Editing Mercury (planet) (section)

==== Impact basins and craters ====
[[File:PIA19421-Mercury-Craters-MunchSanderPoe-20150416.jpg|thumb|left|{{anchor|Munch|Sander|Poe}}Enhanced-color image of craters [[Munch (crater)|Munch]] (left), [[Sander (crater)|Sander]] (center), and [[Poe (crater)|Poe]] (right) amid volcanic plains (orange) near [[Caloris Basin]]]]

Mercury was heavily bombarded by comets and [[asteroid]]s during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the [[Late Heavy Bombardment]] that ended 3.8 billion years ago.<ref>{{cite journal |last=Strom |first=Robert G. |year=1979 |volume=24 |issue=1 |title=Mercury: a post-Mariner assessment |journal=Space Science Reviews |pages=3–70 |bibcode=1979SSRv...24....3S |doi=10.1007/BF00221842 |s2cid=122563809 }}</ref> Mercury received impacts over its entire surface during this period of intense crater formation,<ref name="DunneCh7" /> facilitated by the lack of any [[atmosphere]] to slow impactors down.<ref>{{cite journal |last1=Broadfoot |first1=A. Lyle |first2=Shailendra |last2=Kumar |first3=Michael J. S. |last3=Belton |author-link3=Michael J. Belton |first4=Michael B. |last4=McElroy |author-link4=Michael McElroy (scientist) |title=Mercury's Atmosphere from Mariner&nbsp;10: Preliminary Results |journal=Science |volume=185 |issue=4146 |date=July 12, 1974 |pages=166–169 |doi=10.1126/science.185.4146.166 |pmid=17810510 |bibcode=1974Sci...185..166B|s2cid=7790470 }}</ref> During this time Mercury was [[volcano|volcanically]] active; basins were filled by [[magma]], producing smooth plains similar to the maria found on the Moon.<ref>{{cite book | date=1997 | doi=10.3133/i2596 | title=Geology of the solar system | series=IMAP 2596 | publisher=U.S. Geological Survey }}</ref><ref>{{cite journal |last1=Head |first1=James W. |author-link1=James W. Head |last2=Solomon |first2=Sean C. |author-link2=Sean Solomon |title=Tectonic Evolution of the Terrestrial Planets |journal=Science |year=1981 |volume=213 |issue=4503 |pages=62–76 |doi=10.1126/science.213.4503.62 |pmid=17741171 |bibcode=1981Sci...213...62H |hdl=2060/20020090713 |url=http://www.planetary.brown.edu/pdfs/323.pdf |citeseerx=10.1.1.715.4402 |access-date=October 25, 2017 |archive-date=July 21, 2018 |archive-url=https://web.archive.org/web/20180721153426/http://www.planetary.brown.edu/pdfs/323.pdf |url-status=dead }}</ref> One of the most unusual craters is [[Apollodorus (crater)|Apollodorus]], or "the Spider", which hosts a series of radiating troughs extending outwards from its impact site.<ref>{{cite web |title=Scientists see Mercury in a new light |url=https://www.sciencedaily.com/releases/2008/02/080201093149.htm |website=Science Daily |date=February 28, 2008 |access-date=April 7, 2008 |archive-date=December 5, 2020 |archive-url=https://web.archive.org/web/20201205202019/https://www.sciencedaily.com/releases/2008/02/080201093149.htm |url-status=live }}</ref>

[[Craters on Mercury]] range in diameter from small bowl-shaped cavities to [[multi-ringed impact basin]]s hundreds of kilometers across. They appear in all states of degradation, from relatively fresh rayed craters to highly degraded crater remnants. Mercurian craters differ subtly from lunar craters in that the area blanketed by their ejecta is much smaller, a consequence of Mercury's stronger surface gravity.<ref name="Spudis01">{{cite journal |first=Paul D. |last=Spudis |author-link=Paul Spudis |title=The Geological History of Mercury |journal=Workshop on Mercury: Space Environment, Surface, and Interior, Chicago |issue=1097 |year=2001 |page=100 |bibcode=2001mses.conf..100S}}</ref> According to [[International Astronomical Union]] rules, each new crater must be named after an artist who was famous for more than fifty years, and dead for more than three years, before the date the crater is named.<ref name="Ritzel" />

{{multiple image |direction=horizontal |align=right |total_width=400
|image1=The Mighty Caloris (PIA19213).png | caption1=Overhead view of Caloris Basin
|image2=PIA19450-PlanetMercury-CalorisBasin-20150501.jpg | caption2=Perspective view of Caloris Basin – high (red); low (blue)
}}

The largest known crater is [[Caloris Planitia]], or Caloris Basin, with a diameter of {{convert|1550|km|mi|abbr=on}}.<ref name="newscientist30012008">{{cite news |url=https://www.newscientist.com/article/dn13257-bizarre-spider-scar-found-on-mercurys-surface.html |title=Bizarre spider scar found on Mercury's surface |date=January 30, 2008 |publisher=NewScientist.com news service |first=David |last=Shiga |access-date=September 4, 2017 |archive-date=December 10, 2014 |archive-url=https://web.archive.org/web/20141210213025/http://www.newscientist.com/article/dn13257-bizarre-spider-scar-found-on-mercurys-surface.html |url-status=live }}</ref> The impact that created the Caloris Basin was so powerful that it caused [[lava]] eruptions and left a concentric mountainous ring ~{{convert|2|km|mi|abbr=on}} tall surrounding the [[impact crater]]. The floor of the Caloris Basin is filled by a geologically distinct flat plain, broken up by ridges and fractures in a roughly polygonal pattern. It is not clear whether they were volcanic lava flows induced by the impact or a large sheet of impact melt.<ref name="Spudis01" />

At the [[antipodes|antipode]] of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". One hypothesis for its origin is that shock waves generated during the Caloris impact traveled around Mercury, converging at the basin's antipode (180 degrees away). The resulting high stresses fractured the surface.<ref>{{cite journal |last1=Schultz |first1=Peter H. |author-link1=Peter H. Schultz |last2=Gault |first2=Donald E. |year=1975 |title=Seismic effects from major basin formations on the moon and Mercury |journal=Earth, Moon, and Planets |volume=12 |issue=2 |pages=159–175 |doi=10.1007/BF00577875 |bibcode=1975Moon...12..159S|s2cid=121225801 }}</ref> Alternatively, it has been suggested that this terrain formed as a result of the convergence of ejecta at this basin's antipode.<ref>{{cite journal |last1=Wieczorek |first1=Mark A. |last2=Zuber |first2=Maria T. |author-link2=Maria Zuber |title=A Serenitatis origin for the Imbrian grooves and South Pole-Aitken thorium anomaly |journal=Journal of Geophysical Research |year=2001 |volume=106 |issue=E11 |pages=27853–27864 |url=http://www.agu.org/pubs/crossref/2001/2000JE001384.shtml |access-date=May 12, 2008 |doi=10.1029/2000JE001384 |bibcode=2001JGR...10627853W |doi-access=free |archive-date=May 12, 2011 |archive-url=https://web.archive.org/web/20110512152936/http://www.agu.org/pubs/crossref/2001/2000JE001384.shtml |url-status=live }}</ref>

[[File:EW1027346412Gnomap.png|thumb|Tolstoj basin is along the bottom of this image of Mercury's limb]]
Overall, 46 impact basins have been identified.<ref>{{cite journal | title=Large impact basins on Mercury: Global distribution, characteristics, and modification history from MESSENGER orbital data | last1=Fassett | first1=Caleb I. | last2=Head | first2=James W. | last3=Baker | first3=David M. H. | last4=Zuber | first4=Maria T. | last5=Smith | first5=David E. | last6=Neumann | first6=Gregory A. | last7=Solomon | first7=Sean C. | last8=Klimczak | first8=Christian | last9=Strom | first9=Robert G. | last10=Chapman | first10=Clark R. | last11=Prockter | first11=Louise M. | last12=Phillips | first12=Roger J. | last13=Oberst | first13=Jürgen | last14=Preusker | first14=Frank | journal=Journal of Geophysical Research | volume=117 | id=E00L08 | date=October 2012 | at=15 pp. | doi=10.1029/2012JE004154 | bibcode=2012JGRE..117.0L08F | doi-access=free }}</ref> A notable basin is the {{convert|400|km|mi|abbr=on|adj=mid}}-wide, multi-ring [[Tolstoj Basin]] that has an ejecta blanket extending up to {{convert|500|km|mi|abbr=on}} from its rim and a floor that has been filled by smooth plains materials. [[Beethoven Basin]] has a similar-sized ejecta blanket and a {{convert|625|km|mi|abbr=on|adj=mid}}-diameter rim.<ref name="Spudis01" /> Like the Moon, the surface of Mercury has likely incurred the effects of [[space weathering]] processes, including solar wind and [[micrometeorite]] impacts.<ref>{{cite journal |title=Albedo of Immature Mercurian Crustal Materials: Evidence for the Presence of Ferrous Iron |journal=Lunar and Planetary Science |volume=39 |issue=1391 |year=2008 |page=1750 |last1=Denevi |first1=Brett W. |author-link1=Brett Denevi |last2=Robinson |first2=Mark S. |bibcode=2008LPI....39.1750D}}</ref>