Editing Impact event (section)

=== Biospheric effects ===
The effect of impact events on the biosphere has been the subject of scientific debate. Several theories of impact-related mass extinction have been developed. In the past 500 million years there have been five generally accepted <!-- there is some scientific disagreement over the number, so I have inserted the qualifier of common or "general acceptance" --> major mass extinctions that on average extinguished half of all [[species]].<ref name="Keller"/> One of the largest mass extinctions to have affected [[life|life on Earth]] was the [[Permian-Triassic extinction event|Permian-Triassic]], which ended the [[Permian]] period 250 million years ago and killed off 90 percent of all species;<ref>{{Cite web|url=https://math.ucr.edu/home/baez/extinction/|title=extinction|website=math.ucr.edu}}</ref> life on Earth took 30 million years to recover.<ref name="SahneyBenton2008RecoveryFromProfoundExtinction">{{Citation |url= |author1=Sahney, S. |author2=Benton, M.J. |date=2008 |title=Recovery from the most profound mass extinction of all time |pmid=18198148 |journal=Proceedings of the Royal Society B: Biological Sciences |pmc=2596898 |doi=10.1098/rspb.2007.1370 |volume=275 |issue=1636 |pages=759–765 }}</ref> The cause of the Permian-Triassic extinction is still a matter of debate; the age and origin of proposed impact craters, i.e. the [[Bedout]] High structure, hypothesized to be associated with it are still controversial.<ref name="Muller2005">{{cite journal | last1 = Müller | first1 = R.D. | last2 = Goncharov | first2 = A. | last3 = Kristi | first3 = A. | year = 2005 | title = Geophysical evaluation of the enigmatic Bedout basement high, offshore northwest Australia | journal = Earth and Planetary Science Letters | volume = 237 | issue = 1–2| pages = 265–284 | doi=10.1016/j.epsl.2005.06.014 | bibcode=2005E&PSL.237..264M}}</ref> The [[Cretaceous–Paleogene extinction event|last]] such mass extinction led to the demise of the non-avian [[dinosaur]]s and coincided with a large [[meteorite]] impact; this is the Cretaceous–Paleogene extinction event (also known as the K–T or K–Pg extinction event), which occurred 66 million years ago. There is no definitive evidence of impacts leading to the three other major mass extinctions.

In 1980, physicist [[Luis Walter Alvarez|Luis Alvarez]]; his son, geologist [[Walter Alvarez]]; and nuclear chemists Frank Asaro and Helen V. Michael from the [[University of California, Berkeley]] discovered unusually high concentrations of [[iridium]] in a specific layer of rock [[stratum|strata]] in the Earth's crust. Iridium is an element that is rare on Earth but relatively abundant in many meteorites. From the amount and distribution of iridium present in the 65-million-year-old "iridium layer", the Alvarez team later estimated that an asteroid of {{convert|10|to|14|km|mi|0|abbr=on}} must have collided with Earth. This iridium layer at the [[Cretaceous–Paleogene boundary]] has been found worldwide at 100 different sites. Multidirectionally [[shocked quartz]] (coesite), which is normally associated with large impact events<ref name=Fulgurite>{{cite journal|last1=Carter|first1=Elizabeth|last2=Pasek|first2=Matthew|last3=Smith|first3=Tim|last4=Kee|first4=Terence|last5=Hines|first5=Peter|last6=Howell|first6=G. M. Edwards|title=Rapid Raman mapping of a fulgurite (Paywall)|journal=Analytical and Bioanalytical Chemistry|date=August 2010|volume=397|issue=7|pages=2647–2658|doi=10.1007/s00216-010-3593-z|pmid=20229006|s2cid=23476732}}</ref> or [[atomic bomb]] explosions, has also been found in the same layer at more than 30 sites. [[Soot]] and [[wikt:ash|ash]] at levels tens of thousands times normal levels were found with the above.

Anomalies in chromium isotopic ratios found within the [[K-T boundary]] layer strongly support the impact theory.<ref name="Shukolyukov1998">{{Citation | last1 = Shukolyukov | first1 = A. | last2 = Lugmair | first2 = G. W. | date = 1998 | title = Isotopic Evidence for the Cretaceous-Tertiary Impactor and Its Type | journal = Science | volume = 282 | issue = 5390| pages = 927–930 | doi = 10.1126/science.282.5390.927 | postscript = . | pmid=9794759 | bibcode=1998Sci...282..927S}}</ref> Chromium isotopic ratios are homogeneous within the earth, and therefore these isotopic anomalies exclude a volcanic origin, which has also been proposed as a cause for the iridium enrichment. Further, the chromium isotopic ratios measured in the K-T boundary are similar to the chromium isotopic ratios found in [[carbonaceous chondrite]]s. Thus a probable candidate for the impactor is a carbonaceous asteroid, but a comet is also possible because comets are assumed to consist of material similar to carbonaceous chondrites.

Probably the most convincing evidence for a worldwide catastrophe was the discovery of the crater which has since been named [[Chicxulub Crater]]. This crater is centered on the Yucatán Peninsula of Mexico and was discovered by Tony Camargo and [[Glen Penfield]] while working as [[geophysicist]]s for the Mexican oil company [[PEMEX]].<ref>{{cite web|last=Penfield|first=December 2019 Glen|date=2019-12-01|title=Unlikely Impact|url=https://explorer.aapg.org/story/articleid/55293/unlikely-impact|access-date=2020-08-17|website=AAPG Explorer|language=en-US}}</ref> What they reported as a circular feature later turned out to be a crater estimated to be {{convert|180|km|mi|-1|abbr=on}} in diameter. This convinced the vast majority of scientists that this extinction resulted from a point event that is most probably an extraterrestrial impact and not from increased volcanism and climate change (which would spread its main effect over a much longer time period).

Although there is now general agreement that there was a huge impact at the end of the Cretaceous that led to the iridium enrichment of the K-T boundary layer, remnants have been found of other, smaller impacts, some nearing half the size of the Chicxulub crater, which did not result in any mass extinctions, and there is no clear linkage between an impact and any other incident of mass extinction.<ref name="Keller">{{cite journal | url=http://instruct.uwo.ca/earth-sci/083f/kellerkt.pdf | title=Impacts, volcanism and mass extinction: random coincidence or cause and effect? | author=Keller G. | journal=Australian Journal of Earth Sciences | date=2005 | volume=52 | issue=4–5 | pages=725–757 | doi=10.1080/08120090500170393|bibcode = 2005AuJES..52..725K | s2cid=39063747 }}</ref>

Paleontologists [[David M. Raup]] and [[Jack Sepkoski]] have proposed that an excess of extinction events occurs roughly every 26 million years (though many are relatively minor). This led physicist [[Richard A. Muller]] to suggest that these extinctions could be due to a hypothetical companion star to the Sun called [[Nemesis (hypothetical star)|Nemesis]] periodically disrupting the orbits of comets in the [[Oort cloud]], leading to a large increase in the number of comets reaching the inner Solar System where they might hit Earth. Physicist [[Adrian Melott]] and paleontologist [[Richard Bambach]] have more recently verified the Raup and Sepkoski finding, but argue that it is not consistent with the characteristics expected of a Nemesis-style periodicity.<ref>{{Citation|author = Adrian L. Melott & Richard K. Bambach|title = Nemesis Reconsidered|date=2010|journal = [[Monthly Notices of the Royal Astronomical Society Letters]]|volume =407|issue = 1|pages=L99–L102|arxiv = 1007.0437 |bibcode = 2010MNRAS.407L..99M |doi = 10.1111/j.1745-3933.2010.00913.x |last2 = Bambach| doi-access=free |s2cid = 7911150}}</ref>