Editing Asteroid belt (section)

===Evolution===
[[File:Vesta from Dawn, July 18.jpg|right|thumb|Large main belt asteroid [[4 Vesta]]]]
The asteroids are not pristine samples of the primordial Solar System. They have undergone considerable evolution since their formation, including internal heating (in the first few tens of millions of years), surface melting from impacts, [[space weathering]] from radiation, and bombardment by [[micrometeorites]].<ref>{{cite book | chapter=Asteroid Space Weathering and Regolith Evolution | last1=Clark |first1=B. E. | last2=Hapke | first2=B. | last3=Pieters | first3=C. | last4=Britt | first4=D. | place=University of Arizona | year=2002 | bibcode=2002aste.book..585C|page= 585| title=Asteroids III|doi=10.2307/j.ctv1v7zdn4.44| isbn=978-0-8165-2281-1 }}</ref><ref>{{cite journal|title=The Spectral and Physical Properties of Metal in Meteorite Assemblages: Implications for Asteroid Surface Materials | last=Gaffey | first=Michael J. | year=1996 | bibcode=1986Icar...66..468G|volume=66|pages=468–486|journal=Icarus|issn=0019-1035|doi=10.1016/0019-1035(86)90086-2|issue=3}}</ref><ref>{{cite journal|title= Thermal alteration of asteroids: evidence from meteorites | last=Keil | first=K. | journal=Planetary and Space Science|url=http://www.ingentaconnect.com/content/els/00320633/2000/00000048/00000010/art00054 | year=2000 | volume=48 | issue=10 | pages=887–903 | doi=10.1016/S0032-0633(00)00054-4 | bibcode=2000P&SS...48..887K | access-date=2007-11-08}}</ref><ref>{{cite journal|title=Impact of ions and micrometeorites on mineral surfaces: Reflectance changes and production of atmospheric species in airless solar system bodies| last1=Baragiola | first1=R. A. | last2=Duke | first2=C. A. | last3=Loeffler | first3=M. | last4=McFadden | first4=L. A. | last5=Sheffield | first5=J. | year=2003 | bibcode=2003EAEJA.....7709B|page=7709|journal=EGS – AGU – EUG Joint Assembly}}</ref> Although some scientists refer to the asteroids as residual planetesimals,<ref>{{cite journal| last1=Chapman | first1=C. R. | last2=Williams | first2=J. G. | last3=Hartmann | first3=W. K. | title=The asteroids| year=1978
| journal=Annual Review of Astronomy and Astrophysics
| volume=16| pages=33–75
| doi=10.1146/annurev.aa.16.090178.000341
| bibcode=1978ARA&A..16...33C }}</ref> other scientists consider them distinct.<ref>{{cite web|title=Asteroid 433 Eros and partially differentiated planetesimals: bulk depletion versus surface depletion of sulfur| last=Kracher | first=A. | work=Ames Laboratory | year=2005 | url=http://www.cosis.net/abstracts/EGU05/03788/EGU05-J-03788.pdf|access-date= 2007-11-08| archive-url= https://web.archive.org/web/20071128200221/http://www.cosis.net/abstracts/EGU05/03788/EGU05-J-03788.pdf| archive-date= 28 November 2007 | url-status= live}}</ref>

The current asteroid belt is believed to contain only a small fraction of the mass of the primordial belt. Computer simulations suggest that the original asteroid belt may have contained mass equivalent to the Earth's.<ref>{{cite web| first=Robert | last=Piccioni | url=http://www.guidetothecosmos.com/newsletter-Habitable-Earth.html|title=Did Asteroid Impacts Make Earth Habitable?|publisher=Guidetothecosmos.com|date=2012-11-19|access-date=2013-05-03}}</ref> Primarily because of gravitational perturbations, most of the material was ejected from the belt within about 1&nbsp;million years of formation, leaving behind less than 0.1% of the original mass.<ref name="icarus153" /> Since its formation, the size distribution of the asteroid belt has remained relatively stable; no significant increase or decrease in the typical dimensions of the main-belt asteroids has occurred.<ref>{{Cite news | url=https://uanews.arizona.edu/story/asteroids-caused-early-inner-solar-system-cataclysm | title=Asteroids Caused the Early Inner Solar System Cataclysm | first=Lori | last=Stiles | date=September 15, 2005 | work=UANews | access-date=2018-10-18 | language=en }}</ref>

The 4:1 [[orbital resonance]] with Jupiter, at a radius 2.06&nbsp;[[astronomical unit]]s (AUs), can be considered the inner boundary of the asteroid belt. Perturbations by Jupiter send bodies straying there into unstable orbits. Most bodies formed within the radius of this gap were swept up by Mars (which has an [[aphelion]] at 1.67&nbsp;AU) or ejected by its gravitational perturbations in the early history of the Solar System.<ref>{{cite web
| last1=Alfvén | first1=H.| last2=Arrhenius | first2=G. | year=1976
| url =https://history.nasa.gov/SP-345/ch4.htm
| title =The Small Bodies
| work=SP-345 Evolution of the Solar System
| publisher = NASA
| access-date = 2007-04-12| archive-url= https://web.archive.org/web/20070513081833/https://history.nasa.gov/SP-345/ch4.htm| archive-date= 13 May 2007 | url-status= live}}</ref> The [[Hungaria asteroids]] lie closer to the Sun than the 4:1 resonance, but are protected from disruption by their high inclination.<ref name = hungaria>{{cite journal| title=The Hungaria group of minor planets
| last=Spratt| first=Christopher E.
| journal=Journal of the Royal Astronomical Society of Canada
| volume=84| pages=123–131|date=April 1990
| bibcode=1990JRASC..84..123S }}</ref>

When the asteroid belt was first formed, the temperatures at a distance of 2.7&nbsp;AU from the Sun formed a "[[Frost line (astrophysics)|snow line]]" below the freezing point of water. Planetesimals formed beyond this radius were able to accumulate ice.<ref>{{cite journal
| last1=Lecar | first1=M. | last2=Podolak | first2=M. | last3=Sasselov | first3=D. | last4=Chiang | first4=E.| title=Infrared cirrus&nbsp;– New components of the extended infrared emission
| journal=The Astrophysical Journal
| year=2006
| volume=640
| issue=2
| pages=1115–1118
| bibcode=2006ApJ...640.1115L
| doi=10.1086/500287|arxiv = astro-ph/0602217 |s2cid=18778001}}</ref><ref>{{cite news
| first=Phil
| last=Berardelli
| title=Main-Belt Comets May Have Been Source Of Earths Water
| publisher=Space Daily
| date=March 23, 2006
| url=http://www.spacedaily.com/reports/Main_Belt_Comets_May_Have_Been_Source_Of_Earths_Water.html| access-date=2007-10-27 <!--DASHBot-->}}</ref>
In 2006, a population of [[Main-belt comet|comets]] had been discovered within the asteroid belt beyond the snow line, which may have provided a source of water for Earth's oceans. According to some models, [[outgassing]] of water during the Earth's formative period was insufficient to form the oceans, requiring an external source such as a cometary bombardment.<ref>{{cite web
|last=Lakdawalla
|first=Emily
|date=April 28, 2006
|url=http://www.planetary.org/blog/article/00000551/
|title=Discovery of a Whole New Type of Comet
|publisher=The Planetary Society
|access-date=2007-04-20
|archive-url=https://web.archive.org/web/20070501211319/http://www.planetary.org/blog/article/00000551/
|archive-date=1 May 2007
|url-status=dead
}}</ref>

The outer asteroid belt includes a few icy objects that may have been implanted there during the last few hundred years. One of these objects is the [[quasi-Hilda comet]] 362P/{{mpl|(457175) 2008 GO|98}}, which is thought to be a possible former [[centaur (small Solar System body)|centaur]] that was sent to the outer asteroid belt via a close encounter with Jupiter.<ref name="new">{{cite journal | last2=de la Fuente Marcos |first2=Raúl |last1=de la Fuente Marcos |first1=Carlos |date=1 October 2022 |title=Recent arrivals to the main asteroid belt |journal=[[Celestial Mechanics and Dynamical Astronomy]] |volume=134 |issue=5 | pages=38 |arxiv=2207.07013 |bibcode=2022CeMDA.134...38D |doi=10.1007/s10569-022-10094-4 | doi-access=free }}</ref>