Editing Moon (section)

====Impact craters====
{{Further |List of craters on the Moon}}
[[File:Daedalus crater AS11-41-6151.jpg|alt=A gray, many-ridged surface from high above. The largest feature is a circular ringed structure with high walled sides and a lower central peak: the entire surface out to the horizon is filled with similar structures that are smaller and overlapping.|thumb|A view of a three-kilometer-deep larger crater [[Daedalus (crater)|Daedalus]] on the [[Far side of the Moon|Moon's far side]]]]
A major geologic process that has affected the Moon's surface is [[impact crater]]ing,<ref>{{cite book |last=Melosh |first=H. J. |title=Impact cratering: A geologic process |date=1989 |publisher=[[Oxford University Press]] |isbn=978-0-19-504284-9}}</ref> with craters formed when asteroids and comets collide with the lunar surface. There are estimated to be roughly 300,000 craters wider than {{Convert |1 |km |4=1 |abbr=on}} on the Moon's near side.<ref>{{cite web |title=Moon Facts |url=http://planck.esa.int/science-e/www/object/index.cfm?fobjectid=31412 |work=SMART-1 |publisher=[[European Space Agency]] |date=2010 |access-date=May 12, 2010 |archive-date=March 17, 2012 |archive-url=https://web.archive.org/web/20120317004513/http://planck.esa.int/science-e/www/object/index.cfm?fobjectid=31412 |url-status=dead}}</ref> Lunar craters exhibit a variety of forms, depending on their size. In order of increasing diameter, the basic types are simple craters with smooth bowl shaped interiors and upturned rims, [[complex crater]]s with flat floors, terraced walls and central peaks, [[peak ring]] basins, and [[multi-ring basin]]s with two or more concentric rings of peaks.<ref>[https://www.lpi.usra.edu/exploration/education/hsResearch/moon_101/ImpactCratering.pdf Impact Cratering Notes (LPI)]</ref> The vast majority of impact craters are circular, but some, like [[Cantor (crater)|Cantor]] and [[Janssen (lunar crater)|Janssen]], have more polygonal outlines, possibly guided by underlying faults and joints. Others, such as the [[Messier (crater)|Messier]] pair, [[Schiller (crater)|Schiller]], and [[Daniell (crater)|Daniell]], are elongated. Such elongation can result from highly oblique impacts, [[binary asteroid]] impacts, fragmentation of impactors before surface strike, or closely spaced [[secondary crater|secondary]] impacts.<ref>{{cite journal |last1=Herrick |first1=R.R. |last2=Forsberg-Taylor |first2=N. K. |year=2003 |title=The shape and appearance of craters formed by oblique impact on the Moon and Venus |journal=Meteoritics & Planetary Science |volume=38 |issue=11 |pages=1551–1578 |doi=10.1111/j.1945-5100.2003.tb00001.x |bibcode=2003M&PS...38.1551H |url=https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/j.1945-5100.2003.tb00001.x}}</ref>

The [[lunar geologic timescale]] is based on the most prominent impact events, such as multi-ring formations like [[Nectarian|Nectaris]], [[Lower Imbrian|Imbrium]], and [[Mare Orientale|Orientale]] that are between hundreds and thousands of kilometers in diameter and associated with a broad apron of ejecta deposits that form a regional [[stratigraphy|stratigraphic horizon]].<ref name="geologic" /> The lack of an atmosphere, weather, and recent geological processes mean that many of these craters are well-preserved. Although only a few [[multi-ring basins]] have been definitively dated, they are useful for assigning relative ages. Because impact craters accumulate at a nearly constant rate, counting the number of craters per unit area can be used to estimate the age of the surface.<ref name="geologic" />
However care needs to be exercised with the [[crater counting]] technique due to the potential presence of [[secondary crater]]s. Ejecta from impacts can create secondary craters that often appear in clusters or chains but can also occur as isolated formations at a considerable distance from the impact. These can resemble primary craters, and may even dominate small crater populations, so their unidentified presence can distort age estimates.<ref>{{cite journal |last1=Xiao |first1=Z. |last2=Strom |first2=R.G. |year=2012 |title=Problems determining relative and absolute ages using the small crater population |journal=Icarus |volume=220 |issue=1 |pages=254–267 |doi=10.1016/j.icarus.2012.05.012 |bibcode=2012Icar..220..254X |url=https://www.uni-muenster.de/imperia/md/content/planetology/lectures/ss2015/143897-hottopics/xiao_and_strom_2012.pdf}}</ref>

The radiometric ages of impact-melted rocks collected during the [[Apollo missions]] cluster between 3.8 and 4.1&nbsp;billion years old: this has been used to propose a [[Late Heavy Bombardment]] period of increased impacts.<ref>{{cite journal |last1=Hartmann |first1=William K. |last2=Quantin |first2=Cathy |last3=Mangold |first3=Nicolas |date=2007 |volume=186 |issue=1 |pages=11–23 |journal=[[Icarus (journal)|Icarus]] |title=Possible long-term decline in impact rates: 2. Lunar impact-melt data regarding impact history |doi=10.1016/j.icarus.2006.09.009 |bibcode=2007Icar..186...11H}}</ref>

High-resolution images from the Lunar Reconnaissance Orbiter in the 2010s show a contemporary crater-production rate significantly higher than was previously estimated. A secondary cratering process caused by [[distal ejecta]] is thought to churn the top two centimeters of regolith on a timescale of 81,000 years.<ref>{{cite web |url=https://www.newscientist.com/article/2108929-the-moon-has-hundreds-more-craters-than-we-thought/ |title=The moon has hundreds more craters than we thought |first=Rebecca |last=Boyle |url-status=live |archive-url=https://web.archive.org/web/20161013143743/https://www.newscientist.com/article/2108929-the-moon-has-hundreds-more-craters-than-we-thought/ |archive-date=October 13, 2016}}</ref><ref>{{cite journal |title=Quantifying crater production and regolith overturn on the Moon with temporal imaging |first1=Emerson J. |last1=Speyerer |first2=Reinhold Z. |last2=Povilaitis |first3=Mark S. |last3=Robinson |first4=Peter C. |last4=Thomas |first5=Robert V. |last5=Wagner |date=October 13, 2016 |journal=[[Nature (journal)|Nature]] |volume=538 |issue=7624 |pages=215–218 |doi=10.1038/nature19829 |pmid=27734864 |bibcode=2016Natur.538..215S |s2cid=4443574}}</ref> This rate is 100 times faster than the rate computed from models based solely on direct micrometeorite impacts.<ref>{{cite web |title=Earth's Moon Hit by Surprising Number of Meteoroids |date=October 13, 2016 |publisher=NASA |url=https://www.nasa.gov/press-release/goddard/2016/lro-lunar-cratering |access-date=May 21, 2021 |archive-date=July 2, 2022 |archive-url=https://web.archive.org/web/20220702225136/https://www.nasa.gov/press-release/goddard/2016/lro-lunar-cratering/ |url-status=live}}</ref>