Editing Asteroid belt (section)

=={{anchor|Origin}}Origin==
[[Image:Main belt i vs a.png|thumb|300 px|right|The asteroid belt showing the orbital inclinations versus distances from the Sun, with asteroids in the core region of the asteroid belt in red and other asteroids in blue]]

===Formation===
In 1802, shortly after discovering Pallas, Olbers suggested to Herschel and [[Carl Gauss]] that Ceres and Pallas were [[Phaeton (hypothetical planet)|fragments of a much larger planet]] that once occupied the Mars–Jupiter region, with this planet having suffered an internal explosion or a cometary impact many million years before,<ref>{{cite book|title=Investigating the Origin of the Asteroids and Early Findings on Vesta|series=Historical Studies in Asteroid Research|last1=Cunningham|first1=Clifford J.|year=2017|publisher=Springer International Publishing|doi=10.1007/978-3-319-58118-7|page=1|isbn=978-3-319-58117-0 }}</ref> while [[Odesa]]n astronomer K. N. Savchenko suggested that Ceres, Pallas, Juno, and Vesta were escaped moons rather than fragments of the exploded planet.<ref name="originoftheasteroids">{{cite web | last=Bronshten | first=V. A. | title=Origin of the Asteroids |year=1972 |url=https://archive.org/details/nasa_techdoc_19720017206|publisher = NASA}}</ref> The large amount of energy required to destroy a planet, combined with the belt's low combined mass, which is only about 4% of the mass of  Earth's Moon,<ref name="Krasinskyetal2002" /> does not support these hypotheses. Further, the significant chemical differences between the asteroids become difficult to explain if they come from the same planet.<ref>{{cite web | last1=Masetti | first1=M. | last2=Mukai | first2=K. | date=December 1, 2005| url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/980810a.html| title=Origin of the Asteroid Belt| publisher=NASA Goddard Spaceflight Center| access-date=2007-04-25}}</ref>

A modern hypothesis for the asteroid belt's creation relates to how, in general for the Solar System, [[planetary formation]] is thought to have occurred via a process comparable to the long-standing [[nebular hypothesis]]; a cloud of [[Cosmic dust|interstellar dust]] and gas collapsed under the influence of gravity to form a rotating disc of material that then conglomerated to form the Sun and planets.<ref>{{cite web
| last = Watanabe
| first = Susan
| date = July 20, 2001
| url = http://www.jpl.nasa.gov/news/features.cfm?feature=520
| title = Mysteries of the Solar Nebula
| publisher = NASA
| access-date = 2007-04-02
| archive-date = 2012-01-17
| archive-url = https://web.archive.org/web/20120117093701/http://www.jpl.nasa.gov/news/features.cfm?feature=520
| url-status = dead
}}</ref> During the first few million years of the Solar System's history, an accretion process of sticky collisions caused the clumping of small particles, which gradually increased in size. Once the clumps reached sufficient mass, they could draw in other bodies through gravitational attraction and become planetesimals. This gravitational [[Accretion (astrophysics)|accretion]] led to the formation of the planets.<ref name="Chambers 2004">{{cite journal |title=Planetary accretion in the inner Solar System |journal=Earth and Planetary Science Letters |first=John E. |last=Chambers |volume=233 |issue=3–4 |pages=241–252 |date=July 2004 |doi=10.1016/j.epsl.2004.04.031 |bibcode=2004E&PSL.223..241C}}</ref>

Planetesimals within the region that would become the asteroid belt were strongly [[Perturbation (astronomy)|perturbed]] by Jupiter's gravity.<ref name="icarus153"/> [[Orbital resonance]]s occurred where the [[orbital period]] of an object in the belt formed an integer fraction of the orbital period of Jupiter, perturbing the object into a different orbit; the region lying between the orbits of Mars and Jupiter contains many such orbital resonances. As [[Nice model|Jupiter migrated inward]] following its formation, these resonances would have swept across the asteroid belt, dynamically exciting the region's population and increasing their velocities relative to each other.<ref>{{cite conference
| first = E. R. D.
| last = Scott
| title=Constraints on Jupiter's Age and Formation Mechanism and the Nebula Lifetime from Chondrites and Asteroids
| book-title = Proceedings 37th Annual Lunar and Planetary Science Conference
| publisher = Lunar and Planetary Society
| date = March 13–17, 2006
| location = League City, Texas
| bibcode =2006LPI....37.2367S
}}</ref>  In regions where the average velocity of the collisions was too high, the shattering of planetesimals tended to dominate over accretion,<ref>{{cite journal
| last1=Edgar | first1=R.| last2=Artymowicz | first2=P. | title=Pumping of a Planetesimal Disc by a Rapidly Migrating Planet
| journal=Monthly Notices of the Royal Astronomical Society
| year=2004
| volume=354
| issue=3
| pages=769–772
| doi = 10.1111/j.1365-2966.2004.08238.x
| doi-access=free| bibcode=2004MNRAS.354..769E
|arxiv = astro-ph/0409017 |s2cid=18355985
}}</ref> preventing the formation of a planet. Instead, they continued to orbit the Sun as before, occasionally colliding.<ref name="icarus153">{{cite journal
| last1=Petit | first1=J.-M. | last2=Morbidelli | first2=A. | last3=Chambers | first3=J. 
| title=The Primordial Excitation and Clearing of the Asteroid Belt
| journal=Icarus
| year=2001
| volume=153
| issue=2
| pages=338–347
| url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf
| access-date=2007-03-22| doi = 10.1006/icar.2001.6702
| bibcode=2001Icar..153..338P
| archive-url= https://web.archive.org/web/20070221085835/http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf
| archive-date= 21 February 2007 | url-status= live}}</ref>

During the early history of the Solar System, the asteroids melted to some degree, allowing elements within them to be [[Planetary differentiation|differentiated]] by mass. Some of the progenitor bodies may even have undergone periods of explosive [[volcanism]] and formed [[magma]] oceans. Because of the relatively small size of the bodies, though, the period of melting was necessarily brief compared to the much larger planets, and had generally ended about 4.5&nbsp;billion years ago, in the first tens of millions of years of formation.<ref>{{cite journal
| last1=Taylor | first1=G. J. | last2=Keil | first2=K. | last3=McCoy | first3=T. | last4=Haack | first4=H. | last5=Scott | first5=E. R. D.
| title=Asteroid differentiation&nbsp;– Pyroclastic volcanism to magma oceans
| journal=Meteoritics
| year=1993
| volume=28
| issue=1
| pages=34–52
| bibcode=1993Metic..28...34T
| doi=10.1111/j.1945-5100.1993.tb00247.x}}</ref> In August 2007, a study of [[zircon]] crystals in an Antarctic meteorite believed to have originated from Vesta suggested that it, and by extension the rest of the asteroid belt, had formed rather quickly, within 10&nbsp;million years of the Solar System's origin.<ref>{{cite web|title=U of T researchers discover clues to early solar system | last=Kelly | first=Karen | year=2007|work=University of Toronto|url=http://webapps.utsc.utoronto.ca/ose/story.php?id=665|access-date=2010-07-12|archive-url=https://web.archive.org/web/20120229160619/http://webapps.utsc.utoronto.ca/ose/story.php?id=665|archive-date=2012-02-29|url-status=dead}}</ref>

===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>