Editing Mercury (planet) (section)

== Physical characteristics ==
[[File:Terrestrial planet size comp 2024.png|thumb|upright=1.4|Mercury to scale among the [[Inner Solar System]] [[planetary-mass object]]s beside the Sun, arranged by the order of their orbits outward from the Sun (from left: Mercury, [[Venus]],
[[Earth]], the [[Moon]], [[Mars]] and [[Ceres (dwarf planet)|Ceres]])]]

Mercury is one of four [[terrestrial planet]]s in the [[Solar System]], which means it is a rocky body like Earth. It is the smallest planet in the Solar System, with an [[equator]]ial [[radius]] of {{convert|2439.7|km}}.<ref name="fact"/> Mercury is also [[list of Solar System objects by radius|smaller]]—albeit more massive—than the largest [[natural satellite]]s in the Solar System, [[Ganymede (moon)|Ganymede]] and [[Titan (moon)|Titan]]. Mercury consists of approximately 70% metallic and 30% [[silicate]] material.<ref name="strom" />

=== Internal structure ===
[[File:Mercury with magnetic field.svg|left|thumb|upright=1.0|Mercury's internal structure and magnetic field]]
Mercury appears to have a solid silicate [[Crust (geology)|crust]] and mantle overlying a solid, metallic outer core layer, a deeper liquid core layer, and a solid inner core.<ref>{{cite web |url=https://www.nasa.gov/mission_pages/messenger/media/PressConf20120321.html |title=MESSENGER Provides New Look at Mercury's Surprising Core and Landscape Curiosities |publisher=NASA |editor-first=Tricia |editor-last=Talbert |date=March 21, 2012 |access-date=April 20, 2018 |archive-date=January 12, 2019 |archive-url=https://web.archive.org/web/20190112170032/https://www.nasa.gov/mission_pages/messenger/media/PressConf20120321.html |url-status=dead }}</ref><ref>{{Cite web |url=https://news.agu.org/press-release/scientists-find-evidence-mercury-has-a-solid-inner-core/ |title=Scientists find evidence Mercury has a solid inner core |format=Press release |date=April 17, 2023 |last1=Genova |first1=Antonio |display-authors=et al |website=AGU Newsroom |language=en-US |access-date=April 17, 2019 |archive-date=April 17, 2019 |archive-url=https://web.archive.org/web/20190417162031/https://news.agu.org/press-release/scientists-find-evidence-mercury-has-a-solid-inner-core/ |url-status=live }}</ref> The composition of the iron-rich core remains uncertain, but it likely contains nickel, silicon and perhaps sulfur and carbon, plus trace amounts of other elements.<ref>{{cite book | chapter=The Chemical Composition of Mercury | last1=Nittler | first1=Larry R. | last2=Chabot | first2=Nancy L. | last3=Grove | first3=Timothy L. | last4=Peplowski | first4=Patrick N. | title=Mercury: The View after MESSENGER | editor1-first=Sean C. | editor1-last=Solomon | editor2-first=Larry R. | editor2-last=Nittler | editor3-first=Brian J. | editor3-last=Anderson | isbn=9781316650684 | series=Cambridge Planetary Science Book Series | publication-place=Cambridge, UK | publisher=Cambridge University Press | year=2018 | pages=30–51 | doi=10.1017/9781316650684.003 | arxiv=1712.02187 | bibcode=2018mvam.book...30N | s2cid=119021137 }}</ref> The planet's density is the second highest in the Solar System at 5.427&nbsp;g/cm<sup>3</sup>, only slightly less than Earth's density of 5.515&nbsp;g/cm<sup>3</sup>.<ref name="fact" /> If the effect of [[gravitational compression]] were to be factored out from both planets, the materials of which Mercury is made would be denser than those of Earth, with an uncompressed density of 5.3&nbsp;g/cm<sup>3</sup> versus Earth's 4.4&nbsp;g/cm<sup>3</sup>.<ref>{{cite web |date=May 8, 2003 |url=https://astrogeology.usgs.gov/Projects/BrowseTheGeologicSolarSystem/MercuryBack.html |title=Mercury |publisher=US Geological Survey |access-date=November 26, 2006 |archive-url=https://web.archive.org/web/20060929091534/http://astrogeology.usgs.gov/Projects/BrowseTheGeologicSolarSystem/MercuryBack.html |archive-date=September 29, 2006 |url-status=dead }}</ref> Mercury's density can be used to infer details of its inner structure. Although Earth's high density results appreciably from gravitational compression, particularly at the [[planetary core|core]], Mercury is much smaller and its inner regions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in iron.<ref>{{cite journal |title=On the Internal Structures of Mercury and Venus |last=Lyttleton |first=Raymond A. |author-link=Raymond Lyttleton |journal=Astrophysics and Space Science |volume=5 |issue=1 |pages=18–35 |year=1969 |doi=10.1007/BF00653933 |bibcode=1969Ap&SS...5...18L |s2cid=122572625 }}</ref>

The radius of Mercury's core is estimated to be {{convert|2020|±|30|km|mi|abbr=on}}, based on interior models constrained to be consistent with a [[moment of inertia factor]] of {{val|0.346|0.014}}.<ref name="Margot2012" /><ref name="Hauck_etal_2013" /> Hence, Mercury's core occupies about 57% of its volume; for Earth this proportion is 17%. Research published in 2007 suggests that Mercury has a molten core.<ref name="cornell">{{cite news |first=Lauren |last=Gold |title=Mercury has molten core, Cornell researcher shows |date=May 3, 2007 |work=Chronicle |publisher=Cornell University |url=http://www.news.cornell.edu/stories/May07/margot.mercury.html |access-date=May 12, 2008 |archive-date=June 17, 2012 |archive-url=https://web.archive.org/web/20120617123129/http://www.news.cornell.edu/stories/May07/margot.mercury.html |url-status=live }}</ref><ref name="nrao">{{cite news |last=Finley |first=Dave |date=May 3, 2007 |title=Mercury's Core Molten, Radar Study Shows |publisher=National Radio Astronomy Observatory |url=http://www.nrao.edu/pr/2007/mercury/ |access-date=May 12, 2008 |archive-date=May 3, 2012 |archive-url=https://web.archive.org/web/20120503202505/http://www.nrao.edu/pr/2007/mercury/ |url-status=live }}</ref> The mantle-crust layer is in total {{convert|420|km|mi|abbr=on}} thick.<ref>{{cite journal |last1=Hauck |first1=Steven A. |display-authors=etal |title=The curious case of Mercury's internal structure |journal=Journal of Geophysical Research: Planets |date=May 6, 2013 |volume=118 |issue=6 |pages=1204–1220 |doi=10.1002/jgre.20091 |bibcode=2013JGRE..118.1204H |s2cid=17668886 |url=https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/jgre.20091 |access-date=June 5, 2023 |hdl=1721.1/85633 |hdl-access=free |archive-date=June 5, 2023 |archive-url=https://web.archive.org/web/20230605175115/https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/jgre.20091 |url-status=live }}</ref> Projections differ as to the size of the crust specifically; data from the {{nowrap|''Mariner 10''}} and ''MESSENGER'' probes suggests a thickness of {{convert|35|km|mi|abbr=on}}, whereas an [[Airy isostacy]] model suggests a thickness of {{convert|26|±|11|km|mi|abbr=on}}.<ref name="Padovan2015" /><ref>{{Cite book |last1=Solomon |first1=Sean C. |url=https://books.google.com/books?id=4o92DwAAQBAJ |title=Mercury: The View after MESSENGER |last2=Nittler |first2=Larry R. |last3=Anderson |first3=Brian J. |date=December 20, 2018 |publisher=Cambridge University Press |isbn=978-1-107-15445-2 |pages=534 |language=en |access-date=November 19, 2022 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162217/https://books.google.com/books?id=4o92DwAAQBAJ |url-status=live }}</ref><ref>{{cite journal | title=A thin, dense crust for Mercury | last=Sori | first=Michael M. | journal=Earth and Planetary Science Letters | volume=489 | pages=92–99 | date=May 2018 | doi=10.1016/j.epsl.2018.02.033 | bibcode=2018E&PSL.489...92S | doi-access=free }}</ref> One distinctive feature of Mercury's surface is the presence of numerous narrow ridges, extending up to several hundred kilometers in length. It is thought that these were formed as Mercury's core and mantle cooled and contracted at a time when the crust had already solidified.<ref>{{cite journal |title=Lobate Thrust Scarps and the Thickness of Mercury's Lithosphere |last1=Schenk |first1=Paul M. |last2=Melosh |first2=H. Jay |author-link2=H. Jay Melosh |journal=Abstracts of the 25th Lunar and Planetary Science Conference |volume=1994 |pages=1994LPI....25.1203S |bibcode=1994LPI....25.1203S |date=March 1994 }}</ref><ref>{{cite conference | last1=Watters | first1=T. R. | first2=F. | last2=Nimmo | first3=M. S. | last3=Robinson | title=Chronology of Lobate Scarp Thrust Faults and the Mechanical Structure of Mercury's Lithosphere | conference=Lunar and Planetary Science Conference | page=1886 | year=2004 | bibcode= 2004LPI....35.1886W }}</ref><ref>{{cite journal | journal=Geology | date=November 1998 | volume=26 | issue=11 | pages=991–994 | title=Topography of lobate scarps on Mercury; new constraints on the planet's contraction | first1=Thomas R. | last1=Watters | first2=Mark S. | last2=Robinson | first3=Anthony C. | last3=Cook | doi=10.1130/0091-7613(1998)026<0991:TOLSOM>2.3.CO;2 | bibcode=1998Geo....26..991W }}</ref>

Mercury's core has a higher iron content than that of any other planet in the Solar System, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal–silicate ratio similar to common [[chondrite]] meteorites, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass.<ref name="Benz" /> Early in the Solar System's history, Mercury may have been struck by a [[planetesimal]] of approximately {{frac|1|6}} Mercury's mass and several thousand kilometers across.<ref name="Benz" /> The impact would have stripped away much of the original crust and mantle, leaving the core behind as a relatively major component.<ref name="Benz" /> A similar process, known as the [[giant impact hypothesis]], has been proposed to explain the formation of Earth's Moon.<ref name="Benz" />

Alternatively, Mercury may have formed from the [[solar nebula]] before the Sun's energy output had stabilized. It would initially have had twice its present mass, but as the [[protostar|protosun]] contracted, temperatures near Mercury could have been between 2,500 and 3,500&nbsp;K and possibly even as high as 10,000&nbsp;K.<ref name="CameronAGW1">{{cite journal |title=The partial volatilization of Mercury |last=Cameron |first=Alastair G. W. |author-link=Alastair G. W. Cameron |journal=Icarus |volume=64 |issue=2 |pages=285–294 |year=1985 |doi=10.1016/0019-1035(85)90091-0 |bibcode=1985Icar...64..285C}}</ref> Much of Mercury's surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" that could have been carried away by the [[solar wind]].<ref name="CameronAGW1" /> A third hypothesis proposes that the solar nebula caused [[drag (physics)|drag]] on the particles from which Mercury was [[Accretion (astrophysics)|accreting]], which meant that lighter particles were lost from the accreting material and not gathered by Mercury.<ref>{{cite journal |title=Iron/silicate fractionation and the origin of Mercury |last=Weidenschilling |first=Stuart J. |journal=Icarus |volume=35 |issue=1 |pages=99–111 |year=1987 |doi=10.1016/0019-1035(78)90064-7 |bibcode=1978Icar...35...99W}}</ref>

Each hypothesis predicts a different surface composition, and two space missions have been tasked with making observations of this composition. The first ''[[MESSENGER]]'', which ended in 2015, found higher-than-expected potassium and sulfur levels on the surface, suggesting that the giant impact hypothesis and vaporization of the crust and mantle did not occur because said potassium and sulfur would have been driven off by the extreme heat of these events.<ref name="csmon20110929">{{cite news |url=https://www.csmonitor.com/Science/2011/0929/Messenger-s-message-from-Mercury-Time-to-rewrite-the-textbooks |title=Messenger's message from Mercury: Time to rewrite the textbooks |work=The Christian Science Monitor |first=Mark |last=Sappenfield |date=September 29, 2011 |access-date=August 21, 2017 |archive-date=August 21, 2017 |archive-url=https://web.archive.org/web/20170821214604/https://www.csmonitor.com/Science/2011/0929/Messenger-s-message-from-Mercury-Time-to-rewrite-the-textbooks |url-status=live }}</ref> ''[[BepiColombo]]'', which will arrive at Mercury in 2025, will make observations to test these hypotheses.<ref name="ESA-Bepi">{{cite web |url=http://sci.esa.int/bepicolombo/ |title=BepiColombo |series=Science & Technology |publisher=European Space Agency |access-date=April 7, 2008 |archive-date=March 6, 2018 |archive-url=https://web.archive.org/web/20180306193632/http://sci.esa.int/bepicolombo/ |url-status=live }}</ref> The findings so far would seem to favor the third hypothesis; however, further analysis of the data is needed.<ref name="intra">{{cite news |url=https://www.chemistryworld.com/news/messenger-sheds-light-on-mercurys-formation/3002463.article |title=Messenger sheds light on Mercury's formation |work=Chemistry World |first=Jon |last=Cartwright |date=September 30, 2011 |access-date=August 21, 2017 |archive-date=August 6, 2017 |archive-url=https://web.archive.org/web/20170806063258/https://www.chemistryworld.com/news/messenger-sheds-light-on-mercurys-formation/3002463.article |url-status=live }}</ref>

=== Surface geology ===
{{Main|Geology of Mercury}}
{{multiple image
 | align = right
 | total_width = 360
 | image1   = Mercury render with Blender 01.png
 | image2   = Mercury render with Blender 02.png
 | footer   = Mercury rendered with [[Blender (software)|Blender]] with data from [[NASA]] and the [[United States Geological Survey|USGS]]
}}
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)
}}

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"/>

=== Surface conditions and exosphere ===
{{Main|Atmosphere of Mercury}}
[[File:North pole of Mercury -- NASA.jpg|thumb|Composite of the north pole of Mercury, where NASA confirmed the discovery of a large volume of water ice, in permanently dark craters that are found there.<ref name="NYTimes2012-11-28" />]]
The surface temperature of Mercury ranges from {{Convert|100 to 700|K|C F}}.<ref name=":0">{{cite book |last=Prockter |first=Louise |title=Ice in the Solar System |publisher=Johns Hopkins APL Technical Digest |volume=26 |issue=2 |date=2005 |url=https://www.jhuapl.edu/content/techdigest/pdf/V26-N02/26-02-Prockter.pdf |access-date=July 27, 2009 |archive-date=September 24, 2021 |archive-url=https://web.archive.org/web/20210924085243/https://www.jhuapl.edu/Content/techdigest/pdf/V26-N02/26-02-Prockter.pdf |url-status=live }}</ref> It never rises above 180&nbsp;K at the poles,<ref name="vasa" /> due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. At [[perihelion]], the equatorial [[subsolar point]] is located at latitude 0°W or 180°W, and it climbs to a temperature of about {{val|700|u=K}}. During [[aphelion]], this occurs at 90° or 270°W and reaches only {{val|550|u=K}}.<ref>{{cite book |first=John S. |last=Lewis |date=2004 |title=Physics and Chemistry of the Solar System |page=463 |edition=2nd |publisher=Academic Press |isbn=978-0-12-446744-6}}</ref> On the dark side of the planet, temperatures average {{val|110|u=K}}.<ref name="vasa" /><ref>{{cite journal |last1=Murdock |first1=Thomas L. |last2=Ney |first2=Edward P. |title=Mercury: The Dark-Side Temperature |journal=[[Science (journal)|Science]] |year=1970 |volume=170 |issue=3957 |pages=535–537 |doi=10.1126/science.170.3957.535 |pmid=17799708 |bibcode=1970Sci...170..535M|s2cid=38824994 }}</ref> The intensity of [[sunlight]] on Mercury's surface ranges between 4.59 and 10.61 times the [[solar constant]] (1,370 W·m<sup>−2</sup>).<ref>{{cite book |title=Physics and Chemistry of the Solar System |last=Lewis |first=John S. |publisher=Academic Press |date=2004 |url=https://books.google.com/books?id=ERpMjmR1ErYC&pg=RA1-PA461 |access-date=June 3, 2008 |isbn=978-0-12-446744-6 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162200/https://books.google.com/books?id=ERpMjmR1ErYC&pg=RA1-PA461 |url-status=live }}</ref>

Although daylight temperatures at the surface of Mercury are generally extremely high, observations strongly suggest that ice (frozen water) exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102&nbsp;K, far lower than the global average.<ref>{{cite journal |last1=Ingersoll |first1=Andrew P. |last2=Svitek |first2=Tomas |last3=Murray |first3=Bruce C. |title=Stability of polar frosts in spherical bowl-shaped craters on the Moon, Mercury, and Mars |journal=Icarus |volume=100 |issue=1 |pages=40–47 |year=1992 |bibcode=1992Icar..100...40I |doi=10.1016/0019-1035(92)90016-Z}}</ref> This creates a [[Cold trap (astronomy)|cold trap]] where ice can accumulate. Water ice strongly reflects [[radar]], and observations by the 70-meter [[Goldstone Solar System Radar]] and the [[Very Large Array|VLA]] in the early 1990s revealed that there are patches of high radar [[Reflection (physics)|reflection]] near the poles.<ref name=Slade_et_al_1992>{{cite journal |last1=Slade |first1=Martin A. |last2=Butler |first2=Bryan J. |last3=Muhleman |first3=Duane O. |year=1992 |title=Mercury radar imaging – Evidence for polar ice |journal=[[Science (journal)|Science]] |volume=258 |issue=5082 |pages=635–640 |doi=10.1126/science.258.5082.635 |pmid=17748898 |bibcode=1992Sci...258..635S|s2cid=34009087 }}</ref> Although ice was not the only possible cause of these reflective regions, astronomers thought it to be the most likely explanation.<ref>{{cite web |last=Williams |first=David R. |date=June 2, 2005 |url=http://nssdc.gsfc.nasa.gov/planetary/ice/ice_mercury.html |title=Ice on Mercury |publisher=NASA Goddard Space Flight Center |access-date=May 23, 2008 |archive-date=January 31, 2011 |archive-url=https://web.archive.org/web/20110131225129/http://nssdc.gsfc.nasa.gov/planetary/ice/ice_mercury.html |url-status=live }}</ref> The presence of [[ice|water ice]] was confirmed using ''MESSENGER'' images of craters at the north pole.<ref name="NYTimes2012-11-28">{{cite news |url=https://www.nytimes.com/2012/11/30/science/space/mercury-home-to-ice-messenger-spacecraft-findings-suggest.html |title=On Closest Planet to the Sun, NASA Finds Lots of Ice |work=[[The New York Times]] |first=Kenneth |last=Chang |date=November 29, 2012 |page=A3 |archive-date=November 29, 2012 |archive-url=https://web.archive.org/web/20121129194012/http://www.nytimes.com/2012/11/30/science/space/mercury-home-to-ice-messenger-spacecraft-findings-suggest.html |url-status=live |quote=Sean C. Solomon, the principal investigator for MESSENGER, said there was enough ice there to encase [[Washington, D.C.]], in a frozen block two and a half miles deep.}}</ref>

The icy crater regions are estimated to contain about 10<sup>14</sup>–10<sup>15</sup>&nbsp;kg of ice,<ref name="Zahnle1">{{cite journal |last1=Rawlins |first1=Katherine |last2=Moses |first2=Julianne I. |last3=Zahnle |first3=Kevin J. |author-link3=Kevin J. Zahnle |title=Exogenic Sources of Water for Mercury's Polar Ice |journal=Bulletin of the American Astronomical Society |year=1995 |volume=27 |bibcode=1995DPS....27.2112R |page=1117}}</ref> and may be covered by a layer of [[regolith]] that inhibits [[Sublimation (phase transition)|sublimation]].<ref>{{cite journal |last1=Harmon |first1=John K. |last2=Perillat |first2=Phil J. |last3=Slade |first3=Martin A. |title=High-Resolution Radar Imaging of Mercury's North Pole |journal=Icarus |volume=149 |issue=1 |pages=1–15 |year=2001 |doi=10.1006/icar.2000.6544 |bibcode=2001Icar..149....1H}}</ref> By comparison, the [[Antarctica|Antarctic]] ice sheet on Earth has a mass of about 4{{e|18}}&nbsp;kg, and Mars's south polar cap contains about 10<sup>16</sup>&nbsp;kg of water.<ref name="Zahnle1" /> The origin of the ice on Mercury is not yet known, but the two most likely sources are from [[outgassing]] of water from the planet's interior and deposition by impacts of comets.<ref name="Zahnle1" />

Mercury is too small and hot for its [[gravity]] to retain any significant [[atmosphere]] over long periods of time; it does have a tenuous surface-bounded [[exosphere]]<ref>{{cite journal |last1=Domingue |first1=Deborah L. |last2=Koehn |first2=Patrick L. |display-authors=2 |last3=Killen |first3=Rosemary M. |last4=Sprague |first4=Ann L. |last5=Sarantos |first5=Menelaos |last6=Cheng |first6=Andrew F. |last7=Bradley |first7=Eric T. |last8=McClintock |first8=William E. |title=Mercury's Atmosphere: A Surface-Bounded Exosphere |journal=Space Science Reviews |volume=131 |issue=1–4 |pages=161–186 |year=2009 |doi=10.1007/s11214-007-9260-9 |bibcode=2007SSRv..131..161D |s2cid=121301247 |name-list-style=vanc}}</ref> at a surface pressure of less than approximately 0.5&nbsp;nPa (0.005 picobars).<ref name="fact" /> It includes [[hydrogen]], [[helium]], [[oxygen]], [[sodium]], [[calcium]], [[potassium]], [[magnesium]], [[silicon]], and [[hydroxide]], among others.<ref name=Milillo_et_al_2005/><ref name=Berezhnoy2018/> This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. [[Hydrogen atom]]s and [[helium atom]]s probably come from the solar wind, [[diffusion|diffusing]] into Mercury's [[magnetosphere]] before later escaping back into space. The [[radioactive decay]] of elements within Mercury's crust is another source of helium, as well as sodium and potassium. Water vapor is present, released by a combination of processes such as comets striking its surface, [[sputtering]] creating water out of hydrogen from the solar wind and oxygen from rock, and sublimation from reservoirs of water ice in the permanently shadowed polar craters. The detection of high amounts of water-related ions like O<sup>+</sup>, OH<sup><span style="color:black;">−</span></sup>, and [[hydronium|H<sub>3</sub>O<sup>+</sup>]] was a surprise.<ref>{{cite book |editor-first=Faith |editor-last=Vilas |editor-first2=Clark R. |editor-last2=Chapman |editor-first3=Mildred |editor-last3=Shapley Matthews |last1=Hunten |first1=Donald M. |last2=Shemansky |first2=Donald Eugene |last3=Morgan |first3=Thomas Hunt |date=1988 |publisher=University of Arizona Press |isbn=978-0-8165-1085-6 |chapter=The Mercury atmosphere |title=Mercury |chapter-url=https://www.researchgate.net/publication/23869200_The_Mercury_atmosphere <!-- broken: http://www.uapress.arizona.edu/onlinebks/Mercury/MercuryCh17.pdf --> |url=https://uapress.arizona.edu/book/mercury |access-date=February 19, 2020 |archive-date=February 19, 2020 |archive-url=https://web.archive.org/web/20200219213720/https://uapress.arizona.edu/book/mercury |url-status=live }}</ref><ref>{{cite news |first=Emily |last=Lakdawalla |date=July 3, 2008 |title=MESSENGER Scientists "Astonished" to Find Water in Mercury's Thin Atmosphere |publisher=The Planetary Society |url=https://www.planetary.org/blogs/emily-lakdawalla/2008/0703_MESSENGER_Scientists_Astonished_to.html |access-date=May 18, 2009 |archive-date=April 4, 2017 |archive-url=https://web.archive.org/web/20170404043436/http://www.planetary.org/blogs/emily-lakdawalla/2008/0703_MESSENGER_Scientists_Astonished_to.html |url-status=live }}</ref> Because of the quantities of these ions that were detected in Mercury's space environment, scientists surmise that these molecules were blasted from the surface or exosphere by the solar wind.<ref>{{cite journal |last1=Zurbuchen |first1=Thomas H. |last2=Raines |first2=Jim M. |display-authors=2 |last3=Gloeckler |first3=George |last4=Krimigis |first4=Stamatios M. |last5=Slavin |first5=James A. |last6=Koehn |first6=Patrick L. |last7=Killen |first7=Rosemary M. |last8=Sprague |first8=Ann L. |last9=McNutt Jr. |first9=Ralph L. |last10=Solomon |first10=Sean C. |title=MESSENGER Observations of the Composition of Mercury's Ionized Exosphere and Plasma Environment |journal=Science |volume=321 |issue=5885 |pages=90–92 |year=2008 |doi=10.1126/science.1159314 |pmid=18599777 |bibcode=2008Sci...321...90Z |s2cid=206513512 |name-list-style=vanc}}</ref><ref>{{cite news |publisher=University of Michigan |date=June 30, 2008 |title=Instrument Shows What Planet Mercury Is Made Of |url=http://newswise.com/articles/view/542209/ |access-date=May 18, 2009 |archive-date=March 22, 2012 |archive-url=https://web.archive.org/web/20120322021728/http://newswise.com/articles/view/542209/ |url-status=live }}</ref>

Sodium, potassium, and calcium were discovered in the atmosphere during the 1980s–1990s, and are thought to result primarily from the vaporization of surface rock struck by micrometeorite impacts<ref name="Killen2007">{{cite journal |last1=Killen |first1=Rosemary |title=Processes that Promote and Deplete the Exosphere of Mercury |year=2007 |journal=[[Space Science Reviews]] |volume=132 |issue=2–4 |pages=433–509 |doi=10.1007/s11214-007-9232-0 |ref=Killen2007 |bibcode=2007SSRv..132..433K |last2=Cremonese |first2=Gabrielle |display-authors=2 |last3=Lammer |first3=Helmut |last4=Orsini |first4=Stefano |last5=Potter |first5=Andrew E. |last6=Sprague |first6=Ann L. |last7=Wurz |first7=Peter |last8=Khodachenko |first8=Maxim L. |last9=Lichtenegger |first9=Herbert I. M. |s2cid=121944553 |url=https://boris.unibe.ch/25351/ |access-date=October 16, 2022 |archive-date=October 9, 2022 |archive-url=https://web.archive.org/web/20221009053004/https://boris.unibe.ch/25351/ |url-status=live }}</ref> including presently from [[Comet Encke]].<ref>{{cite journal |first1=Rosemary M. |last1=Killen |first2=Joseph M. |last2=Hahn |title=Impact Vaporization as a Possible Source of Mercury's Calcium Exosphere |journal=Icarus |date=December 10, 2014 |doi=10.1016/j.icarus.2014.11.035 |bibcode=2015Icar..250..230K |volume=250 |pages=230–237 |hdl=2060/20150010116}}</ref> In 2008, magnesium was discovered by ''MESSENGER''.<ref name="McClintock2009">{{cite journal |last1=McClintock |first1=William E. |last2=Vervack |first2=Ronald J. |last3=Bradley |first3=E. Todd |last4=Killen |first4=Rosemary M. |last5=Mouawad |first5=Nelly |last6=Sprague |first6=Ann L. |last7=Burger |first7=Matthew H. |last8=Solomon |first8=Sean C. |last9=Izenberg |first9=Noam R. |title=MESSENGER Observations of Mercury's Exosphere: Detection of Magnesium and Distribution of Constituents |journal=Science |year=2009 |volume=324 |doi=10.1126/science.1172525 |pages=610–613 |bibcode=2009Sci...324..610M |pmid=19407195 |issue=5927 |s2cid=5578520 |display-authors=2 }}</ref> Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic poles. This would indicate an interaction between the magnetosphere and the planet's surface.<ref name="chaikin1" />

According to NASA, Mercury is not a suitable planet for Earth-like life. It has a [[exosphere|surface boundary exosphere]] instead of a layered atmosphere, extreme temperatures, and high solar radiation. It is unlikely that any living beings can withstand those conditions.<ref>{{cite web|url= https://solarsystem.nasa.gov/planets/mercury/in-depth/|title= Mercury|date= October 19, 2021|publisher= NASA|accessdate= July 4, 2022|archive-date= July 5, 2022|archive-url= https://web.archive.org/web/20220705191357/https://solarsystem.nasa.gov/planets/mercury/in-depth/|url-status= live}}</ref> Some parts of the subsurface of Mercury may have been [[Planetary habitability|habitable]], and perhaps [[life form]]s, albeit likely primitive [[microorganism]]s, may have existed on the planet.<ref name="NYT-20200324">{{cite news |last=Hall |first=Shannon |title=Life on the Planet Mercury? 'It's Not Completely Nuts' – A new explanation for the rocky world's jumbled landscape opens a possibility that it could have had ingredients for habitability. |url=https://www.nytimes.com/2020/03/24/science/mercury-life-water.html |archive-url=https://web.archive.org/web/20200324150021/https://www.nytimes.com/2020/03/24/science/mercury-life-water.html |archive-date=March 24, 2020 |url-access=subscription |url-status=live |date=March 24, 2020 |work=[[The New York Times]] |access-date=March 26, 2020 }}</ref><ref name="SR-20200316">{{cite journal | last1=Rodriguez | first1=J. Alexis P. | last2=Leonard | first2=Gregory J. | last3=Kargel | first3=Jeffrey S. | last4=Domingue | first4=Deborah | last5=Berman | first5=Daniel C. | last6=Banks | first6=Maria | last7=Zarroca | first7=Mario | last8=Linares | first8=Rogelio | last9=Marchi | first9=Simone | last10=Baker | first10=Victor R. | last11=Webster | first11=Kevin D. | last12=Sykes | first12=Mark |title=The Chaotic Terrains of Mercury Reveal a History of Planetary Volatile Retention and Loss in the Innermost Solar System |date=March 16, 2020 |journal=[[Scientific Reports]] |volume=10 |issue=4737 |page=4737 |doi=10.1038/s41598-020-59885-5 |pmid=32179758 |pmc=7075900 |bibcode=2020NatSR..10.4737R }}</ref><ref>{{cite news | title=Vast Collapsed Terrains on Mercury Might be Windows Into Ancient – Possibly Habitable – Volatile-Rich Materials | work=Planetary Science Institute | date=March 16, 2020 | url=https://www.psi.edu/news/mercurychaos | access-date=August 27, 2022 | archive-date=August 28, 2022 | archive-url=https://web.archive.org/web/20220828041010/https://www.psi.edu/news/mercurychaos | url-status=live }}</ref>

=== Magnetic field and magnetosphere ===
{{Main|Mercury's magnetic field}}
[[File:Mercury Magnetic Field NASA.jpg|thumb|Graph showing relative strength of Mercury's magnetic field]]

Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, [[magnetic field]]. According to measurements taken by {{nowrap|''Mariner 10''}}, it is about 1.1% the strength of [[Earth's magnetic field|Earth's]]. The magnetic-field strength at Mercury's equator is about {{nowrap|300 [[Tesla (unit)|nT]]}}.<ref>{{cite book |title=Astronomy: The Solar System and Beyond |first=Michael A. |last=Seeds |date=2004 |isbn=978-0-534-42111-3 |publisher=Brooks Cole |edition=4th}}</ref><ref>{{cite web |last=Williams |first=David R. |date=January 6, 2005 |url=http://nssdc.gsfc.nasa.gov/planetary/planetfact.html |title=Planetary Fact Sheets |publisher=NASA National Space Science Data Center |access-date=August 10, 2006 |archive-date=September 25, 2008 |archive-url=https://web.archive.org/web/20080925071832/http://nssdc.gsfc.nasa.gov/planetary/planetfact.html |url-status=live }}</ref> Like that of Earth, Mercury's magnetic field is [[dipolar]]<ref name="chaikin1">{{cite book |first1=J. Kelly |last1=Beatty |last2=Petersen |first2=Carolyn Collins |last3=Chaikin |first3=Andrew |title=The New Solar System |date=1999 |publisher=Cambridge University Press |isbn=978-0-521-64587-4}}</ref> and nearly aligned with the planet's spin axis (10° dipolar tilt, compared to 11° for Earth).<ref name="qq">{{cite web |date=January 30, 2008 |url=https://messenger.jhuapl.edu/Explore/Science-Images-Database/gallery-image-152.html |title=Mercury's Internal Magnetic Field |publisher=NASA |access-date=April 21, 2021 |archive-date=April 21, 2021 |archive-url=https://web.archive.org/web/20210421163118/https://messenger.jhuapl.edu/Explore/Science-Images-Database/gallery-image-152.html |url-status=live }}</ref> Measurements from both the {{nowrap|''Mariner 10''}} and ''MESSENGER'' space probes have indicated that the strength and shape of the magnetic field are stable.<ref name="qq" />

It is likely that this magnetic field is generated by a [[Dynamo theory|dynamo]] effect, in a manner similar to the magnetic field of Earth.<ref>{{cite web |last=Gold |first=Lauren |date=May 3, 2007 |url=http://www.news.cornell.edu/stories/May07/margot.mercury.html |title=Mercury has molten core, Cornell researcher shows |publisher=Cornell University |access-date=April 7, 2008 |archive-date=June 17, 2012 |archive-url=https://web.archive.org/web/20120617123129/http://www.news.cornell.edu/stories/May07/margot.mercury.html |url-status=live }}</ref><ref>{{cite journal |last=Christensen |first=Ulrich R. |title=A deep dynamo generating Mercury's magnetic field |journal=Nature |year=2006 |volume=444 |pages=1056–1058 |doi=10.1038/nature05342 |pmid=17183319 |issue=7122 |bibcode=2006Natur.444.1056C |s2cid=4342216 |url=https://resolver.sub.uni-goettingen.de/purl?gro-2/65564 |access-date=October 29, 2023 |archive-date=March 1, 2024 |archive-url=https://web.archive.org/web/20240301162201/https://publications.goettingen-research-online.de/handle/2/65564 |url-status=live }}</ref> This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong [[tidal heating]] effects caused by the planet's high orbital eccentricity would serve to keep part of the core in the liquid state necessary for this dynamo effect.<ref name="Spohn2001">{{cite journal |last1=Spohn |first1=Tilman |last2=Sohl |first2=Frank |last3=Wieczerkowski |first3=Karin |last4=Conzelmann |first4=Vera |title=The interior structure of Mercury: what we know, what we expect from BepiColombo |journal=Planetary and Space Science |volume=49 |issue=14–15 |pages=1561–1570 |doi=10.1016/S0032-0633(01)00093-9 |bibcode=2001P&SS...49.1561S |year=2001 }}</ref><ref>{{cite journal | title=The tides of Mercury and possible implications for its interior structure | last1=Padovan | first1=Sebastiano | last2=Margot | first2=Jean-Luc | last3=Hauck | first3=Steven A. | last4=Moore | first4=William B. | last5=Solomon | first5=Sean C. | journal=Journal of Geophysical Research: Planets | volume=119 | issue=4 | pages=850–866 | date=April 2014 | doi=10.1002/2013JE004459 | bibcode=2014JGRE..119..850P | s2cid=56282397 }}</ref>

Mercury's magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. The planet's magnetosphere, though small enough to fit within Earth,<ref name="chaikin1" /> is strong enough to trap solar wind [[plasma (physics)|plasma]]. This contributes to the space weathering of the planet's surface.<ref name="qq" /> Observations taken by the {{nowrap|''Mariner 10''}} spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts of energetic particles in the planet's magnetotail indicate a dynamic quality to the planet's magnetosphere.<ref name="chaikin1" />

During its second flyby of the planet on October 6, 2008, ''MESSENGER'' discovered that Mercury's magnetic field can be extremely "leaky". The spacecraft encountered magnetic "tornadoes"—twisted bundles of magnetic fields connecting the planetary magnetic field to interplanetary space—that were up to {{nowrap|800 km}} wide or a third of the radius of the planet. These twisted magnetic flux tubes, technically known as [[flux transfer event]]s, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface via [[magnetic reconnection]].<ref name="NASA060209" /> This also occurs in Earth's magnetic field. The ''MESSENGER'' observations showed the reconnection rate was ten times higher at Mercury, but its proximity to the Sun only accounts for about a third of the reconnection rate observed by ''MESSENGER''.<ref name="NASA060209">{{cite web |first=Bill |last=Steigerwald |date=June 2, 2009 |title=Magnetic Tornadoes Could Liberate Mercury's Tenuous Atmosphere |publisher=NASA Goddard Space Flight Center |url=http://www.nasa.gov/mission_pages/messenger/multimedia/magnetic_tornadoes.html |access-date=July 18, 2009 |archive-date=May 18, 2012 |archive-url=https://web.archive.org/web/20120518035510/http://www.nasa.gov/mission_pages/messenger/multimedia/magnetic_tornadoes.html |url-status=dead }}</ref>