Mercury (planet)
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Mercury isclosest planet toSun. This issecond-smallest planet within Earth's solar system. Uponcelestial sphere, Mercury ranges from -0.45.5,apparent magnitude; Mercurysufficiently "close" toSun that telescopes rarely examine it. Mercury has no natural satellites. The only spacecraftapproach Mercury was Mariner 10 (1974-75); only 40-45% ofplanet has been mapped. The astronomical symbolMercury iscircle on top ofshort vertical line withcross below andsemicircle abovecircle.
Physical characteristics
Atmosphere
Mercury has only trace amountsan atmosphere. The atmosphereMercuryextremely thin; indeed, gas moleculesMercury's atmosphere collide withsurface ofplanet more frequently thancollideeach other;most purposes Mercury should be considered as being airless. The "atmosphere"primarily composedoxygen, potassium,sodium.
The atoms that compose Mercury's atmospherecontinually being lostspace, withaverage "lifespan" ofpotassium or sodium being approximately ~3 hours (duringMercurian day --only half that at perihelion). The lost atmospherecontinually replenished by several mechanisms; solar wind captured byplanetary magnetic field, vapor produced by micrometeor impacts, direct thermal evaporation ofpolar ice, and/or outgassing.
Temperaturesunlight
The mean surface temperatureMercury452K, butranges from 90-700K; by comparison,temperature on Earth varies by only ~11K (with respect onlysolar radiation; not climate or season). The sunlight on Mercury's surface6.3 times more intense than that on Earth,total irradiance3566W/m2.
Terrain
Mercury's cratered surface appears very similarLuna. Mercury's most distinctive surface feature (of what has been photographed)Caloris Basin,impact crater ~1350kmdiameter. The planetmarkedscarps, which apparently formed billionsyears ago as Mercury's core cooledshrank causingcrustwrinkle. The majorityMercury's surfacecoveredplainstwo distinct ages;younger plainsless heavily crateredprobably formed when lava flows buried earlier terrain. In addition, Mercury has "significant" tidal bulges.
See also: Listcraters on Mercury
Interior composition
The planet hasrelatively large iron core (even when comparedEarth)is, therefore, much denser; Mercury's compositionapproximately 70% metallic30% silicate. The average density5430kg/m3; whichslightly less than Earth's density. The reason that Mercury,so much iron, has less density than Earth;that,overall massEarth compressesplanetcreateshigh density. Mercury only has 5.5%Earth's mass. The iron core fills 42% ofplanetary volume (Earth's core only fills 17%). Surroundingcore is600km mantle.
Mercurian orbit
The orbitMercuryeccentric, ranging from 46M-70Mkmradius. The slow precessionthis orbit aroundsun could not be completely explained by Newtonian Classical Mechanics,for some timewas thought that another planet might be presentan orbit even closer tosun (sometimes referredas Vulcan)accountthis perturbation. Einstein's General TheoryRelativity providedexplanationthis small discrepancy instead, however.
Mercurian rotation
At certain points on Mercury's surface, an observer (standing upon one oftidal bulges) would be ableseeSun rise about halfway, then reverseset, then rise again; all withinsame Mercurian day. Thisbecause approximately four days priorperihelion, Mercury's orbital velocity exactly equals its rotational velocity; such thatSun's apparent motion ceases; and, at perihelion, Mercury's orbital velocity exceedsrotational velocity; thus,Sun appearsretrograde. Four days after perihelion,Sun's normal apparent motion resumes.
Until radar observations1965 proved otherwisewas thought that Mercury was tidally locked withSun, rotating onceeach orbitkeepingsame face directed towardsSun at all times. Instead, Mercury has3:2 spin-orbit-coupling, rotating three timesevery two revolutions aroundSun;eccentrityMercury's orbit makes this resonance stable. The original reason astronomers thoughtwas tidally locked was because whenever Mercury was best placedobservation,was always atsame pointits 3:2 resonance, so showingsame face, which would be alsocase ifwas totally locked. Mercury rotates 59 times slower than Earth.
BecauseMercury's 3:2 spin-orbit-coupling; althoughsidereal day (the periodrotation) lasts ~58.7 Earth days,solar day (the length between two meridian transitss ofSun) lasts ~176 Earth days.
The Mercurian magnetosphere
Despite its slow rotation, Mercury hasrelatively strong magnetosphere;1% ofmagnetic field strength generated by Earth. Itpossible that this magnetic fieldgenerated inmanner similarEarth's, bydynamocirculating liquid core material; current estimates suggest that Mercury's corenot hot enoughliquefy nickel-iron, but itpossible that materials withlower melting point such as sulfur may be responsible. Italso possible that Mercury's magnetic field isremnantan eariler dynamo effect that has now ceased,magnetic field becoming "frozen"solidified magnetic materials.
Why Mercury has so much iron
Mercury hashigher iron percentage than any other object withinsystem. Several theories have been proposedexplain Mercury's high metallicity.
One theory suggests that Mercury originally hadmetal-silicate ratio similarcommon chondrite meteors andmass approximately 2.25 times its current mass, but that early insolar system's history Mercury was struck byplanetesimalapproximately 1/6 that mass. The impact would have stripped away much oforiginal crustmantle; leavingcore behind. A similar theory has been proposedexplainformationEarth's Moon, see giant impact theory. Alternately, Mercury may have formed very early inhistory ofsolar nebula, beforeSun's energy output had stabilized. Mercury starts outapproximately twice its current massthis theory; but, asprotostar contracted, temperatures near Mercury could have been between 2500-3500K;possibly even as high as 10000K. MuchMercury's surface rock would have vaporized at such temperatures, forming an atmosphere"rock vapor" which would have been carried away bynebular wind. A third theory, similar tosecond, argues thatouter layersMercury were "eroded" bysolar wind overlonger periodtime.
Ice on Mercury
Radar observations first indicated,1992, that therefrozen water ice at Mercury's north pole. Such waterbelievedexist atpermanently shaded bottomscraters, where itdeposited by comets and/or gases fromplanetary core.
The ExplorationMercury
Mercury has been known since at leasttime ofSumerians (3rd millennium BC), who calledUbu-idim-gud-ud. The earliest recorded detailed observations were made byBabylonians, who calledgu-ad or gu-utu. It was given two names byancient Greeks, Apollo when visible inmorning skyHermes when visible inevening, but Greek astronomers knew thattwo names referred tosame body. Heraclitus even believed that MercuryVenus orbitedSun, notEarth. ObservationMercuryseverely complicated by its proximity toSun; itonly visible from Earth at sunrise or sunset.
The only spacecraftapproach Mercury was Mariner 10 (1974-75)
A missionMercury has been approved by NASA, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry,Ranging), which will launch2004reach Mercury2009.
Japan andESA
Japanplanningjoint mission withEuropean Space Agency that would befirstlandprobe on Mercury. The mission entails three probes, two that would orbitone that would land,maptopographystudyorigins ofplanet. Russian Soyuz rockets would launchprobes starting2010. The probes would reach Mercury about four years later,onethem landing onplanet, andother two orbitingcharting its surface foryear.
External links
References
- DiscoveringEssential Universe by Neil F. Comins (2001)
| The Solar System |
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| Sun | Mercury | Venus | Earth | Mars | Asteroid belt | Jupiter | Saturn | Uranus | Neptune | Pluto |
| Trans-Neptunian objects | Astronomical objects |
