Editing Mercury (planet) (section)

=== 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>