and Planetary Science Conference
 | date=March 15–19, 2004 | location=League City, Texas
 | bibcode=2004LPI....35.1616H }}</ref>

The density of the only two well-characterized P-type asteroids, [[87 Sylvia]] and [[107 Camilla]] P-type asteroids appears to be low, at {{val|1.3|u=g/cm3}} &ndash; lower even than [[C-type asteroid]]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 {{e|19}} kg, and so are unlikely to have much internal porosity affecting their densities.<ref name=VLT>P. Vernazza et al. (2021) VLT/SPHERE imaging survey of the largest main-belt asteroids: Final results and synthesis. ''Astronomy & Astrophysics'' 54, A56 </ref>

The outer part of the main asteroid belt beyond 2.6 [[Astronomical Unit|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.<ref>{{cite journal
  | author=Lazzarin, M.
  | author2=Barbieri, C.
  | author3=Barucci, M. A.
  | title=Visible Spectroscopy of Dark, Primitive Asteroids |date=December 1995
  | journal=Astronomical Journal | volume=110
  | pages=3058 | doi=10.1086/117747 | bibcode=1995AJ....110.3058L
| doi-access=free}}</ref> The distribution of P-type asteroids peaks at an orbital distance of 4 AU.<ref>{{cite book
  | first=Harry Y. | last=McSween | date=1999
  | title=Meteorites and their parent planets
  | edition=2nd | publisher=Cambridge University Press
  | page=101 | isbn=0-521-58751-4 }}</ref>

== References ==
{{reflist|2
| refs =
<ref name="SBDB">{{cite web
  |title=JPL Small-Body Database Search Engine: spec. type <nowiki>=</nowiki> P (Tholen) 
  |publisher=JPL Solar System Dynamics
  |url=http://ssd.jpl.nasa.gov/sbdb_query.cgi?obj_group=all;obj_kind=all;obj_numbered=all;OBJ_field=0;ORB_field=0;c1_group=OBJ;c1_item=Ay;c1_op=%3D;c1_value=P;table_format=HTML;max_rows=100;format_option=comp;c_fields=AcBhBgBjBiBnBsAiArApAxAy;.cgifields=format_option;.cgifields=obj_kind;.cgifields=obj_group;.cgifields=obj_numbered;.cgifields=ast_orbit_class;.cgifields=table_format;.cgifields=com_orbit_class&query=1&c_sort=AyA
  |access-date=2015-06-17}}</ref>

}}

==See also==
*[[Asteroid spectral types]]

{{Asteroids}}
{{Small Solar System bodies}}

{{DEFAULTSORT:P-Type Asteroid}}
[[Category:Asteroid spectral classes]]
[[Category:P-type asteroids (Tholen)|*]]