Editing Martian meteorite (section)

==Origin==
The majority of SNC meteorites are quite young compared to most other meteorites and seem to imply that [[volcano|volcanic]] activity was present on Mars only a few hundred million years ago. The young formation ages of Martian meteorites was one of the early recognized characteristics that suggested their origin from a planetary body such as Mars. Among Martian meteorites, only ALH 84001 and NWA 7034 have radiometric ages older than about 1400&nbsp;Ma (Ma&nbsp;= million years). All nakhlites, as well as Chassigny and NWA 2737, give similar if not identical formation ages around 1300&nbsp;Ma, as determined by various radiometric dating techniques.<ref name="Nyquist 2001 105–164">{{cite journal|last=Nyquist|first=L.E.|display-authors=etal|title=Ages and geologic histories of martian meteorites|journal=Space Science Reviews|date=2001|volume=96|pages=105–164|bibcode = 2001SSRv...96..105N |doi=10.1023/A:1011993105172|citeseerx=10.1.1.117.1954|s2cid=10850454}}</ref><ref>{{cite journal|last=Park|first=J.|display-authors=etal|title=39Ar-40Ar ages of martian nakhlites|journal=Geochim. Cosmochim. Acta|date=2009|volume=73|issue=7|pages=2177–2189|bibcode = 2009GeCoA..73.2177P |doi=10.1016/j.gca.2008.12.027|url=https://zenodo.org/record/1258979}}</ref> Formation ages determined for many shergottites are variable and much younger, mostly ~150–575&nbsp;Ma.<ref name="Nyquist 2001 105–164"/><ref>{{cite journal|last=Borg|first=L.E.|display-authors=etal|title=Constraints on the U-Pbisotopic systematics of Mars inferred from a combined U-Pb, Rb-Sr, and Sm-Nd isotopic study of the martian meteorite Zagami|journal=Geochim. Cosmochim. Acta|date=2005|volume=69|issue=24|pages=5819–5830|bibcode=2005GeCoA..69.5819B|doi=10.1016/j.gca.2005.08.007}}</ref><ref>{{cite journal|last=Shih|first=C-Y|display-authors=etal|title=Rb-Sr and Sm-Nd dating of olivine-phyric shergottite Yamato 980459: Petrogenesis of depleted shergottites|journal=Antarctic Meteorite Research|date=2005|volume=18|pages=46–65|bibcode= 2005AMR....18...46S }}</ref><ref>{{cite journal|last=Nyquist|first=L.E.|display-authors=etal|title=Concordant Rb-Sr, Sm-Nd, and Ar-Ar ages for Northwest Africa 1460: A 446 Ma old basaltic shergottite related to "lherzolitic" shergottites|journal=Geochim. Cosmochim. Acta|date=2009|volume=73|issue=14|pages=4288–4309|bibcode=2009GeCoA..73.4288N|doi=10.1016/j.gca.2009.04.008|url=https://zenodo.org/record/1258981}}</ref>

The chronological history of shergottites is not totally understood, and a few scientists have suggested that some may actually have formed prior to the times given by their radiometric ages,<ref>{{cite journal|last=Bouvier|first=A.|display-authors=etal|title= The case for old basaltic shergottites|journal=Earth Planet. Sci. Lett.|date=2008|volume=266|issue=1–2|pages=105–124|bibcode= 2008E&PSL.266..105B|doi=10.1016/j.epsl.2007.11.006}}</ref> a suggestion not accepted by most scientists. Formation ages of SNC meteorites are often linked to their cosmic-ray exposure (CRE) ages, as measured from the nuclear products of interactions of the meteorite in space with energetic [[cosmic ray]] particles. Thus, all measured nakhlites give essentially identical CRE ages of approximately 11 Ma, which when combined with their possible identical formation ages indicates ejection of nakhlites into space from a single location on Mars by a single impact event.<ref name="Nyquist 2001 105–164" /> Some of the shergottites also seem to form distinct groups according to their CRE ages and formation ages, again indicating ejection of several different shergottites from Mars by a single impact. However, CRE ages of shergottites vary considerably (~0.5–19&nbsp;Ma),<ref name="Nyquist 2001 105–164" /> and several impact events are required to eject all the known shergottites. It had been asserted that there are no large young craters on Mars that are candidates as sources for the Martian meteorites, but subsequent studies claimed to have a likely source for [[Allan Hills 84001|ALH 84001]],<ref>{{Cite news |url=https://www.newscientist.com/article.ns?id=dn8004 |title=Birthplace of famous Mars meteorite pinpointed |date=September 16, 2005 |work=New Scientist |first=David L. |last=Chandler |accessdate=September 8, 2006 |url-status=dead |archiveurl=https://web.archive.org/web/20060113195930/https://www.newscientist.com/article.ns/?id=dn8004 |archivedate=2006-01-13}}</ref> and a possible source for other shergottites.<ref>{{cite journal|author=McEwen, A.S.|url=http://www.mars.asu.edu/christensen/classdocs/mcewen_zunil_Icarus_2005.pdf|title=The rayed crater Zunil and interpretations of small impact craters on Mars|journal=[[Icarus (journal)|Icarus]]|volume=176|issue=2|pages=351–381|date=2005|access-date=2006-09-08|doi= 10.1016/j.icarus.2005.02.009|bibcode=2005Icar..176..351M|last2=Preblich|first2=B|last3=Turtle|first3=E|last4=Artemieva|first4=N|author4-link=Natalia Artemieva|last5=Golombek|first5=M|last6=Hurst|first6=M|last7=Kirk|first7=R|last8=Burr|first8=D|last9=Christensen|first9=P}}.</ref>

In a 2014 paper, several researchers claimed that all shergottites meteorites come from the [[Mojave (crater)|Mojave Crater]] on Mars.<ref name="Werner2014"/>

===Age estimates based on cosmic ray exposure===
[[File:Martian meteorite crafted into a small pendant.jpg|thumb|right|A Martian meteorite crafted into a small pendant and suspended from a gold necklace.]]
The amount of time spent in transit from Mars to Earth can be estimated by measurements of the effect of cosmic radiation on the meteorites, particularly on isotope ratios of [[noble gas]]es. The meteorites cluster in families that seem to correspond to distinct impact events on Mars. It is thought that the meteorites all originate in relatively few impacts every few million years on  Mars. The impactors would be kilometers in diameter and the craters they form on Mars tens of kilometers in diameter. Models of impacts on Mars are consistent with these findings.<ref name="Eugster">O. Eugster, G. F. Herzog, K. Marti, M. W. Caffee [http://www.lpi.usra.edu/books/MESSII/9004.pdf Irradiation Records, Cosmic-Ray Exposure Ages, and Transfer Times of Meteorites, see section 4.5 Martian Meteorites] LPI, 2006</ref>

Ages since impact determined so far include<ref>L.E. NYQUIST, D.D. BOGARD1, C.-Y. SHIH, A. GRESHAKE, D. STÖFFLER [http://seismo.berkeley.edu/~manga/nyquist.pdf AGES AND GEOLOGIC HISTORIES OF MARTIAN METEORITES] 2001</ref><ref>Tony Irving [http://www.imca.cc/mars/martian-meteorites.htm Martian Meteorites – has graphs of ejection ages] – site maintained by Tony Irving for up to date information on Martian meteorites</ref>
{| class="wikitable"
|-
! Type !! Age ([[Year#mya|mya]])
|-
| Dhofar 019, olivine-phyric shergottite|| 19.8 ± 2.3<ref name=Eugster/>
|-
|  ALH 84001, orthopyroxenite||15.0 ± 0.8<ref name=Eugster/>
|-
| Dunite (Chassigny) || 11.1 ± 1.6<ref name=Eugster/>
|-
| Six nakhlites || 10.8 ± 0.8<ref name=":0" /><ref name=Eugster/>
|-
| Lherzolites  || 3.8–4.7<ref name=Eugster/>
|-
| Six basaltic shergottites || 2.4–3.0<ref name=Eugster/>
|-
| Five olivine-phyric shergottites || 1.2 ± 0.1<ref name=Eugster/>
|- 
| EET 79001 || 0.73 ± 0.15<ref name=Eugster/>
|}