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{{featured article}}
<!--
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{{Infobox planet
| bgcolour = #D8BBA6
| name = Mercury
| symbol = <span style="font-size:200%; font-weight:bold">&#9791;</span>
| image = ]
| caption = ] false color image of Mercury
| orbit_ref =<ref name=horizons>{{cite web
| date=April 7, 2008 | first=Donald K. | last=Yeomans
| url=http://ssd.jpl.nasa.gov/?horizons
| title=HORIZONS System
| publisher=NASA JPL | accessdate=2008-04-07 }}</ref>
| epoch = ]
| aphelion = 69,816,900 km<br />0.466&nbsp;697 ]
| perihelion = 46,001,200 km<br />0.307&nbsp;499 AU
| semimajor = 57,909,100 km<br />0.387&nbsp;098 AU
| eccentricity = 0.205&nbsp;630<ref name="nssdcMercury" />
| period = 87.969&nbsp;1 ]<br/>(0.240&nbsp;846 ])<br/>0.5 Mercury ]
| synodic_period = 115.88&nbsp;d<ref name="nssdcMercury">{{cite web|title=Mercury Fact Sheet|url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html|publisher=] Goddard Space Flight Center | date=November 30, 2007 |accessdate=2008-05-28}}</ref>
| avg_speed = 47.87 km/s<ref name="nssdcMercury" />
| inclination = 7.005° to ]<br>3.38° to ]<br>6.34° to ]<ref name=meanplane>{{cite web
|date=2009-04-03
|title=The MeanPlane (Invariable plane) of the Solar System passing through the barycenter
|url=http://home.comcast.net/~kpheider/MeanPlane.gif
|accessdate=2009-04-03}} (produced with written by Aldo Vitagliano; see also ])</ref>
| asc_node = 48.331°
| arg_peri = 29.124°
| mean_anomaly = 174.796°
| satellites = None
| physical_characteristics = yes
diameter = 4,880 km
| mean_radius = 2,439.7&nbsp;±&nbsp;1.0&nbsp;km<ref name=nasa>{{cite web
| date=May 28, 2009 | first=Kirk | last=Munsell
| coauthors=Smith, Harman; Harvey, Samantha
| url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Mercury&Display=Facts
| title=Mercury: Facts & Figures
| work=Solar System Exploration
| publisher=NASA | accessdate=2008-04-07 }}
</ref><ref name=Seidelmann2007>{{cite journal
| last= Seidelmann| first= P. Kenneth
| coauthors= Archinal, B. A.; A’hearn, M. F.; et al.
| title= Report of the IAU/IAGWorking Group on cartographic coordinates and rotational elements: 2006
| journal= Celestial Mechanics and Dynamical Astronomy
| volume=90 | pages=155–180 | year=2007
| doi=10.1007/s10569-007-9072-y
| url=http://adsabs.harvard.edu/doi/10.1007/s10569-007-9072-y
| accessdate=2007-08-28 }}</ref><br />0.3829 Earths
| flattening = 0<ref name=Seidelmann2007/>
| surface_area = 7.48{{e|7}}&nbsp;km²<ref name=nasa/><br />0.147 Earths
| volume = 6.083{{e|10}}&nbsp;km³<ref name=nasa/><br />0.056 Earths
| mass = 3.3022{{e|23}}&nbsp;kg<ref name=nasa/><br />0.055 Earths
| density = 5.427&nbsp;g/cm³<ref name=nasa/>
| surface_grav = 3.7&nbsp;]<br />0.38 ]<ref name=nasa/>
| escape_velocity = 4.25&nbsp;km/s<ref name=nasa/>
| sidereal_day = 58.646 day<br>1407.5&nbsp;]<ref name=nasa/>
| rot_velocity = {{convert|10.892|km/h|m/s|abbr=on}}
| axial_tilt = 2.11′&nbsp;±&nbsp;0.1′<ref name=Margot2007>{{cite journal| last=Margot | first=L.J.| coauthors=Peale, S. J.; Jurgens, R. F.; Slade, M. A.; Holin, I. V.| title=Large Longitude Libration of Mercury Reveals a Molten Core| journal=Science| year=2007 | volume=316 | pages=710&ndash;714| doi=10.1126/science.1140514
| url=http://adsabs.harvard.edu/abs/2007Sci...316..710M| pmid=17478713}}</ref>
| right_asc_north_pole = 18 h 44 min 2 s<br/>281.01°<ref name="nssdcMercury" />
| declination = 61.45°<ref name="nssdcMercury" />
| albedo = 0.119 (])<br/>
0.106 (])<ref name="nssdcMercury" />
| magnitude = &minus;2.3 to 5.7<ref name=ephemeris/><ref name="nssdcMercury" />
| angular_size = 4.5" &ndash; 13"<ref name="nssdcMercury" />
| temperatures = yes
| temp_name1 = 0°N, 0°W <!-- Vasavada et al. 1999-->
| min_temp_1 = 100 K <!-- (-173 °C) -->
| mean_temp_1 = 340 K <!-- (67 °C) -->
| max_temp_1 = 700 K <!-- (427 °C) -->
| temp_name2 = 85°N, 0°W
| min_temp_2 = 80 K <!-- (-193 °C) -->
| mean_temp_2 = 200 K <!-- (-73 °C) -->
| max_temp_2 = 380 K <!-- (106.85 °C) -->
| pronounce = {{IPA-en|ˈmɜrkjəri||en-us-Mercury.ogg}}
| adjectives = Mercurian, Mercurial<ref>{{cite web
| url=http://www.merriam-webster.com/dictionary/mercurial
| publisher=Merriam-Webster Online
| title=mercurial | accessdate=2008-06-12 }}</ref>
| atmosphere = yes
| surface_pressure = trace
| atmosphere_composition = 42% Molecular ]<br />29.0% ]<br />22.0% ]<br />6.0% ]<br />0.5% ]<br />Trace amounts of ], ], ], ], ], ], & ]<ref name="nssdcMercury" />
}}


'''Mercury''' is the innermost and smallest ] in the ],<ref>] was once considered the smallest, but, as of 2008, it is classified as a ].</ref> ]ing the ] once every 87.969 days. The orbit of Mercury has the highest ] of all the Solar System planets, and it has the smallest ]. It completes three rotations about the axis for every two orbits. The ] of Mercury's orbit precesses around the Sun at an excess of 43&nbsp;]s per century; a phenomenon that was explained in the 20th century by ]'s ].<ref name=wudka>{{cite web
|date=1998-09-24
|title=Precession of the perihelion of Mercury
|publisher=Department of Physics and Astronomy at the University of California, Riverside
|author=Jose Wudka
|url=http://physics.ucr.edu/~wudka/Physics7/Notes_www/node98.html
|accessdate=2009-03-04}}</ref> Mercury is bright when viewed from ], ranging from −2.3 to 5.7 in ], but is not easily seen as its greatest ] is only 28.3°. Since Mercury is normally lost in the glare of the Sun, unless there is a ], Mercury can only be viewed in morning or evening ].


<blockquote>
Comparatively little is known about Mercury; ground-based telescopes reveal only an illuminated crescent with limited detail. The first of two ] to visit the planet was ], which mapped only about 45% of the planet’s surface from 1974 to 1975. The second is the ], which mapped another 30% during its flyby of January 14, 2008. A final flyby took place in September 2009. MESSENGER is scheduled to attain orbital insertion around Mercury in 2011, and will then survey and map the entire planet.
{| class="wikitable"

Mercury is similar in appearance to the ]: it is heavily ] with regions of smooth plains, has no ]s and no substantial ]. However, unlike the moon, it has a large ] ], which generates a ] about 1% as strong as that of the ].<ref>{{cite web |url=http://www-spc.igpp.ucla.edu/personnel/russell/papers/merc_mag/|title=Mercury magnetic field|publisher=C. T. Russell & J. G. Luhmann|accessdate=2007-03-16}}</ref> It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about 90 to {{nowrap|700 ]}} (−183&nbsp;°C to 427&nbsp;°C, −297&nbsp;°F to 801&nbsp;°F),<ref name="ESAs&t">{{cite web|url=http://sci.esa.int/science-e/www/category/index.cfm?fcategoryid=4586|title=Background Science|publisher=European Space Agency|accessdate=2008-05-23}}</ref> with the ] being the hottest and the bottoms of craters near the ] being the coldest. |Mercury is similar in appearance to the ]: it is heavily ] with regions of smooth plains, has no ]s and no substantial ]. However, unlike the moon, it has a large ] ], which generates a ] about 1% as strong as that of the ].<ref>{{cite web |url=http://www-spc.igpp.ucla.edu/personnel/russell/papers/merc_mag/|title=Mercury magnetic field|publisher=C. T. Russell & J. G. Luhmann|accessdate=2007-03-16}}</ref> It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about 90 to {{nowrap|700 ]}} (−183&nbsp;°C to 427&nbsp;°C, −297&nbsp;°F to 801&nbsp;°F),<ref name="ESAs&t">{{cite web|url=http://sci.esa.int/science-e/www/category/index.cfm?fcategoryid=4586|title=Background Science|publisher=European Space Agency|accessdate=2008-05-23}}</ref> with the ] being the hottest and the bottoms of craters near the ] being the coldest.


] of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called ]; the other visible only at sunset, which they called ].<ref name="Dunne">{{cite book|title=The Voyage of Mariner&nbsp;10 — Mission to Venus and Mercury|author=Dunne, J. A. and Burgess, E.|chapterurl=http://history.nasa.gov/SP-424/ch1.htm|publisher=NASA History Office|year=1978|chapter=Chapter One|url=http://history.nasa.gov/SP-424/}}</ref> The English name for the planet comes from the ], who named it after the ] ], which they equated with the Greek ] (Ἑρμῆς). The ] for Mercury is a stylized version of Hermes' ].<ref>{{cite book|title=Astronomy: A Textbook|first=John Charles|last=Duncan|year=1946|publisher=Harper & Brothers|pages=125|quote=The symbol for Mercury represents the Caduceus, a wand with two serpents twined ] of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called ]; the other visible only at sunset, which they called ].<ref name="Dunne">{{cite book|title=The Voyage of Mariner&nbsp;10 — Mission to Venus and Mercury|author=Dunne, J. A. and Burgess, E.|chapterurl=http://history.nasa.gov/SP-424/ch1.htm|publisher=NASA History Office|year=1978|chapter=Chapter One|url=http://history.nasa.gov/SP-424/}}</ref> The English name for the planet comes from the ], who named it after the ] ], which they equated with the Greek ] (Ἑρμῆς). The ] for Mercury is a stylized version of Hermes' ].<ref>{{cite book|title=Astronomy: A Textbook|first=John Charles|last=Duncan|year=1946|publisher=Harper & Brothers|pages=125|quote=The symbol for Mercury represents the Caduceus, a wand with two serpents twined
around it, which was carried by the messenger of the gods.}}</ref> around it, which was carried by the messenger of the gods.}}</ref>


==Internal structure==
Mercury is one of four ]s in the ], and is a rocky body like the Earth. It is the smallest planet in the Solar System, with an ]ial ] of 2,439.7&nbsp;km.<ref name="nssdcMercury" /> Mercury is even ]&mdash;albeit more massive&mdash;than the ] ]s in the Solar System, ] and ]. Mercury consists of approximately 70% ]lic and 30% ] material.<ref name="strom" /> Mercury's density is the second highest in the Solar System at 5.427&nbsp;g/cm³, only slightly less than Earth’s density of 5.515&nbsp;g/cm³.<ref name="nssdcMercury" /> If the effect of ] were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3&nbsp;g/cm³ versus Earth’s 4.4&nbsp;g/cm³.<ref>{{cite web
| author=staff | date=May 8, 2003
| url=http://astrogeology.usgs.gov/Projects/BrowseTheGeologicSolarSystem/MercuryBack.html
|title=Mercury
|publisher=U.S. Geological Survey
|accessdate=2006-11-26 }}</ref>

Mercury’s density can be used to infer details of its inner structure. While the Earth’s high density results appreciably from gravitational compression, particularly at the ], Mercury is much smaller and its inner regions are not nearly as strongly 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
| author=Lyttleton, R. A.
| journal=Astrophysics and Space Science
| volume=5
| issue=1
| pages=18
| year=1969
| accessdate=2008-04-16
| doi=10.1007/BF00653933 }}</ref>

]

Geologists estimate that Mercury’s core occupies about 42% of its volume; for Earth this proportion is 17%. Recent research strongly suggests 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 | publisher=Cornell University
| url=http://www.news.cornell.edu/stories/May07/margot.mercury.html
| work=Chronicle Online | accessdate =2008-05-12 }}</ref><ref name=nrao/> Surrounding the core is a 500&ndash;700&nbsp;km ] consisting of silicates.<ref>{{cite journal | author=Spohn, Tilman; Sohl, Frank; Wieczerkowski, Karin; Conzelmann, Vera | title=The interior structure of Mercury: what we know, what we expect from BepiColombo | journal=Planetary and Space Science | volume=49 | issue=14&ndash;15 | pages=1561&ndash;1570 | doi=10.1016/S0032-0633(01)00093-9 | bibcode=2001P&SS...49.1561S | year=2001 }}</ref><ref>Gallant, R. 1986. ''The National Geographic Picture Atlas of Our Universe''. National Geographic Society, 2nd edition.</ref> Based on data from the ''Mariner&nbsp;10'' mission and Earth-based observation, Mercury’s ] is believed to be 100–300&nbsp;km thick.<ref name="anderson1">{{cite journal
| author=J.D. Anderson, et al. | title=Shape and Orientation of Mercury from Radar Ranging Data
| publisher=Jet Propulsion Laboratory, California Institute of Technology | date=July 10, 1996
| doi=10.1006/icar.1996.0242 | journal=Icarus | volume=124 | pages=690 }}</ref> One distinctive feature of Mercury’s surface is the presence of numerous narrow ridges, and these can extend up to several hundred kilometers. It is believed 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
| author = Schenk, P.; Melosh, H. J.;
| journal = Abstracts of the 25th Lunar and Planetary Science Conference
| volume = 1994
| pages = 1994LPI....25.1203S
| accessdate =2008-06-03
| url = http://adsabs.harvard.edu/abs/1994LPI....25.1203S }}
</ref><!-- CHRONOLOGY OF LOBATE SCARP THRUST FAULTS AND THE MECHANICAL STRUCTURE OF
MERCURY’S LITHOSPHERE T. R. Watters , F. Nimmo and M. S. Robinson http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1886.pdf OR Geology; November 1998; v. 26; no. 11; p. 991-994, Topography of lobate scarps on Mercury; new constraints on the planet's contraction Thomas R. Watters, Mark S. Robinson, and Anthony C. Cook OR might be the better refs-->

Mercury's core has a higher iron content than that of any other major 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 ] meteors, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass.<ref name="Benz">
{{cite journal
| title=Collisional stripping of Mercury’s mantle
| author=Benz, W.; Slattery, W. L.; Cameron, A. G. W.
| journal=Icarus
| volume=74
| issue=3
| pages=516–528
| year=1988
| accessdate=2008-04-16
| doi=10.1016/0019-1035(88)90118-2 }}</ref> However, early in the solar system’s history, Mercury may have been struck by a ] of approximately 1/6 that mass and several hundred 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 has been proposed to explain the formation of Earth’s ] (''see ]'').<ref name="Benz" />

Alternatively, Mercury may have formed from the ] before the Sun's ] output had stabilized. The planet would initially have had twice its present mass, but as the ] contracted, temperatures near Mercury could have been between 2,500 and 3,500&nbsp;K (Celsius equivalents about 273 degrees less), and possibly even as high as 10,000&nbsp;K.<ref name="CameronAGW1">
{{cite journal | title = The partial volatilization of Mercury | author = Cameron, A. G. W. | journal = Icarus | volume = 64 | issue = 2| pages = 285–294 | year = 1985 | doi = 10.1016/0019-1035(85)90091-0 }}</ref> Much of Mercury’s surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" which could have been carried away by the ].<ref name="CameronAGW1" />

A third hypothesis proposes that the ] caused ] on the particles from which Mercury was ], which meant that lighter particles were lost from the accreting material.<ref>{{cite journal
| title = Iron/silicate fractionation and the origin of Mercury
| author = Weidenschilling, S. J.
| journal = Icarus
| volume = 35
| issue = 1
| pages = 99–111
| year = 1987
| accessdate = 2008-04-16
| doi = 10.1016/0019-1035(78)90064-7}}</ref> Each of these hypotheses predicts a different surface composition, and two upcoming space missions, ] and ], both aim to make observations to test them.<ref name="MSGRgrayzeck">{{cite web| first=Ed | last=Grayzeck | url=http://messenger.jhuapl.edu/| title=MESSENGER Web Site | publisher=Johns Hopkins University |accessdate=2008-04-07 }}</ref><ref name="ESA pages">{{cite web| url=http://sci.esa.int/science-e/www/area/index.cfm?fareaid=30| title=BepiColombo | work=ESA Science & Technology| publisher=European Space Agency| accessdate=2008-04-07 }}</ref>
{{clear}}

==Surface geology==
]
] is just below center. An extensive ] emanates from the ] near the top.]]

{{main|Geology of Mercury}}
Mercury’s surface is overall very similar in appearance to that of the Moon, showing extensive ]-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. Since our knowledge of ] has been based on the 1975 ] flyby and ] observations, it is the least understood of the terrestrial planets.<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/
| accessdate=2008-05-12 }}</ref> As data from the recent ] flyby is processed this knowledge will increase. For example, an unusual crater with radiating troughs has been discovered which scientists are calling "the spider."<ref>{{cite news
| author=Staff | title=Scientists see Mercury in a new light
| url=http://www.sciencedaily.com/releases/2008/02/080201093149.htm
| publisher=Science Daily | date=February 28, 2008
| accessdate=2008-04-07 }}</ref>

] features refer to areas of markedly different reflectivity, as seen by telescopic observation. Mercury possesses ] (also called "]s"), Moon-like ], Montes (mountains), ]e, or plains, ] (]), and ] (]).<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 | accessdate=2008-04-11 }}</ref><ref name="DunneCh7">{{cite book|title=The Voyage of Mariner&nbsp;10 — Mission to Venus and Mercury|author=Dunne, J. A. and Burgess, E.|chapterurl=http://history.nasa.gov/SP-424/ch7.htm|publisher=NASA History Office|year=1978|chapter=Chapter Seven|url=http://history.nasa.gov/SP-424/|accessdate=2008-05-28}}</ref>

Mercury was heavily bombarded by ]s and ]s during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the ] that came to an end 3.8 billion years ago.<ref>{{cite journal|author=Strom, Robert|month=September | year=1979 |volume=24|title=Mercury: a post-Mariner assessment|journal=Space Science Reviews|pages=3–70}}</ref> During this period of intense crater formation, the planet received impacts over its entire surface,<ref name="DunneCh7" /> facilitated by the lack of any ] to slow impactors down.<ref>{{cite journal|last=Broadfoot|first=A. L.|coauthors=S. Kumar, M. J. S. Belton, and M. B. McElroy|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}}</ref> During this time the planet was ] active; basins such as the ] were filled by ] from within the planet, which produced smooth plains similar to the ] found on the Moon.<ref>{{cite web
| author=Staff
| date=August 5, 2003
| url=http://astrogeology.usgs.gov/Projects/BrowseTheGeologicSolarSystem/MercuryBack.html
| title=Mercury
| publisher=U.S. Geological Survey
| accessdate=2008-04-07 }}
</ref><ref>{{cite journal
| last=Head | first=James W.
| coauthors=Solomon, Sean C.
| title=Tectonic Evolution of the Terrestrial Planets
| journal=Science
| year=1981 | volume=213
| issue=4503 | pages=62–76
| url=http://www.sciencemag.org/cgi/content/abstract/213/4503/62
| doi=10.1126/science.213.4503.62
| accessdate=2008-04-07
| pmid=17741171 }}</ref>

Data from the October 2008 flyby of MESSENGER gave researchers a greater appreciation for the jumbled nature of Mercury's surface. Mercury's surface is more heterogeneous than either Mars or earth's Moon, both of which contain significant stretches of similar geology, such as maria and plateaus.<ref>Jefferson Morris, "Laser Altimetry", ] Vol 169 No 18, 10 Nov. 2008, p. 18: "Mercury's crust is more analogous to a marbled cake than a layered cake."</ref>
{{clear}}
===Impact basins and craters===
] is one of the largest impact features in the Solar System.]]

]s on Mercury range in diameter from small bowl-shaped cavities to multi-ringed ]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=P. D. | last=Spudis | title=The Geological History of Mercury
| journal=Workshop on Mercury: Space Environment, Surface, and Interior, Chicago
| year=2001 |pages=100 | url=http://adsabs.harvard.edu/abs/2001mses.conf..100S | accessdate=2008-06-03 }}</ref>

The largest known craters are {{dp|Caloris Basin}}, with a diameter of 1,550&nbsp;km,<ref name="newscientist30012008">{{cite news
| url=http://space.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}}</ref> and the ] with an outer-ring diameter of 2,300&nbsp;km.<ref name=Ksa06>{{cite journal|author = L. V. Ksanfomality|title=Earth-based optical imaging of Mercury| journal= Advances in Space Research |volume= 38|pages= 594|year= 2006|url= http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2006AdSpR..38..594K&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=461152a03222956|doi=10.1016/j.asr.2005.05.071}}</ref> The impact that created the Caloris Basin was so powerful that it caused ] eruptions and left a concentric ring over 2&nbsp;km tall surrounding the ]. At the ] 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 the planet, converging at the basin’s antipode (180 degrees away). The resulting high stresses fractured the surface.<ref>{{cite journal
|last=Schultz
|first=Peter H.
|authorlink=
|coauthors=Gault, Donald E.
|year=1975
|month=
|title=Seismic effects from major basin formations on the moon and Mercury
|journal=Earth, Moon, and Planets
|volume=12
|issue= 2
|pages=159–175
|id=
|doi = 10.1007/BF00577875
|url=http://adsabs.harvard.edu/abs/1975Moon...12..159S
|accessdate=2008-04-16
}}</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
| last=Wieczorek | first=Mark A. | coauthors=Zuber, Maria T.
| 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
| accessdate=2008-05-12
| doi=10.1029/2000JE001384 }}</ref>

Overall, about 15 impact basins have been identified on the imaged part of Mercury. A notable basin is the 400&nbsp;km wide, multi-ring ] which has an ejecta blanket extending up to 500&nbsp;km from its rim and a floor that has been filled by smooth plains materials. ] has a similar-sized ejecta blanket and a 625&nbsp;km diameter rim.<ref name=Spudis01/> Like the ], the surface of Mercury has likely incurred the effects of ] processes, including ] and ] 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 | year=2008 | pages=1750
| last=Denevi | first=B. W. | coauthors=Robinson, M. S.
| url=http://adsabs.harvard.edu/abs/2008LPI....39.1750D | accessdate=2008-06-03 }}</ref>
{{clear}}

===Plains===
] impact at its antipodal point.]]
There are two geologically distinct plains regions on Mercury.<ref name=Spudis01/><ref name=WagWolIva01>{{cite journal|author=R.J. Wagner ''et al.''|title=Application of an Updated Impact Cratering Chronology Model to Mercury's Time-Stratigraphic System|journal=Workshop on Mercury: Space Environment, Surface, and Interior, Chicago|year=2001|pages=106|url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001mses.conf..106W&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=4613707b1d22308}}</ref> Gently rolling, hilly 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 30&nbsp;km in diameter.<ref name=WagWolIva01/> It is not clear whether they are of volcanic or impact origin.<ref name=WagWolIva01/> The inter-crater plains are distributed roughly uniformly over the entire surface of the planet.

Smooth plains are widespread flat areas which fill depressions of various sizes and bear a strong resemblance to the lunar maria. Notably, they fill a wide ring surrounding the Caloris Basin. 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 localisation and rounded, lobate shape of these plains strongly support volcanic origins.<ref name=Spudis01/> All the Mercurian smooth plains formed significantly later than the Caloris basin, as evidenced by appreciably smaller crater densities than on the Caloris ejecta blanket.<ref name=Spudis01/> 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 are volcanic lavas induced by the impact, or a large sheet of impact melt.<ref name=Spudis01/>

One unusual feature of the planet’s surface is the numerous compression folds, or ], which crisscross the plains. As the planet’s interior cooled, it may have contracted and its surface began to deform, creating these features. The folds can be seen on top of other features, such as craters and smoother plains, indicating that the folds are more recent.<ref>{{cite journal
|last=Dzurisin |first=D. |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 |pages=4883–4906
|url=http://adsabs.harvard.edu/abs/1978JGR....83.4883D |accessdate=2008-06-03
|doi=10.1029/JB083iB10p04883 }}</ref> Mercury’s surface is flexed by significant ]s raised by the ]—the Sun’s tides on Mercury are about 17 times stronger than the Moon’s on Earth.<ref>{{cite journal
|last=Van Hoolst |first=Tim |coauthors=Jacobs, Carla |year=2003
|title=Mercury’s tides and interior structure
|journal=Journal of Geophysical Research
|volume=108 |issue=E11 |pages=7
|doi=10.1029/2003JE002126 |accessdate=2008-04-16 }}</ref>
{{clear}}

==Surface conditions and "atmosphere" (exosphere)==
]
], ], and ]]]

The ] surface ] of Mercury is 442.5&nbsp;],<ref name="nssdcMercury" /> but it ranges from 100&nbsp;K to 700&nbsp;K<ref>{{cite book
| author=Prockter, Louise
| title=Ice in the Solar System
| publisher=Johns Hopkins APL Technical Digest
| volume=Volume 26 | issue=number 2 | year=2005
| url=http://www.jhuapl.edu/techdigest/td2602/Prockter.pdf
| accessdate=2009-07-27 }}</ref> due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. The subsolar point reaches about 700&nbsp;K during ] then drops to 550&nbsp;K at ].<ref>{{cite book
| first=John S. | last=Lewis | year=2004
| title=Physics and Chemistry of the Solar System | page=463
| edition=2nd | publisher=Academic Press | isbn=012446744X }}</ref> On the dark side of the planet, temperatures average 110&nbsp;K.<ref>{{cite journal
| last=Murdock | first=T. L. | coauthors=Ney, E. P.
| title=Mercury: The Dark-Side Temperature
| journal=] | year=1970
| volume=170 | issue=3957 | pages=535–537 | url=http://www.sciencemag.org/cgi/content/abstract/170/3957/535
| doi=10.1126/science.170.3957.535 | accessdate=2008-04-09
| pmid=17799708 }}</ref> The intensity of ] on Mercury’s surface ranges between 4.59 and 10.61 times the ] (1,370 W·m<sup><small>&minus;2</small></sup>).<ref>{{cite book
| title=Physics and Chemistry of the Solar System
| author=Lewis, John S. | page=461
| publisher=Academic Press | year=2004 | url=http://books.google.co.uk/books?id=ERpMjmR1ErYC&pg=RA1-PA461&lpg=RA1-PA461&dq=solar-constant+mercury+-wikipedia+-wiki+-encyclopedia&source=web&ots=5jprP6dXYk&sig=iJEN0OU01yxgxnZhPcG17z-exYw&hl=en#PRA1-PA461,M1
| accessdate=2008-06-03}}</ref>

Despite the generally extremely high temperature of its surface, observations strongly suggest that ] 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
| author=Ingersoll, Andrew P.; Svitek, Tomas; Murray, 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&ndash;47
| month=November | year=1992 | bibcode=1992Icar..100...40I
| doi=10.1016/0019-1035(92)90016-Z }}</ref> Water ice strongly reflects ], and observations by the 70 m ] telescope and the ] in the early 1990s revealed that there are patches of very high radar ] near the poles.<ref>{{cite journal
| last=Slade | first=M. A.
| coauthors=Butler, B. J.; Muhleman, D. O. | year=1992
| title=Mercury radar imaging — Evidence for polar ice
| journal=]
| volume=258 | issue=5082 | pages=635–640
| doi=10.1126/science.258.5082.635 | accessdate=2008-04-16
| pmid=17748898 }}</ref> While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.<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
| accessdate=2008-05-23 }}</ref>

The icy regions are believed to contain about 10<sup>14</sup>–10<sup>15</sup>&nbsp;kg of ice,<ref name="Zahnle1">{{cite journal
| last=Rawlins |first=K
| coauthors=Moses, J. I.; Zahnle, K.J.
| 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
| pages=1117 }}</ref> and may be covered by a layer of ] that inhibits sublimation.<ref>{{cite journal
| author=Harmon, J. K.; Perillat, P. J.; Slade, M. A.
| title=High-Resolution Radar Imaging of Mercury's North Pole
| journal=Icarus | volume=149 | issue=1 | pages=1&ndash;15
| year=2001 | month=January | doi=10.1006/icar.2000.6544 }}</ref> By comparison, the ] ice sheet on Earth has a mass of about 4{{e|18}}&nbsp;kg, and ]' 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 ] of water from the planet’s interior or deposition by impacts of ]s.<ref name="Zahnle1" />

Mercury is too small for its ] to retain any significant ] over long periods of time; however, it does have a "tenuous surface-bounded ]"<ref>{{cite journal
| author=Domingue, Deborah L. ''et al.''
| title=Mercury's Atmosphere: A Surface-Bounded Exosphere
| journal=Space Science Reviews | volume=131
| issue=1&ndash;4 | pages=161&ndash;186 | year=2009
| month=August | doi=10.1007/s11214-007-9260-9
| url=http://adsabs.harvard.edu/abs/2007SSRv..131..161D
}}</ref> containing ], ], ], ], ] and ]. This exosphere is not stable—atoms are continuously lost and replenished from a variety of sources. Hydrogen and helium atoms probably come from the ], ] into Mercury’s magnetosphere before later escaping back into space. ] of elements within Mercury’s crust is another source of helium, as well as sodium and potassium. ] found high proportions of calcium, helium, ], ], oxygen, potassium, ] and sodium. Water vapor is present, being released by a combination of processes such as: comets striking its surface, ] creating water out of hydrogen from the ] 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>-</sup>, and H<sub>2</sub>O<sup>+</sup> was a surprise.<ref>{{cite book
| author=Hunten, D. M.; Shemansky, D. E.; Morgan, T. H.
| year=1988 | publisher=University of Arizona Press
| isbn=0-8165-1085-7 | chapter=The Mercury atmosphere
| title=Mercury | chapterurl=www.uapress.arizona.edu/onlinebks/Mercury/MercuryCh17.pdf
| accessdate=2009-05-18
}}</ref><ref>{{cite news
| first=Emily | last=Lakdawalla | date=July 3, 2008
| title=MESSENGER Scientists 'Astonished' to Find Water in Mercury's Thin Atmosphere | url=http://www.planetary.org/news/2008/0703_MESSENGER_Scientists_Astonished_to.html
| accessdate=2009-05-18 }}</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
| author=Zurbuchen, Thomas H. ''et al.''
| title=MESSENGER Observations of the Composition of Mercury’s Ionized Exosphere and Plasma Environment
| journal=Science | volume=321 | issue=5885
| pages=90&ndash;92 | month=July | year=2008
| doi=10.1126/science.1159314 | pmid=18599777
}}</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/
| accessdate=2009-05-18 }}</ref>

Sodium and potassium were discovered in the atmosphere during the 1980s, and are believed to result primarily from the vaporization of surface rock struck by micrometeorite impacts. Due to the ability of these materials to diffuse sunlight, Earth-based observers can readily detect their composition in the atmosphere. Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic dipoles. This would indicate an interaction between the magnetosphere and the planet's surface.<ref name="chaikin1" />

==Magnetic field and magnetosphere==
]

Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, ]. According to measurements taken by {{nowrap|Mariner 10}}, it is about 1.1% as strong as the Earth’s. The magnetic field strength at the Mercurian equator is about {{nowrap|300 ]}}.<ref>{{cite book
| title=Astronomy: The Solar System and Beyond
| first=Michael A. | last=Seeds | year=2004
| isbn=0534421113 | 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
| accessdate=2006-08-10 }}</ref> Like that of Earth, Mercury's magnetic field is ] in nature.<ref name="chaikin1">{{cite book
| first=J. Kelly | last=Beatty | coauthors=Petersen, Carolyn Collins; Chaikin, Andrew
| title=The New Solar System | year=1999
| publisher=Cambridge University Press | isbn=0521645875 }}</ref> Unlike Earth, however, Mercury's poles are nearly aligned with the planet's spin axis.<ref name="qq">{{cite web
| author=Staff | date=January 30, 2008 | url=http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?page=2&gallery_id=2&image_id=152
| title=Mercury’s Internal Magnetic Field
| publisher=NASA | accessdate=2008-04-07}}</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 way of a ] 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
| accessdate=2008-04-07 }}</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 }}</ref> This dynamo effect would result from the circulation of the planet's iron-rich liquid core. Particularly strong tidal effects caused by the planet's high orbital eccentricity would serve to keep the core in the liquid state necessary for this dynamo effect.<ref>{{cite journal
| last=Spohn | first=T.
| coauthors=Sohl, F.; Wieczerkowski, K.; Conzelmann, V.
| title=The interior structure of Mercury: what we know, what we expect from BepiColombo
| journal=Planetary and Space Science | year=2001
| volume=49 | issue=14–15 | pages=1561–1570
| doi=10.1016/S0032-0633(01)00093-9 }}</ref>

Mercury’s magnetic field is strong enough to deflect the ] around the planet, creating a ]. The planet's magnetosphere, though small enough to fit within the Earth,<ref name="chaikin1" /> is strong enough to trap solar wind plasma. This contributes to the ] 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 were detected in the planet's magnetotail, which indicates 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 'tornadoes' form when magnetic fields carried by the solar wind connect to Mercury's magnetic field. As the solar wind blows past Mercury's field, these joined magnetic fields are carried with it and twist up into vortex-like structures. These twisted magnetic flux tubes, technically known as ]s, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface.<ref name="NASA060209"/>

The process of linking interplanetary and planetary magnetic fields, called ], is common throughout the cosmos. It occurs in Earth's magnetic field, where it generates magnetic tornadoes as well. However, the MESSENGER observations show the reconnection rate is ten times higher at Mercury. Mercury's 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
| accessdate=2009-07-18 }}</ref>

==Orbit and rotation==
]

Mercury has the most ] orbit of all the planets; its eccentricity is 0.21 with its distance from the Sun ranging from 46 to 70 million kilometers. It takes 88 days to complete an orbit.
The diagram on the right illustrates the effects of the eccentricity, showing Mercury’s orbit overlaid with a circular orbit having the same ]. The higher velocity of the planet when it is near perihelion is clear from the greater distance it covers in each 5-day interval. The size of the spheres, inversely proportional to their distance from the Sun, is used to illustrate the varying heliocentric distance. This varying distance to the Sun, combined with a 3:2 ] of the planet’s rotation around its axis, result in complex variations of the surface temperature.<ref name=strom>{{cite book
| first=Robert G. | last=Strom
| coauthors=Sprague, Ann L. | year=2003
| title=Exploring Mercury: the iron planet
| publisher=Springer | isbn=1852337311 }}</ref>

A solar day on Mercury lasts about 176 Earth days, which is about twice as long as Mercury's orbital period, roughly 88 Earth Days. As a result, a Mercury year is about 0.5 Mercury days long, and one Mercury day lasts approximately two Mercury years.<ref name="compare">{{cite web
| title=Space Topics: Compare the Planets: Mercury, Venus, Earth, The Moon, and Mars
| publisher=Planetary Society
| url= http://www.planetary.org/explore/topics/compare_the_planets/terrestrial.html
| accessdate=2007-04-12 }}</ref>

Mercury’s orbit is inclined by 7° to the plane of Earth’s orbit (the ]), as shown in the diagram on the right. As a result, ]s of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between the Earth and the Sun. This occurs about every seven years on average.<ref>{{cite web
| last=Espenak | first=Fred | date=April 21, 2005
| url=http://eclipse.gsfc.nasa.gov/transit/catalog/MercuryCatalog.html
| title=Transits of Mercury
| publisher=NASA/Goddard Space Flight Center
| accessdate=2008-05-20 }}</ref>

]

Mercury’s ] is almost zero,<ref name="JPLweather">{{cite web|url=http://solarsystem.nasa.gov/scitech/display.cfm?ST_ID=725|title=Weather, Weather, Everywhere?|author=Samantha Harvey|publisher=] Jet Propulsion Laboratory|date=April 24, 2008 |accessdate=2008-05-23}}</ref><ref name="Cosmic1">{{cite book|title=Cosmic Perspectives in Space Physics|author=S. Biswas|publisher=Springer|year=2000|pages=176}}</ref> with the best measured value as low as 0.027°.<ref name=Margot2007/> This is significantly smaller than that of ], which boasts the second smallest axial tilt of all planets at 3.1 degrees. This means that to an observer at Mercury’s poles, the center of the Sun never rises more than 2.1′ above the horizon.<ref name=Margot2007/>

At certain points on Mercury’s surface, an observer would be able to see the Sun rise about halfway, then reverse and set before rising again, all within the same ]. This is because approximately four days prior to ], Mercury’s angular ] exactly equals its angular ] so that the Sun’s ] ceases; at perihelion, Mercury’s angular orbital velocity then exceeds the angular rotational velocity. Thus, the Sun appears to move in a ] direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.<ref name="strom" />

{{clear}}
===Spin–orbit resonance===
]

For many years it was thought that Mercury was synchronously ] with the Sun, ] once for each orbit and keeping the same face directed towards the Sun at all times, in the same way that the same side of the Moon always faces the Earth. However, ] observations in 1965 proved that the planet has a 3:2 spin–orbit resonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury’s orbit makes this resonance stable—at perihelion, when the solar tide is strongest, the Sun is nearly still in Mercury’s sky.<ref>{{cite journal
| last=Liu | first=Han-Shou | coauthors=O'Keefe, John A.
| title=Theory of Rotation for the Planet Mercury
| journal=Science | year=1965 | volume=150
| issue=3704 | pages=1717
| doi=10.1126/science.150.3704.1717
| pmid=17768871 }}</ref>

The original reason astronomers thought it was synchronously locked, was that whenever Mercury was best placed for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the same face. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period with respect to Earth. Due to Mercury’s 3:2 spin–orbit resonance, a ] (the length between two ] ]s of the Sun) lasts about 176 Earth days.<ref name="strom" /> A ] (the period of rotation) lasts about 58.7 Earth days.<ref name="strom" />

Simulations indicate that the ] of Mercury varies ] from nearly zero (circular) to more than 0.45 over millions of years due to ] from the other planets.<ref name="strom" /><ref name=Correia2009>{{cite journal
|last=Correia |first=Alexandre C.M
|coauthors=Laskar, Jacques
|title=Mercury's capture into the 3/2 spin-orbit resonance including the effect of core-mantle friction
|journal=Icarus |year=2009
|doi=10.1016/j.icarus.2008.12.034
|url=http://arxiv.org/abs/0901.1843
|accessdate=2009-03-03 }}</ref> This is thought to explain Mercury’s 3:2 spin-orbit resonance (rather than the more usual 1:1), since this state is more likely to arise during a period of high eccentricity.<ref name="Correia">{{cite journal
| last=Correia | first=Alexandre C. M.
| coauthors=Laskar, Jacques | year=2004
| title=Mercury’s capture into the 3/2 spin–orbit resonance as a result of its chaotic dynamics
| journal=] | volume=429
| pages=848–850 | doi=10.1038/nature02609 }}</ref>

===Advance of perihelion===
{{main|Tests of general relativity#Perihelion_precession_of_Mercury}}
In 1859, the French ] and astronomer ] reported that the slow ] of Mercury’s orbit around the Sun could not be completely explained by ] and perturbations by the known planets. He suggested, among possible explanations, that another planet (or perhaps instead a series of smaller 'corpuscules') might exist in an orbit even closer to the Sun than that of Mercury, to account for this perturbation.<ref>U. Le Verrier (1859), (in French), , Comptes rendus hebdomadaires des séances de l'Académie des sciences (Paris), vol. 49 (1859), pp.379-383. (At p.383 in the same volume Le Verrier's report is followed by another, from Faye, enthusiastically recommending to astronomers to search for a previously undetected intra-mercurial object.)</ref> (Other explanations considered included a slight oblateness of the Sun.) The success of the search for ] based on its perturbations of the orbit of ] led astronomers to place faith in this possible explanation, and the hypothetical planet was even named ]. However, no such planet was ever found.<ref>{{cite book
| first=Richard | last=Baum |coauthors=Sheehan, William
| title = In Search of Planet Vulcan, The Ghost in Newton's Clockwork Machine
| year = 1997 | isbn=0-306-45567-6
| publisher=Plenum Press
| location=New York }}</ref>

The precession of Mercury is 5600 arc seconds per century. Newtonian mechanics, taking into account all the effects from the other planets, predicts a precession of 5557 seconds of arc per century.<ref>{{cite journal
| first=G. M. | last=Clemence
| title=The Relativity Effect in Planetary Motions
| journal=Reviews of Modern Physics | volume=19
| issue=4 | pages=361&ndash;364 | month=October
| year=1947 | doi=10.1103/RevModPhys.19.361 }}</ref> In the early 20th century, ]’s ] provided the explanation for the observed precession. The effect is very small: the Mercurian relativistic perihelion advance excess is just 42.98 ]s per century, therefore it requires a little over twelve million orbits for a full excess turn. Similar, but much smaller, effects operate for other planets, being 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for ].<ref>{{cite journal
| last=Gilvarry | first=J. J.
| title=Relativity Precession of the Asteroid Icarus
| journal=Physical Review
| year=1953 | volume=89 | issue=5 | pages=1046
| doi=10.1103/PhysRev.89.1046
| url=http://prola.aps.org/abstract/PR/v89/i5/p1046_1
| accessdate=2008-05-22
| format=subscription required }}
</ref><ref>{{cite web
| author=Anonymous
| url=http://www.mathpages.com/rr/s6-02/6-02.htm
| title=6.2 Anomalous Precession
| work=Reflections on Relativity
| publisher=MathPages | accessdate=2008-05-22 }}</ref>

==Coordinate system==

Longitude on Mercury increases in the westerly direction. A small crater named ] provides the reference point for measuring longitude. By definition, the center of Hun Kal is 20° west longitude.<ref>{{cite web|url=http://astrogeology.usgs.gov/Projects/WGCCRE/constants/iau2000_table1.html|accessdate=22 October 2009|title=USGS Astrogeology: Rotation and pole position for the Sun and planets (IAU WGCCRE)}}</ref>

==Observation==
Mercury’s ] varies between about −2.3—brighter than ]—and 5.7, with the extremes being when Mercury is very close to the Sun in the sky.<ref name=ephemeris>{{cite web
| last=Espenak | first=Fred | date=July 25, 1996
| url=http://eclipse.gsfc.nasa.gov/TYPE/mercury2.html
| title=Twelve Year Planetary Ephemeris: 1995&ndash;2006
| work=NASA Reference Publication 1349
| publisher=NASA | accessdate=2008-05-23 }}</ref> Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun’s glare for much of the time. Mercury can be observed for only a brief period during either morning or evening twilight. The ] cannot observe Mercury at all, due to safety procedures which prevent its pointing too close to the Sun.<ref>{{cite journal
| last=Baumgardner | first=Jeffrey
| coauthors=Mendillo, Michael; Wilson, Jody K.
| title=A Digital High-Definition Imaging System for Spectral Studies of Extended Planetary Atmospheres. I. Initial Results in White Light Showing Features on the Hemisphere of Mercury Unimaged by ''Mariner'' 10
| journal=The Astronomical Journal | year=2000 | volume=119
| pages=2458&ndash;2464 | doi=10.1086/301323 }}</ref>

Like the Moon, Mercury exhibits ] as seen from Earth, being "new" at ] and "full" at ]. The planet is rendered invisible on both of these occasions by virtue of its rising and setting in concert with the Sun in each case. The first and last quarter phases occur at greatest ] east and west, respectively, when Mercury's separation from the Sun ranges anywhere from 17.9° at ] to 27.8° at ].<ref name=elongation>{{cite web
|title=Mercury Chaser's Calculator
|publisher=Fourmilab Switzerland
|author=John Walker
|url=http://www.fourmilab.ch/images/3planets/elongation.html
|accessdate=2008-05-29}} (look at 1964 and 2013)</ref><ref name=MercHorizons>{{cite web
|title=Mercury Elognation and Distance
|url=http://home.comcast.net/~kpheider/Mercury.txt
|accessdate=2008-05-30}} &mdash;Numbers generated using the Solar System Dynamics Group, .</ref> At greatest elongation west, Mercury rises at its earliest before the Sun, and at greatest elongation east, it sets at its latest after the Sun.<ref name="RASC2007">{{cite book|title=Observer's Handbook 2007|author=Patrick Kelly, ed.|publisher=]|year=2007|isbn=0-9738109-3-9}}</ref>

Mercury attains inferior conjunction every 116 days on average,<ref name="nssdcMercury" /> but this interval can range from 105 days to 129 days due to the planet’s eccentric orbit. Mercury can come as close as 77.3&nbsp;million km to the Earth.<ref name="nssdcMercury" /> In 871, the nearest approach was the first in about 41,000 years to be closer than 82.2&nbsp;Gm, something that has happened 68 times since then, as of 2008. After much longer gaps, the next approach to within 82.1&nbsp;Gm is in 2679, and to 82&nbsp;Gm in 4487. But it will not be closer to Earth than 80&nbsp;Gm until 28,622.{{clarifyme|date=March 2009|please provide instructions to help verifiers; preceding numbers not found on following cites; I roughly verified home.comcast.net/~kpheider/Mercury.txt against NASA's ssd.jpl.nasa.gov/horizons.cgi as correct which would support "NASA's simulations show at least six closer approaches between 1961 and 2154 and an approach closer than 82,100,000km on 2679-Jun-11" but how does that support all the previous sentences? NASA reports "No ephemeris for target "Mercury" after A.D. 3000-MAY-05"}}<ref name=multisource>Mercury Closest Approaches to Earth generated with:<br>1. ()<br>2. <br>3. {{Hide in print|<br>(3 sources are provided to prevent <nowiki>{{OR}}</nowiki> concerns and to support general long-term trends)}}</ref> In its period of ] as seen from Earth can vary from 8 to 15 days on either side of inferior conjunction. This large range arises from the planet’s high ].<ref name=strom />

Mercury is more often easily visible from Earth’s ] than from its ]; this is because its maximum possible elongations west of the Sun always occur when it is early autumn in the Southern Hemisphere, while its maximum possible eastern elongations happen during late winter in the Southern Hemisphere.<ref name="RASC2007" /> In both of these cases, the angle Mercury strikes with the ] is maximized, allowing it to rise several hours before the Sun in the former instance and not set until several hours after sundown in the latter in countries located at southern temperate zone latitudes, such as ] and ].<ref name="RASC2007" /> By contrast, at northern temperate latitudes, Mercury is never above the horizon of a more-or-less fully dark night sky. Mercury can, like several other planets and the brightest stars, be seen during a total ].<ref name=eclipse>{{cite web
|date=January 22, 2003
|title=Total Solar Eclipse of 2006 March 29
|publisher=Department of Physics at Fizik Bolumu in Turkey
|author=Tunç Tezel
|url=http://www.physics.metu.edu.tr/~aat/TSE2006/TSE2006.html
|accessdate=2008-05-24}}</ref>

Mercury is brightest as seen from Earth when it is at a ], between either quarter phase and full. Although the planet is further away from Earth when it is gibbous than when it is a crescent, the greater illuminated area visible more than compensates for the greater distance.<ref name=ephemeris/> The opposite is true for Venus, which appears brightest when it is a thin crescent, because it is much closer to Earth than when gibbous.<ref>{{cite web
| last=Espenak | first=Fred| year=1996
| url=http://sunearth.gsfc.nasa.gov/eclipse/TYPE/venus2.html
| title=NASA Reference Publication 1349; Venus: Twelve year planetary ephemeris, 1995&ndash;2006
| work=Twelve Year Planetary Ephemeris Directory
| publisher=NASA | accessdate=2008-05-24}}</ref>

==Studies==
===Ancient astronomers===
The earliest known recorded observations of Mercury are from the ] tablets. These observations were most likely made by an ]n astronomer around the 14th century BC.<ref>{{cite journal | title=The Latitude and Epoch for the Origin of the Astronomical Lore in MUL.APIN | first=Bradley E. | last=Schaefer | journal=American Astronomical Society Meeting 210, #42.05 | year=2007 | month=May | url=http://cdsads.u-strasbg.fr/abs/2007AAS...210.4205S | publisher=American Astronomical Society}}</ref> The ] name used to designate Mercury on the MUL.APIN tablets is transcribed as UDU.IDIM.GU<sub>4</sub>.UD ("the jumping planet").{{Ref_label|B|b|none}}<ref>{{cite journal
| first=Hermann | last=Hunger |coauthors=Pingree, David
| title=MUL.APIN: An Astronomical Compendium in Cuneiform
| journal=Archiv für Orientforschung | volume=24
| publisher=Verlag Ferdinand Berger & Sohne Gesellschaft MBH
| location=Austria | year=1989 | pages=146 }}</ref> Babylonian records of Mercury date back to the 1st millennium BC. The Babylonians called the planet ] after the messenger to the Gods in their ].<ref name="JHU history">{{cite web
| year=2008 | author=Staff
| url=http://btc.montana.edu/messenger/elusive_planet/ancient_cultures_2.php
| title=MESSENGER: Mercury and Ancient Cultures
| publisher=NASA JPL | accessdate=2008-04-07 }}</ref>

The ] of ]'s time knew the planet as Στίλβων (''Stilbon''), meaning "the gleaming", and Ἑρμάων (''Hermaon'').<ref>{{cite book |author= H.G. Liddell and R. Scott |coauthors=''rev.'' H.S. Jones and R. McKenzie |title=Greek–English Lexicon, with a Revised Supplement |edition=9th edition |year=1996 |publisher=Clarendon Press |location=Oxford |isbn=0-19-864226-1 |pages=690 and 1646 }}</ref> Later Greeks called the planet ] when it was visible in the morning sky and ] when visible in the evening. Around the 4th century BC, however, Greek astronomers came to understand that the two names referred to the same body. The Romans named the planet after the swift-footed Roman messenger god, ] (Latin ''Mercurius''), which they equated with the Greek ], because it moves across the sky faster than any other planet.<ref name="Dunne" /><ref>{{cite book
| first=Eugène Michel | last=Antoniadi
| coauthors=Translated from French by Moore, Patrick
| year=1974 | title=The Planet Mercury
| publisher=Keith Reid Ltd | location=Shaldon, Devon
| pages=9&ndash;11 }}</ref>

In ], Mercury was known as Ch'en-Hsing, the Hour Star. It was associated with the direction north and the phase of water in the ].<ref>{{cite book
| first=David H. | last=Kelley
| coauthors=Milone, E. F.; Aveni, Anthony F. | year=2004
| title=Exploring Ancient Skies: An Encyclopedic Survey of Archaeoastronomy
| publisher=Birkhäuser | isbn=0387953108 }}</ref> ] used the name ] for Mercury, and this god was thought to preside over ].<ref>{{cite book
| first=R.M. | last=Pujari
| coauthors=Kolhe, Pradeep; Kumar, N. R. | year=2006
| title=Pride of India: A Glimpse Into India's Scientific Heritage
| publisher=Samskrita Bharati | isbn=8187276274 }}</ref> The god ] (or Woden) of ] was associated with the planet Mercury and the ].<ref>{{cite book
| first=Michael E. | last=Bakich | year=2000
| title=The Cambridge Planetary Handbook
| publisher=Cambridge University Press
| isbn=0521632803 }}</ref> The ] may have represented Mercury as an owl (or possibly four owls; two for the morning aspect and two for the evening) that served as a messenger to the ].<ref>{{cite book
| first=Susan | last=Milbrath | year=1999
| title=Star Gods of the Maya: Astronomy in Art, Folklore and Calendars
| publisher=University of Texas Press
| isbn=0292752261 }}</ref>

===Ground-based telescopic research===
]. Mercury is the small dot in the lower center, in front of the sun. The dark area on the left of the solar disk is a sunspot.]]

The first ] observations of Mercury were made by ] in the early 17th century. Although he observed ] when he looked at Venus, his telescope was not powerful enough to see the phases of Mercury. In 1631 ] made the first observations of the ] of a planet across the Sun when he saw a transit of Mercury predicted by ]. In 1639 ] used a telescope to discover that the planet had ]al phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited around the Sun.<ref name=strom/>

A very rare event in astronomy is the passage of one planet in front of another (]), as seen from Earth. Mercury and Venus occult each other every few centuries, and the event of May 28, 1737 is the only one historically observed, having been seen by ] at the ].<ref>{{cite journal |last=Sinnott |first=RW |authorlink= |coauthors=Meeus, J |year=1986 |month= |title=John Bevis and a Rare Occultation |journal=Sky and Telescope |volume=72 |issue= |pages=220 |id= |url=http://adsabs.harvard.edu/abs/1986S&T....72..220S |accessdate= |quote= }}</ref> The next occultation of Mercury by Venus will be on December 3, 2133.<ref>{{cite book
| first=Timothy | last=Ferris | year=2003
| title=Seeing in the Dark: How Amateur Astronomers
| publisher=Simon and Schuster
| isbn=0684865807 }}</ref>

The difficulties inherent in observing Mercury mean that it has been far less studied than the other planets. In 1800 ] made observations of surface features, claiming to have observed 20&nbsp;km high mountains. ] used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°.<ref name="sao188r">{{cite journal
| last=Colombo | first=G. | coauthors=Shapiro, I. I.
| title=The Rotation of the Planet Mercury
| journal=SAO Special Report #188R
| url=http://adsabs.harvard.edu/abs/1965SAOSR.188.....C
| accessdate=2008-05-23 }}</ref> In the 1880s ] mapped the planet more accurately, and suggested that Mercury’s rotational period was 88 days, the same as its orbital period due to ].<ref>{{cite journal
|last=Holden |first=E. S. |year=1890
|title=Announcement of the Discovery of the Rotation Period of Mercury
|journal=Publications of the Astronomical Society of the Pacific |volume=2 |issue=7 |pages=79
|url=http://adsabs.harvard.edu/abs/1890PASP....2...79H |accessdate=2008-06-03
|doi=10.1086/120099 }}</ref> This phenomenon is known as ] and is shown by Earth’s Moon. The effort to map the surface of Mercury was continued by ], who published a book in 1934 that included both maps and his own observations.<ref name="chaikin1" /> Many of the planet's surface features, particularly the ], take their names from Antoniadi's map.<ref>{{cite book|url=http://history.nasa.gov/SP-423/sp423.htm|title=Atlas of Mercury|publisher=] Office of Space Sciences|author=Merton E. Davies, et al.|year=1978|chapter=Surface Mapping|chapterurl=http://history.nasa.gov/SP-423/surface.htm|accessdate=2008-05-28}}</ref>

In June 1962 ] scientists at the ] of the ] lead by ] became first to bounce ] signal off Mercury and receive it, starting radar observations of the planet.<ref>{{cite journal
|first= J. V. |last=Evans
| coauthors=Brockelman, R. A.; Henry, J. C.; Hyde, G. M.; Kraft, L. G.; Reid, W. A.; Smith, W. W.
| title=Radio Echo Observations of Venus and Mercury at 23 cm Wavelength
| year=1965 | journal=Astronomical Journal | volume=70
| url=http://articles.adsabs.harvard.edu/abs/1965AJ.....70..486E/0000487.000.html
| pages=487&ndash;500 | accessdate=2008-05-23
| doi=10.1086/109772 }}</ref><ref>{{cite book
| last=Moore | first=Patrick
| title=The Data Book of Astronomy | page=483 | year=2000
| publisher=CRC Press | location=New York
| url=http://books.google.com/books?lr=&as_brr=3&q=kotelnikov+1962+mercury&btnG=Search+Books
| isbn=0750306203}}</ref><ref>{{cite book
| title=To See the Unseen: A History of Planetary Radar Astronomy
| url=http://history.nasa.gov/SP-4218/sp4218.htm
| first=Andrew J. | last=Butrica
| publisher=] History Office, Washington D.C.
| year=1996 | chapter=Chapter 5
| chapterurl=http://history.nasa.gov/SP-4218/ch5.htm }}</ref> Three years later radar observations by Americans ] and R. Dyce using 300-meter ] ] in ] showed conclusively that the planet’s rotational period was about 59 days.<ref>{{cite journal
| last=Pettengill | first=G. H. |coauthors=Dyce, R. B.
| title=A Radar Determination of the Rotation of the Planet Mercury
| journal=] | volume=206
| issue= 1240 | pages= 451–2 | year= 1965
|doi=10.1038/2061240a0 }}</ref><ref> at Eric Weisstein's 'World of Astronomy'</ref> The theory that Mercury’s rotation was synchronous had become widely held, and it was a surprise to astronomers when these radio observations were announced. If Mercury were tidally locked, its dark face would be extremely cold, but measurements of radio emission revealed that it was much hotter than expected. Astronomers were reluctant to drop the synchronous rotation theory and proposed alternative mechanisms such as powerful heat-distributing winds to explain the observations.<ref>{{cite book
| first=Bruce C. | last=Murray
| coauthors=Burgess, Eric | year=1977
| title=Flight to Mercury
| publisher=Columbia University Press
| isbn=0231039964 }}</ref>

Italian astronomer ] noted that the rotation value was about two-thirds of Mercury’s orbital period, and proposed that a different form of tidal locking had occurred in which the planet’s orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance.<ref>{{cite journal
| last=Colombo | first=G. | title=Rotational Period of the Planet Mercury
| journal=Nature | volume=208 | pages=575 | year=1965
| doi = 10.1038/208575a0 | accessdate=2009-05-30 | url=http://adsabs.harvard.edu/abs/1965Natur.208..575C }}</ref> Data from Mariner&nbsp;10 subsequently confirmed this view.<ref>{{cite web
| month=October | year=1976 | author=Davies, Merton E. et al.
| url=http://history.nasa.gov/SP-423/mariner.htm
| title=Mariner&nbsp;10 Mission and Spacecraft
| work=SP-423 Atlas of Mercury
| publisher=NASA JPL | accessdate=2008-04-07 }}</ref> This means that Schiaparelli's and Antoniadi's maps were not "wrong". Instead, the astronomers saw the same features during every ''second'' orbit and recorded them, but disregarded those seen in the meantime, when Mercury's other face was toward the Sun, since the orbital geometry meant that these observations were made under poor viewing conditions.<ref name="sao188r" />

Ground-based observations did not shed much further light on the innermost planet, and it was not until the first space probe flew past Mercury that many of its most fundamental properties became known. However, recent technological advances have led to improved ground-based observations. In 2000, high-resolution ] observations were conducted by the ] 1.5 meter {{dp|Hale telescope}}. They provided the first views that resolved surface features on the parts of Mercury which were not imaged in the Mariner mission.<ref>{{cite journal
| last=Dantowitz | first=R. F. | coauthors=Teare, S. W.; Kozubal, M. J.
| title=Ground-based High-Resolution Imaging of Mercury
| journal=Astronomical Journal | volume=119 | pages=2455&ndash;2457 | year= 2000
| url=http://ukads.nottingham.ac.uk/cgi-bin/nph-bib_query?bibcode=2000AJ....119.2455D&amp;db_key=AST
| doi = 10.1016/j.asr.2005.05.071 }}</ref> Later imaging has shown evidence of a huge double-ringed impact basin even larger than the ] in the non-Mariner-imaged hemisphere. It has informally been dubbed the '']''.<ref name=Ksa06/>
Most of the planet has been mapped by the Arecibo radar telescope, with 5&nbsp;km resolution, including polar deposits in shadowed craters of what may be water ice.<ref name=Harm06>{{cite journal|author =Harmon, J. K. et al.|title= Mercury: Radar images of the equatorial and midlatitude zones| journal= Icarus |volume= 187|pages= 374|year= 2007|url= http://adsabs.harvard.edu/abs/2007Icar..187..374H | doi = 10.1016/j.icarus.2006.09.026 <!--Retrieved from CrossRef by DOI bot-->}}</ref>

{{clear}}

===Research with space probes===
{{main|Exploration of Mercury}}

Reaching Mercury from Earth poses significant technical challenges, since the planet orbits so much closer to the Sun than does the Earth. A Mercury-bound spacecraft launched from Earth must travel over 91 million kilometers into the Sun’s ] ]. Mercury has an ] of 48&nbsp;km/s, while Earth’s orbital speed is 30&nbsp;km/s. Thus the spacecraft must make a large change in ] (]) to enter into a ] that passes near Mercury, as compared to the delta-v required for other planetary missions.<ref name="DunneCh4">{{cite book|title=The Voyage of Mariner&nbsp;10 — Mission to Venus and Mercury|author=Dunne, J. A. and Burgess, E.|chapterurl=http://history.nasa.gov/SP-424/ch4.htm|publisher=NASA History Office|year=1978|chapter=Chapter Four|url=http://history.nasa.gov/SP-424/|accessdate=2008-05-28}}</ref>

The ] liberated by moving down the Sun’s ] becomes ]; requiring another large delta-v change to do anything other than rapidly pass by Mercury. In order to land safely or enter a stable orbit the spacecraft would rely entirely on rocket motors. ] is ruled out because the planet has very little atmosphere. A trip to Mercury actually requires more rocket fuel than that required to ] the solar system completely. As a result, only two space probes have visited the planet so far.<ref name="JPLprofile1">{{cite web|url=http://solarsystem.jpl.nasa.gov/planets/profile.cfm?Object=Mercury&Display=OverviewLong|title=Mercury|publisher=] Jet Propulsion Laboratory|date=May 5, 2008 |accessdate=2008-05-29}}</ref> A proposed alternative approach would use a ] to attain a Mercury-synchronous orbit around the Sun.<ref>{{ cite journal
| last=Leipold | first=M.
| coauthors=Seboldt, W.; Lingner, S.; Borg, E.; Herrmann, A.; Pabsch, A.; Wagner, O.; Bruckner, J.
| title=Mercury sun-synchronous polar orbiter with a solar sail
| year=1996 | month=July | journal=Acta Astronautica
| volume=39 |issue=1 | pages = 143&ndash;151 | doi=10.1016/S0094-5765(96)00131-2 }}</ref>

====Mariner 10====
{{main|Mariner 10}}
]
]

The first spacecraft to visit Mercury was ]’s ] (1974–75).<ref name="Dunne" /> The spacecraft used the gravity of ] to adjust its orbital velocity so that it could approach Mercury, making it both the first spacecraft to use this ] effect and the first NASA mission to visit multiple planets.<ref name="DunneCh4" /> Mariner&nbsp;10 provided the first close-up images of Mercury’s surface, which immediately showed its heavily cratered nature, and revealed many other types of geological features, such as the giant scarps which were later ascribed to the effect of the planet shrinking slightly as its iron core cools.<ref>{{cite web
| month=October | year=1976 | first=Tony | last=Phillips
| url=http://www.nasa.gov/vision/universe/solarsystem/20oct_transitofmercury.html
| title=NASA 2006 Transit of Mercury | work=SP-423 Atlas of Mercury
| publisher=NASA | accessdate=2008-04-07 }}</ref> Unfortunately, due to the length of Mariner&nbsp;10's orbital period, the same face of the planet was lit at each of Mariner&nbsp;10’s close approaches. This made observation of both sides of the planet impossible,<ref>{{cite web|url=http://sci.esa.int/science-e/www/category/index.cfm?fcategoryid=4586|title=BepiColumbo - Background Science|publisher=European Space Agency|accessdate=2008-05-30}}</ref> and resulted in the mapping of less than 45% of the planet’s surface.<ref name="USATMessenger">{{cite news|url=http://www.usatoday.com/tech/news/2004-08-16-mercury-may-shrink_x.htm|title=MESSENGER to test theory of shrinking Mercury|publisher=USA Today|author=Tariq Malik|date=August 16, 2004 |accessdate=2008-05-23}}</ref>

On March 27, 1974, two days before its first flyby of Mercury, Mariner 10's instruments began registering large amounts of unexpected ultraviolet radiation in the vicinity of Mercury. This led to the tentative identification of ]. Shortly afterward, the source of the excess UV was identified as the star 31 ]is, and Mercury's moon passed into astronomy's history books as a footnote.

The spacecraft made three close approaches to Mercury, the closest of which took it to within 327&nbsp;km of the surface.<ref name="AtlasM10">{{cite book|url=http://history.nasa.gov/SP-423/sp423.htm|title=Atlas of Mercury|publisher=] Office of Space Sciences|author=Merton E. Davies, et al.|year=1978|chapter=Mariner&nbsp;10 Mission and Spacecraft|chapterurl=http://history.nasa.gov/SP-423/mariner.htm|accessdate=2008-05-30}}</ref> At the first close approach, instruments detected a magnetic field, to the great surprise of planetary geologists—Mercury’s rotation was expected to be much too slow to generate a significant ] effect. The second close approach was primarily used for imaging, but at the third approach, extensive magnetic data were obtained. The data revealed that the planet’s magnetic field is much like the Earth’s, which deflects the ] around the planet. However, the origin of Mercury’s magnetic field is still the subject of several competing theories.<ref name="Ness1">{{ cite journal | last = Ness| first = Norman F. | year = 1978| month = March|title=Mercury - Magnetic field and interior| journal = Space Science Reviews | volume = 21| pages = 527–553| bibcode = 1978SSRv...21..527N | url = http://adsabs.harvard.edu/full/1978SSRv...21..527N| accessdate = 2008-05-23|doi=10.1007/BF00240907}}</ref>

Just a few days{{Quantify|date=March 2009}} after its final close approach, Mariner&nbsp;10 ran out of fuel. Since its orbit could no longer be accurately controlled, mission controllers instructed the probe to shut itself down on March 24, 1975.<ref name="DunneCh8">{{cite book|title=The Voyage of Mariner&nbsp;10 — Mission to Venus and Mercury|author=Dunne, J. A. and Burgess, E.|chapterurl=http://history.nasa.gov/SP-424/ch8.htm|publisher=NASA History Office|year=1978|chapter=Chapter Eight|url=http://history.nasa.gov/SP-424/}}</ref> Mariner&nbsp;10 is thought to be still orbiting the Sun, passing close to Mercury every few months.<ref>{{cite web
| date=April 2, 2008 | first=Ed | last=Grayzeck
| url=http://nssdc.gsfc.nasa.gov/nmc/spacecraftDisplay.do?id=1973-085A
| title=Mariner&nbsp;10 | work=NSSDC Master Catalog
| publisher=NASA | accessdate=2008-04-07 }}</ref>

====MESSENGER====
{{main|MESSENGER}}

]

A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004, from the ] aboard a ] rocket. It made a fly-by of the Earth in August 2005, and of Venus in October 2006 and June 2007 in order to place it onto the correct trajectory to reach an orbit around Mercury.<ref>{{cite web|year=2005|url = http://www.spaceref.com/news/viewsr.html?pid=18956| title = MESSENGER Engine Burn Puts Spacecraft on Track for Venus|publisher = SpaceRef.com | accessdate = 2006-03-02}}</ref> A first fly-by of Mercury occurred on January 14, 2008, a second on October 6, 2008,<ref name="MessCountdown">{{cite web|url= http://messenger.jhuapl.edu/gallery/sciencePhotos/image.php?gallery_id=2&image_id=115|title= Countdown to MESSENGER's Closest Approach with Mercury|date= January 14, 2008 | publisher= Johns Hopkins University Applied Physics Laboratory |accessdate= 2008-05-30}}</ref> and a third on September 29, 2009.<ref>{{cite web | title=MESSENGER Gains Critical Gravity Assist for Mercury Orbital Observations
| url=http://messenger.jhuapl.edu/news_room/details.php?id=136
| date=September 30, 2009 | publisher=MESSENGER Mission News
| accessdate=2009-09-30}}</ref> Most of the hemisphere not imaged by Mariner&nbsp;10 has been mapped during these fly-bys. The probe will then enter an elliptical orbit around the planet in March 2011; the nominal mapping mission is one terrestrial year.<ref name="MessCountdown" />

<!--]-->
The mission is designed to shed light on six key issues: Mercury’s high density, its geological history, the nature of its ], the structure of its core, whether it really has ice at its poles, and where its tenuous atmosphere comes from. To this end, the probe is carrying imaging devices which will gather much higher resolution images of much more of the planet than Mariner&nbsp;10, assorted ]s to determine abundances of elements in the crust, and ]s and devices to measure velocities of charged particles. Detailed measurements of tiny changes in the probe’s velocity as it orbits will be used to infer details of the planet’s interior structure.<ref name="MSGRgrayzeck" />

====BepiColombo====
{{main|BepiColombo}}
The ] is planning a joint mission with ] called ], which will orbit Mercury with two probes: one to map the planet and the other to study its ].<ref name="ESAColumboGoAhead">{{cite web
| title=ESA gives go-ahead to build BepiColombo
| date=February 26, 2007 | publisher=]
| url=http://www.esa.int/esaSC/SEMC8XBE8YE_index_0.html
| accessdate=2008-05-29 }}</ref> Once launched, the spacecraft bus is expected to reach Mercury in 2019.<ref name="Bepitelegraph1">{{cite news
| url=http://www.telegraph.co.uk/earth/main.jhtml?view=DETAILS&grid=&xml=/earth/2008/01/18/scimerc118.xml
| title=Star Trek-style ion engine to fuel Mercury craft
| author=Fleming, Nic | publisher=The Telegraph
| date=January 18, 2008 | accessdate=2008-05-23 }}</ref> The bus will release a ] probe into an elliptical orbit, then chemical rockets will fire to deposit the mapper probe into a circular orbit. Both probes will operate for a terrestrial year.<ref name="ESAColumboGoAhead" /> The mapper probe will carry an array of spectrometers similar to those on MESSENGER, and will study the planet at many different wavelengths including ], ], ] and ].<ref>{{cite web
| title=Objectives | publisher=European Space Agency | url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=31350
| date=February 21, 2006 | accessdate=2008-05-29 }}</ref>

==In culture==
In western ] ] is the ruling planet of ] and ]. That is, the supposed astrological influence of the planet was greatest when it was observed in these constellations.<ref>{{cite book
| first=Roger | last=Beck | pages=84&ndash;87
| title=A Brief History of Ancient Astrology
| publisher=Wiley-Blackwell | year=2007
| isbn=1405110740 }}</ref>

On ]s of Mercury created by astronomers before the detailed mapping of recent decades, the ''Solitudo Hermae Trismegisti'' (''Wilderness of ]'') was identified as a major feature of the planet Mercury, covering about one-fourth of the planet in the SE quadrant.<ref></ref>
<ref></ref>
<ref></ref>

''Mercury, ]'' is a movement in ]'s '']''.

==See also==
{{portal|Solar System|Solar system.jpg}}

* ]
* ]

==Notes==
<div class="references-small">
<ol type="a">
<li>{{Note_label|A|a|none}}1/30 of a degree is the fractional equivalent to 2.1 arcminutes.</li>
<li>{{Note_label|B|b|none}}Some sources precede the cuniform transcription with "MUL". "MUL" is a cuneiform sign that was used in the Sumerian language to designate a star or planet, but it is not considered part of the actual name. The "4" is a reference number in the Sumero-Akkadian transliteration system to designate which of several syllables a certain cuneiform sign is most likely designating.</li>
</ol>
</div>

==References==
{{reflist|colwidth=30em}}

==External links==
{{SpecialChars}}
{{Spoken Misplaced Pages|En-Mercury(Planet).ogg|2008-01-16}}
{{sisterlinks|Mercury}}
* —About Space
*
*
*
* at
*
*
*
*
*
* A kid’s guide to Mercury.
*
*
* World’s search engine that supports ], ], and other applications.
* flash animation
*

{{Solar System}}
{{Mercury (planet)}}

]
]

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{{Link FA|ca}}
{{Link FA|cs}}
{{Link FA|de}}
{{Link FA|hu}}
{{Link FA|sl}}
{{Link FA|sv}}


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Revision as of 08:48, 30 January 2010


Mercury is similar in appearance to the Moon: it is heavily cratered with regions of smooth plains, has no natural satellites and no substantial atmosphere. However, unlike the moon, it has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth. It is an exceptionally dense planet due to the large relative size of its core. Surface temperatures range from about 90 to 700 K (−183 °C to 427 °C, −297 °F to 801 °F), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

Recorded observations of Mercury date back to at least the first millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes. The English name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated with the Greek Hermes (Ἑρμῆς). The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.


  1. "Mercury magnetic field". C. T. Russell & J. G. Luhmann. Retrieved 2007-03-16.
  2. "Background Science". European Space Agency. Retrieved 2008-05-23.
  3. Dunne, J. A. and Burgess, E. (1978). "Chapter One". The Voyage of Mariner 10 — Mission to Venus and Mercury. NASA History Office. {{cite book}}: External link in |chapterurl= (help); Unknown parameter |chapterurl= ignored (|chapter-url= suggested) (help)CS1 maint: multiple names: authors list (link)
  4. Duncan, John Charles (1946). Astronomy: A Textbook. Harper & Brothers. p. 125. The symbol for Mercury represents the Caduceus, a wand with two serpents twined around it, which was carried by the messenger of the gods. {{cite book}}: line feed character in |quote= at position 80 (help)