Editing Mercury (planet) (section)

=== Surface geology ===
{{Main|Geology of Mercury}}
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Mercury's surface is similar in appearance to that of the Moon, showing extensive [[Lunar mare|mare]]-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. It is more [[heterogeneous]] than the surface of [[Mars]] or the Moon, both of which contain significant stretches of similar geology, such as [[Lunar mare|maria]] and plateaus.<ref name="awst169_18_18" /> [[Albedo]] features are areas of markedly different reflectivity, which include impact craters, the resulting ejecta, and [[ray system]]s. Larger albedo features correspond to higher reflectivity plains.<ref>{{cite conference | title=Albedo Features of Mercury | last1=Hughes | first1=E. T. | last2=Vaughan | first2=W. M. | conference=43rd Lunar and Planetary Science Conference, held March 19–23, 2012 at The Woodlands, Texas | volume=1659 | id=2151 | date=March 2012 | bibcode=2012LPI....43.2151H }}</ref> Mercury has "[[wrinkle-ridge]]s" (dorsa), Moon-like [[highland]]s, mountains (montes), plains (planitiae), escarpments (rupes), and valleys ([[Vallis (planetary geology)|valles]]).<ref>{{cite web |last=Blue |first=Jennifer |date=April 11, 2008 |url=http://planetarynames.wr.usgs.gov/ |title=Gazetteer of Planetary Nomenclature |publisher=US Geological Survey |access-date=April 11, 2008 |archive-date=April 12, 2012 |archive-url=https://web.archive.org/web/20120412082057/http://planetarynames.wr.usgs.gov/ |url-status=live }}</ref><ref name="DunneCh7">{{cite book |title=The Voyage of Mariner&nbsp;10 – Mission to Venus and Mercury |last1=Dunne |first1=James A. |last2=Burgess |first2=Eric |author-link2=Eric Burgess |chapter-url=https://history.nasa.gov/SP-424/ch7.htm |publisher=NASA History Office |date=1978 |chapter=Chapter Seven |url=https://history.nasa.gov/SP-424/ |access-date=May 28, 2008 |archive-date=November 17, 2017 |archive-url=https://web.archive.org/web/20171117190025/https://history.nasa.gov/SP-424/ |url-status=dead }}</ref>

[[File:Unmasking the Secrets of Mercury.jpg|thumb|left|[[MESSENGER#Scientific instruments|MASCS]] spectrum scan of Mercury's surface by ''MESSENGER'']]
The planet's mantle is chemically heterogeneous, suggesting the planet went through a [[magma ocean]] phase early in its history. Crystallization of minerals and convective overturn resulted in a layered, chemically heterogeneous crust with large-scale variations in chemical composition observed on the surface. The crust is low in iron but high in sulfur, resulting from the stronger early [[Chemical reduction|chemically reducing]] conditions than is found on other terrestrial planets. The surface is dominated by iron-poor [[pyroxene]] and [[olivine]], as represented by [[enstatite]] and [[forsterite]], respectively, along with sodium-rich [[plagioclase]] and minerals of mixed magnesium, calcium, and iron-sulfide. The less reflective regions of the crust are high in carbon, most likely in the form of graphite.<ref>{{cite journal | title=The Surface Composition of Mercury | first1=Larry R. | last1=Nittler | first2=Shoshana Z. | last2=Weider | journal=Elements | year=2019 | volume=15 | issue=1 | pages=33–38 | doi=10.2138/gselements.15.1.33 | bibcode=2019Eleme..15...33N | s2cid=135051680 }}</ref><ref>{{cite journal | title=The Role of Reducing Conditions in Building Mercury | first1=Camille | last1=Cartier | first2=Bernard J. | last2=Wood | journal=Elements | volume=15 | number=1 | pages=39–45 | date=February 2019 | doi=10.2138/gselements.15.1.39 | bibcode=2019Eleme..15...39C | s2cid=135268415 }}</ref>

Names for features on Mercury come from a variety of sources and are set according to the [[IAU]] [[planetary nomenclature]] system. Names coming from people are limited to the deceased. Craters are named for artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field. Ridges, or dorsa, are named for scientists who have contributed to the study of Mercury. Depressions or [[fossa (geology)|fossae]] are named for works of architecture. Montes are named for the word "hot" in a variety of languages. [[Plain]]s or planitiae are named for [[Mercury (god)|Mercury]] in various languages. [[Escarpment]]s or [[rupēs]] are named for ships of scientific expeditions. Valleys or valles are named for abandoned cities, towns, or settlements of antiquity.<ref>{{cite web |url=http://planetarynames.wr.usgs.gov/Page/Categories |title=Categories for Naming Features on Planets and Satellites |publisher=US Geological Survey |access-date=August 20, 2011 |archive-date=July 8, 2014 |archive-url=https://web.archive.org/web/20140708063522/http://planetarynames.wr.usgs.gov/Page/Categories |url-status=live }}</ref>

==== 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)
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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>

==== Plains ====
There are two geologically distinct plains regions on Mercury.<ref name="Spudis01" /><ref name="WagWolIva01" /> Gently rolling, hilly [[Inter-crater plains on Mercury|plains in the regions between craters]] are Mercury's oldest visible surfaces,<ref name="Spudis01" /> predating the heavily cratered terrain. These inter-crater plains appear to have obliterated many earlier craters, and show a general paucity of smaller craters below about {{convert|30|km|mi|abbr=on}} in diameter.<ref name="WagWolIva01" />

Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance to lunar maria. Unlike lunar maria, the smooth plains of Mercury have the same albedo as the older inter-crater plains. Despite a lack of unequivocally volcanic characteristics, the localization and rounded, lobate shape of these plains strongly support volcanic origins.<ref name="Spudis01" /> All the smooth plains of Mercury formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket.<ref name="Spudis01" />

==== Compressional features ====
An unusual feature of Mercury's surface is the numerous compression folds, or [[rupes]], that crisscross the plains. These exist on the Moon, but are much more prominent on Mercury.<ref>{{cite journal | title=Wrinkle ridges on Mercury and the Moon within and outside of mascons | last1=Schleicher | first1=Lisa S. | last2=Watters | first2=Thomas R. | last3=Martin | first3=Aaron J. | last4=Banks | first4=Maria E. | journal=Icarus | volume=331 | pages=226–237 | date=October 2019 | doi=10.1016/j.icarus.2019.04.013 | bibcode=2019Icar..331..226S | s2cid=150072193 }}</ref> As Mercury's interior cooled, it contracted and its surface began to deform, creating [[wrinkle ridge]]s and [[lobate scarp]]s associated with [[thrust fault]]s. The scarps can reach lengths of {{convert|1000|km|mi|abbr=on}} and heights of {{convert|3|km|mi|abbr=on}}.<ref name = "Choi2016.09">{{cite web |url=http://www.space.com/34199-earthquakes-rock-mercury-today.html |title=Mercuryquakes May Currently Shake Up the Tiny Planet |last=Choi |first=Charles Q. |date=September 26, 2016 |website=[[Space.com]] |access-date=September 28, 2016 |archive-date=September 28, 2016 |archive-url=https://web.archive.org/web/20160928040406/http://www.space.com/34199-earthquakes-rock-mercury-today.html |url-status=live }}</ref> These compressional features can be seen on top of other features, such as craters and smooth plains, indicating they are more recent.<ref name="Dzurisin1978">{{cite journal |last=Dzurisin |first=Daniel |date=October 10, 1978 |title=The tectonic and volcanic history of Mercury as inferred from studies of scarps, ridges, troughs, and other lineaments |journal=Journal of Geophysical Research |volume=83 |issue=B10 |pages=4883–4906 |bibcode=1978JGR....83.4883D |doi=10.1029/JB083iB10p04883}}</ref> Mapping of the features has suggested a total shrinkage of Mercury's radius in the range of ~{{convert|1–7|km|mi|abbr=on}}.<ref name="Watters2016">{{cite journal |last1=Watters |first1=Thomas R. |last2=Daud |first2=Katie |last3=Banks |first3=Maria E. |last4=Selvans |first4=Michelle M. |last5=Chapman |first5=Clark R. |last6=Ernst |first6=Carolyn M. |title=Recent tectonic activity on Mercury revealed by small thrust fault scarps |journal=Nature Geoscience |date=September 26, 2016 |doi=10.1038/ngeo2814 |volume=9 |issue=10 |pages=743–747 |bibcode=2016NatGe...9..743W}}</ref> Most activity along the major thrust systems probably ended about 3.6–3.7&nbsp;billion years ago.<ref>{{cite journal | title=Dating long thrust systems on Mercury: New clues on the thermal evolution of the planet | first1=L. | last1=Giacomini | first2=M. | last2=Massironi | first3=V. | last3=Galluzzi | first4=S. | last4=Ferrari | first5=P. | last5=Palumbo | journal=Geoscience Frontiers | volume=11 | issue=3 | date=May 2020 | pages=855–870 | doi=10.1016/j.gsf.2019.09.005 | bibcode=2020GeoFr..11..855G | s2cid=210298205 | doi-access=free }}</ref> Small-scale thrust fault scarps have been found, tens of meters in height and with lengths in the range of a few kilometers, that appear to be less than 50 million years old, indicating that compression of the interior and consequent surface geological activity continue to the present.<ref name = "Choi2016.09" /><ref name= "Watters2016" />

====Volcanism====
[[File:Picasso crater.png|thumb|[[Picasso (crater)|Picasso crater]]—the large arc-shaped pit located on the eastern side of its floor is postulated to have formed when subsurface magma subsided or drained, causing the surface to collapse into the resulting void.]]
There is evidence for [[pyroclastic flow]]s on Mercury from low-profile [[shield volcano]]es.<ref name="Kerber 2009">{{cite journal |title=Explosive volcanic eruptions on Mercury: Eruption conditions, magma volatile content, and implications for interior volatile abundances |journal=Earth and Planetary Science Letters |date=August 15, 2009 |last1=Kerber |first1=Laura |last2=Head |first2=James W. |last3=Solomon |first3=Sean C. |last4=Murchie |first4=Scott L. |last5=Blewett |first5=David T. | volume=285 | issue=3–4 | pages=263–271 | doi=10.1016/j.epsl.2009.04.037 | bibcode=2009E&PSL.285..263K }}</ref><ref name="Volcanism 2011">{{cite journal |title=Flood Volcanism in the Northern High Latitudes of Mercury Revealed by ''MESSENGER'' |journal=Science |date=September 30, 2011 |last1=Head |first1=James W. |last2=Chapman |first2=Clark R. |last3=Strom |first3=Robert G. |last4=Fassett |first4=Caleb I. |last5=Denevi |first5=Brett W. |volume=333 |issue=6051 |pages=1853–1856 |doi=10.1126/science.1211997 |bibcode=2011Sci...333.1853H |pmid=21960625 |s2cid=7651992 |url=https://authors.library.caltech.edu/72395/2/Head.SOM.pdf |access-date=August 20, 2019 |archive-date=July 19, 2018 |archive-url=https://web.archive.org/web/20180719031049/https://authors.library.caltech.edu/72395/2/Head.SOM.pdf |url-status=live }}</ref><ref name="becca">{{cite journal |last1=Thomas |first1=Rebecca J. |last2=Rothery |first2=David A. |last3=Conway |first3=Susan J. |last4=Anand |first4=Mahesh |title=Long-lived explosive volcanism on Mercury |journal=Geophysical Research Letters |date=September 16, 2014 |volume=41 |issue=17 |pages=6084–6092 |doi=10.1002/2014GL061224 |bibcode=2014GeoRL..41.6084T |s2cid=54683272 |url=http://oro.open.ac.uk/40782/ |access-date=July 19, 2017 |archive-date=August 22, 2017 |archive-url=https://web.archive.org/web/20170822012357/http://oro.open.ac.uk/40782/ |url-status=live }}</ref> Fifty-one pyroclastic deposits have been identified,<ref name="Groudge 2014">{{cite journal |title=Global inventory and characterization of pyroclastic deposits on Mercury: New insights into pyroclastic activity from MESSENGER orbital data |journal=Journal of Geophysical Research |date=March 2014 |last1=Groudge |first1=Timothy A. |last2=Head |first2=James W. |doi=10.1002/2013JE004480 |volume=119 |issue=3 |pages=635–658 |bibcode=2014JGRE..119..635G |s2cid=14393394 |url=http://www.planetary.brown.edu/pdfs/4334.pdf |access-date=August 25, 2019 |archive-date=July 18, 2019 |archive-url=https://web.archive.org/web/20190718161242/http://www.planetary.brown.edu/pdfs/4334.pdf |url-status=dead }}</ref> where 90% of them are found within impact craters.<ref name="Groudge 2014"/> A study of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on Mercury over a prolonged interval.<ref name="Groudge 2014"/>

A "rimless depression" inside the southwest rim of the Caloris Basin consists of at least nine overlapping volcanic vents, each individually up to {{convert|8|km|mi|abbr=on}} in diameter. It is thus a "[[compound volcano]]".<ref name="Rothery 2014">{{cite journal |title=Prolonged eruptive history of a compound volcano on Mercury: Volcanic and tectonic implications |journal=Earth and Planetary Science Letters |date=January 1, 2014 |last1=Rothery |first1=David A. |last2=Thomas |first2=Rebeca J. |last3=Kerber |first3=Laura |volume=385 |pages=59–67 |bibcode=2014E&PSL.385...59R |doi=10.1016/j.epsl.2013.10.023 |url=http://oro.open.ac.uk/38842/1/Rothery2.pdf |access-date=August 20, 2019 |archive-date=March 6, 2020 |archive-url=https://web.archive.org/web/20200306081432/http://oro.open.ac.uk/38842/1/Rothery2.pdf |url-status=live }}</ref> The vent floors are at least {{convert|1|km|mi|abbr=on}} below their brinks and they bear a closer resemblance to volcanic craters sculpted by explosive eruptions or modified by collapse into void spaces created by magma withdrawal back down into a conduit.<ref name="Rothery 2014"/> Scientists could not quantify the age of the volcanic complex system but reported that it could be on the order of a billion years.<ref name="Rothery 2014"/>