Mercury (Planet) - Wikipedia, The Free Encyclopedia

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Mercury

Composite image of Mercury taken by

MESSENGER

Designations

Pronunciation i/mrkjri/

Adjective Mercurian, Mercurial[1]

Orbital characteristics[4]

Epoch J2000

Aphelion 69,816,900 km

0.466 697 AU

Perihelion 46,001,200 km

0.307 499 AU

Semi-major

axis

57,909,100 km

0.387 098 AU

Eccentricity 0.205 630[2]

Orbital period 87.969 1 d

(0.240 846 a)

0.5 Mercury solar day

Synodic period 115.88 d[2]

Average

orbital speed

47.87 km/s[2]

Mean anomaly 174.796

Inclination 7.005 to Ecliptic

3.38 to Sun's equator

6.34 to Invariable plane[3]

Longitude of 48.331

Mercury (planet)From Wikipedia, the free encyclopedia

Mercury is the smallest and closest to the Sun of the

eight planets in the Solar System,[a] with an orbital periodof about 88 Earth days. Seen from the Earth, it appears tomove around its orbit in about 116 days, which is muchfaster than any other planet. This rapid motion may haveled to it being named after the Roman deity Mercury, thefast-flying messenger to the gods. Because it has almostno atmosphere to retain heat, Mercury's surfaceexperiences the greatest temperature variation of all theplanets, ranging from 100 K (173 C; 280 F) at nightto 700 K (427 C; 800 F) during the day at someequatorial regions. The poles are constantly below 180 K(93 C; 136 F). Mercury's axis has the smallest tilt of

any of the Solar System's planets (about 130 of a degree),

but it has the largest orbital eccentricity.[a] At aphelion,Mercury is about 1.5 times as far from the Sun as it is atperihelion. Mercury's surface is heavily cratered andsimilar in appearance to the Moon, indicating that it hasbeen geologically inactive for billions of years.

Mercury does not experience seasons in the same way asmost other planets, such as the Earth. It is locked so itrotates in a way that is unique in the Solar System. Asseen relative to the fixed stars, it rotates exactly three

times for every two revolutions[b] it makes around itsorbit. As seen from the Sun, in a frame of reference thatrotates with the orbital motion, it appears to rotate onlyonce every two Mercurian years. An observer onMercury would therefore see only one day every twoyears.

Because Mercury's orbit lies within Earth's orbit (as doesVenus's), it can appear in Earth's sky in the morning or theevening, but not in the middle of the night. Also, likeVenus and the Moon, it displays a complete range ofphases as it moves around its orbit relative to the Earth.Although Mercury can appear as a very bright objectwhen viewed from Earth, its proximity to the Sun makes itmore difficult to see than Venus.

Contents

1 Rotational peculiarity, and consequences2 Internal structure

3 Surface geology



Longitude of

ascending node

48.331

Argument of

perihelion

29.124

Satellites None

Physical characteristics

Mean radius 2,439.7 1.0 km[5][6]

0.3829 Earths

Flattening 0[6]

Surface area 7.48 107 km2[5]

0.147 Earths

Volume 6.083 1010 km3[5]

0.056 Earths

Mass 3.3022 1023 kg[5]

0.055 Earths

Mean density 5.427 g/cm3[5]

Equatorial

surface gravity

3.7 m/s2

0.38 g[5]

Escape velocity 4.25 km/s[5]

Sidereal

rotation period

58.646 day

1407.5 h[5]

Equatorial

rotation

velocity

10.892 km/h (3.026 m/s)

Axial tilt 2.11 0.1[7]

North pole

right ascension

18 h 44 min 2 s

281.01[2]

North pole

declination

61.45[2]

Albedo 0.068 (Bond)[8]

0.142 (geom.)[8]

Surface temp. min mean max0N, 0W [11] 100 K 340 K 700 K

85N, 0W[11]80 K 200 K 380 K

Apparent

magnitude

2.6[9] to 5.7[2][10]

Angular

diameter

4.5" 13"[2]

Atmosphere[2]

Surface

pressure

trace

Composition 42% Molecular oxygen

3.1 Impact basins and craters3.2 Plains

4 Surface conditions and "atmosphere"

(exosphere)

5 Magnetic field and magnetosphere

6 Orbit and rotation

6.1 Spinorbit resonance

6.2 Advance of perihelion

7 Observation

7.1 Naked-eye viewing

8 Observation history

8.1 Ancient astronomers8.2 Ground-based telescopic research

8.3 Research with space probes

8.3.1 Mariner 10

8.3.2 MESSENGER8.3.3 BepiColombo

9 See also

10 Notes11 References

12 External links

Rotational peculiarity, andconsequences

Mercury makes three rotations about its axis for everytwo revolutions around its orbit, as seen relative to thefixed stars. As seen from the Sun, in a frame of referencethat rotates with the orbital motion, it appears to rotate

only once during two orbital revolutions.[c] This exactratio is maintained by a gravitational resonance.Correspondingly, an observer on Mercury would see justone passage of the Sun across the sky, and therefore oneday, every two Mercurian years.



29.0% sodium

22.0% hydrogen

6.0% helium

0.5% potassium

Trace amounts of argon,

nitrogen, carbon dioxide,

water vapor, xenon,

krypton, and neon

Terrestrial planets: Mercury, Venus,

Earth, and Mars (to scale)

One year passes during each night, while the Sun is belowthe horizon, so the surface temperature is very low.During the other year of each day, the Sun appears tomove slowly in the sky from the eastern to the western

horizon,[d] while the planet makes a complete revolutionaround the Sun, passing through both perihelion andaphelion. At perihelion, the intensity of sunlight onMercury's surface is more than twice the intensity ataphelion. There are places on Mercury's surface fromwhich the Sun is visible high in the sky every day at thetime of the perihelion. These places receive intense solarirradiance at that time, and therefore become very hot. Places where the Sun is high in the sky during daytimeaphelion have lower temperatures.

This difference between perihelion and aphelion temperatures is increased by the variation of the speed of theSun's apparent motion in the Mercurian sky. When it is close to perihelion, Mercury travels faster around itsorbit than it does near aphelion, following Kepler's second law. Near perihelion, Mercury's orbital angularvelocity becomes great enough to roughly equal its rotational velocity, relative to the fixed stars. Temporarily, asseen from the Sun, Mercury resembles the Moon as seen from the Earth, apparently not rotating because itsorbital and rotational angular velocities, relative to the fixed stars, are equal. Therefore, seen from the Sunaround the time of Mercury's perihelion, the planet's rotation appears to pause, and as seen from Mercury, the

Sun's motion across the sky also appears to pause.[e] This prolongs and enhances the solar heating of the placeswhere the Sun appears high in the sky at Mercury's daytime perihelion. The converse happens around the timeof aphelion, when the Sun appears to move faster than usual in the Mercurian sky.

At any given place on the planet's surface, there is a cycle of temperature variations that is repeated every day.The varying angle of elevation and apparent speed of the Sun in the sky interact with the changing intensity ofsunlight, caused by Mercury's varying distance from the Sun, to cause places at different latitudes and longitudesto experience different patterns of temperature variation during the daily cycle.

Internal structure

Mercury is one of four terrestrial planets in the Solar System, and is arocky body like the Earth. It is the smallest planet in the Solar System,

with an equatorial radius of 2,439.7 km.[2] Mercury is even smalleralbeit more massivethan the largest natural satellites in the SolarSystem, Ganymede and Titan. Mercury consists of approximately

70% metallic and 30% silicate material.[12] Mercury's density is the

second highest in the Solar System at 5.427 g/cm3, only slightly less

than Earth's density of 5.515 g/cm3.[2] If the effect of gravitationalcompression were to be factored out, the materials of which Mercury

is made would be denser, with an uncompressed density of 5.3 g/cm3 versus Earth's 4.4 g/cm3.[13]

Mercury's density can be used to infer details of its inner structure. Although Earth's high density resultsappreciably from gravitational compression, particularly at the core, Mercury is much smaller and its innerregions are not as compressed. Therefore, for it to have such a high density, its core must be large and rich in

iron.[14]



Internal structure of

Mercury:

1. Crust: 100300 km thick

2. Mantle: 600 km thick

3. Core: 1,800 km radius

Geologists estimate that Mercury's core occupies about 42% of its volume;for Earth this proportion is 17%. Research published in 2007 suggests that

Mercury has a molten core.[15][16] Surrounding the core is a 500700 km

mantle consisting of silicates.[17][18] Based on data from the Mariner 10mission and Earth-based observation, Mercury's crust is believed to be 100

300 km thick.[19] One distinctive feature of Mercury's surface is the presenceof numerous narrow ridges, extending up to several hundred kilometers inlength. 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.[20]

Mercury's core has a higher iron content than that of any other major planet inthe Solar System, and several theories have been proposed to explain this.The most widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteorites, thought to be typical ofthe Solar System's rocky matter, and a mass approximately 2.25 times its

current mass.[21] Early in the Solar System's history, Mercury may have been

struck by a planetesimal of approximately 1/6 that mass and several hundred kilometers across.[21] The impactwould have stripped away much of the original crust and mantle, leaving the core behind as a relatively major

component.[21] A similar process, known as the giant impact hypothesis, has been proposed to explain the

formation of the Moon.[21]

Alternatively, Mercury may have formed from the solar nebula before the Sun's energy output had stabilized.The planet would initially have had twice its present mass, but as the protosun contracted, temperatures near

Mercury could have been between 2,500 and 3,500 K and possibly even as high as 10,000 K.[22] Much ofMercury's surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock

vapor" that could have been carried away by the solar wind.[22]

A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury wasaccreting, which meant that lighter particles were lost from the accreting material and not gathered by

Mercury.[23] Each hypothesis predicts a different surface composition, and two upcoming space missions,

MESSENGER and BepiColombo, both will make observations to test them.[24][25] MESSENGER has foundhigher-than-expected potassium and sulfur levels on the surface, suggesting that the giant impact hypothesis andvaporization of the crust and mantle did not occur because potassium and sulfur would have been driven off bythe extreme heat of these events. The findings would seem to favor the third hypothesis, however further analysis

of the data is needed.[26]

Surface geology

Main article: Geology of Mercury

Mercury's surface is very similar in appearance to that of the Moon, showing extensive mare-like plains andheavy cratering, indicating that it has been geologically inactive for billions of years. Because our knowledge ofMercury's geology has been based on the 1975 Mariner flyby and terrestrial observations, it is the least

understood of the terrestrial planets.[16] As data from the recent MESSENGER flyby is processed thisknowledge will increase. For example, an unusual crater with radiating troughs has been discovered that

scientists called "the spider".[27] It later received the name Apollodorus.[28]

Albedo features are areas of markedly different reflectivity, as seen by telescopic observation. Mercurypossesses dorsa (also called "wrinkle-ridges"), Moon-like highlands, montes (mountains), planitiae (plains),

rupes (escarpments), and valles (valleys).[29][30]



The so-called "Weird Terrain" was

formed at the point antipodal to the

Caloris Basin impact

Names for features on Mercury come from a variety of sources. Names coming from people are limited to thedeceased. Craters are named for artists, musicians, painters, and authors who have made outstanding orfundamental contributions to their field. Ridges, or dorsa, are named for scientists who have contributed to thestudy of Mercury. Depressions or fossae are named for works of architecture. Montes are named for the word"hot" in a variety of languages. Plains or planitiae are named for Mercury in various languages. Escarpments or

rups are named for ships of scientific expeditions. Valleys or valles are named for radio telescope facilities.[31]

Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4.6 billionyears ago, as well as during a possibly separate subsequent episode called the late heavy bombardment that

came to an end 3.8 billion years ago.[32] During this period of intense crater formation, the planet received

impacts over its entire surface,[30] facilitated by the lack of any atmosphere to slow impactors down.[33] Duringthis time the planet was volcanically active; basins such as the Caloris Basin were filled by magma, producing

smooth plains similar to the maria found on the Moon.[34][35]

Data from the October 2008 flyby of MESSENGER gave researchers a greater appreciation for the jumblednature of Mercury's surface. Mercury's surface is more heterogeneous than either Mars's or the Moon's, both of

which contain significant stretches of similar geology, such as maria and plateaus.[36]

Impact basins and craters

Craters on Mercury range in diameter from small bowl-shapedcavities to multi-ringed impact basins hundreds of kilometers across.They appear in all states of degradation, from relatively fresh rayedcraters to highly degraded crater remnants. Mercurian craters differsubtly from lunar craters in that the area blanketed by their ejecta ismuch smaller, a consequence of Mercury's stronger surface

gravity.[37] According to IAU rules, each new crater must benamed after an artist that was famous for more than fifty years, anddead for more than three years, before the date the crater is

named.[38]

The largest known crater is Caloris Basin, with a diameter of

1,550 km.[39] The impact that created the Caloris Basin was sopowerful that it caused lava eruptions and left a concentric ring over2 km tall surrounding the impact crater. At the antipode of theCaloris Basin is a large region of unusual, hilly terrain known as the"Weird Terrain". One hypothesis for its origin is that shock wavesgenerated during the Caloris impact traveled around the planet, converging at the basin's antipode (180 degrees

away). The resulting high stresses fractured the surface.[40] Alternatively, it has been suggested that this terrain

formed as a result of the convergence of ejecta at this basin's antipode.[41]

Overall, about 15 impact basins have been identified on the imaged part of Mercury. A notable basin is the400 km wide, multi-ring Tolstoj Basin that has an ejecta blanket extending up to 500 km from its rim and a floorthat has been filled by smooth plains materials. Beethoven Basin has a similar-sized ejecta blanket and a 625 km

diameter rim.[37] Like the Moon, the surface of Mercury has likely incurred the effects of space weathering

processes, including Solar wind and micrometeorite impacts.[42]

Plains

There are two geologically distinct plains regions on Mercury.[37][43] Gently rolling, hilly plains in the regions



Radar image of Mercury's north pole

There are two geologically distinct plains regions on Mercury.[37][43] Gently rolling, hilly plains in the regions

between craters are Mercury's oldest visible surfaces,[37] 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 km in diameter.[43] It is not clear whether they are of volcanic or impact origin.[43] The inter-

crater plains are distributed roughly uniformly over the entire surface of the planet.[citation needed]

Smooth plains are widespread flat areas that fill depressions of various sizes and bear a strong resemblance tothe lunar maria. Notably, they fill a wide ring surrounding the Caloris Basin. Unlike lunar maria, the smoothplains 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.[37]

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.[37] The floor of the Caloris Basin is filled by ageologically 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.[37]

One unusual feature of the planet's surface is the numerous compression folds, or rupes, that crisscross theplains. As the planet's interior cooled, it may have contracted and its surface began to deform, creating thesefeatures. The folds can be seen on top of other features, such as craters and smoother plains, indicating that the

folds are more recent.[44] Mercury's surface is flexed by significant tidal bulges raised by the Sunthe Sun's

tides on Mercury are about 17 times stronger than the Moon's on Earth.[45]

Surface conditions and "atmosphere" (exosphere)

Main article: Atmosphere of Mercury

The surface temperature of Mercury ranges from 100 K to 700 K[47]

at the most extreme places 0N, 0 or 180W. It never rises above

180 K at the poles,[11] due to the absence of an atmosphere and asteep temperature gradient between the equator and the poles. Thesubsolar point reaches about 700 K during perihelion (0 or 180W),

but only 550 K at aphelion (90 or 270W).[48] On the dark side of

the planet, temperatures average 110 K.[11][49] The intensity ofsunlight on Mercury's surface ranges between 4.59 and 10.61 times

the solar constant (1,370 Wm2).[50]

Although the daylight temperature at the surface of Mercury isgenerally extremely high, observations strongly suggest that ice (frozenwater) exists on Mercury. The floors of deep craters at the poles arenever exposed to direct sunlight, and temperatures there remain

below 102 K; far lower than the global average.[51] Water ice strongly reflects radar, and observations by the70 m Goldstone telescope and the VLA in the early 1990s revealed that there are patches of very high radar

reflection near the poles.[52] Although ice is not the only possible cause of these reflective regions, astronomers

believe it is the most likely.[53]

The icy regions are believed to contain about 10141015 kg of ice,[54] and may be covered by a layer of regolith

that inhibits sublimation.[55] By comparison, the Antarctic ice sheet on Earth has a mass of about 4 1018 kg,

and Mars's south polar cap contains about 1016 kg of water.[54] The origin of the ice on Mercury is not yetknown, but the two most likely sources are from outgassing of water from the planet's interior or deposition by

impacts of comets.[54]



Composite of the north pole of

Mercury, where NASA confirmed the

discovery of a large volume of water

ice, in permanently dark craters that

exist there.[46]

Graph showing relative strength of Mercury's

magnetic field

Mercury is too small and hot for its gravity to retain any significantatmosphere over long periods of time; it does have a "tenuous

surface-bounded exosphere"[56] containing hydrogen, helium, oxygen,sodium, calcium, potassium and others. This exosphere is not stableatoms are continuously lost and replenished from a variety ofsources. Hydrogen and helium atoms probably come from the solarwind, diffusing into Mercury's magnetosphere before later escapingback into space. Radioactive decay of elements within Mercury'scrust is another source of helium, as well as sodium and potassium.MESSENGER found high proportions of calcium, helium, hydroxide,magnesium, oxygen, potassium, silicon and sodium. Water vapor ispresent, released by a combination of processes such as: cometsstriking its surface, sputtering creating water out of hydrogen from thesolar wind and oxygen from rock, and sublimation from reservoirs ofwater ice in the permanently shadowed polar craters. The detection of high amounts of water-related ions like

O+, OH, and H2O+ was a surprise.[57][58] 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.[59][60]

Sodium, potassium and calcium were discovered in the atmosphere during the 19801990s, and are believed to

result primarily from the vaporization of surface rock struck by micrometeorite impacts.[61] In 2008 magnesium

was discovered by MESSENGER probe.[62] Studies indicate that, at times, sodium emissions are localized atpoints that correspond to the planet's magnetic poles. This would indicate an interaction between the

magnetosphere and the planet's surface.[63]

On November 29, 2012, NASA confirmed that images from MESSENGER had detected that craters at thenorth pole contained water ice. Sean C. Solomon was quoted in the New York Times as estimating the volume

of the ice as large enough to "encase Washington, D.C., in a frozen block two and a half miles deep".[46][f]

Magnetic field and magnetosphere

Main article: Mercury's magnetic field

Despite its small size and slow 59-day-long rotation,Mercury has a significant, and apparently global, magneticfield. According to measurements taken by Mariner 10, it isabout 1.1% as strong as the Earth's. The magnetic field

strength at the Mercurian equator is about 300 nT.[64][65]

Like that of Earth, Mercury's magnetic field is dipolar.[63]

Unlike Earth, Mercury's poles are nearly aligned with the

planet's spin axis.[66] Measurements from both theMariner 10 and MESSENGER space probes haveindicated that the strength and shape of the magnetic field

are stable.[66]

It is likely that this magnetic field is generated by way of adynamo effect, in a manner similar to the magnetic field of

Earth.[67][68] 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.[69]



Mercury's magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere.

The planet's magnetosphere, though small enough to fit within the Earth,[63] is strong enough to trap solar wind

plasma. This contributes to the space weathering of the planet's surface.[66] Observations taken by theMariner 10 spacecraft detected this low energy plasma in the magnetosphere of the planet's nightside. Bursts ofenergetic particles were detected in the planet's magnetotail, which indicates a dynamic quality to the planet's

magnetosphere.[63]

During its second flyby of the planet on October 6, 2008, MESSENGER discovered that Mercury's magneticfield can be extremely "leaky". The spacecraft encountered magnetic "tornadoes" twisted bundles of magneticfields connecting the planetary magnetic field to interplanetary space that were up to 800 km wide or a third ofthe radius of the planet. These "tornadoes" form when magnetic fields carried by the solar wind connect toMercury's magnetic field. As the solar wind blows past Mercury's field, these joined magnetic fields are carriedwith it and twist up into vortex-like structures. These twisted magnetic flux tubes, technically known as fluxtransfer events, form open windows in the planet's magnetic shield through which the solar wind may enter and

directly impact Mercury's surface.[70]

The process of linking interplanetary and planetary magnetic fields, called magnetic reconnection, is commonthroughout the cosmos. It occurs in Earth's magnetic field, where it generates magnetic tornadoes as well. TheMESSENGER 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.[70]

Orbit and rotation

Mercury has the most eccentric orbit of all the planets; its eccentricity is 0.21 with its distance from the Sunranging from 46,000,000 to 70,000,000 km (29,000,000 to 43,000,000 mi). It takes 87.969 earth days tocomplete an orbit. The diagram on the right illustrates the effects of the eccentricity, showing Mercury's orbitoverlaid with a circular orbit having the same semi-major axis. The higher velocity of the planet when it is nearperihelion is clear from the greater distance it covers in each 5-day interval. In the diagram the varying distanceof Mercury to The Sun is represented by the size of the planet, which is inversely proportional to Mercury'sdistance from the Sun. This varying distance to the Sun, combined with a 3:2 spinorbit resonance of the

planet's rotation around its axis, result in complex variations of the surface temperature.[12] This resonance

makes a single day on Mercury last exactly two Mercury years, or about 176 Earth days.[71]

Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (the ecliptic), as shown in the diagram onthe right. As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossingthe plane of the ecliptic at the time it lies between the Earth and the Sun. This occurs about every seven years on

average.[72]

Mercury's axial tilt is almost zero,[73] with the best measured value as low as 0.027 degrees.[7] This issignificantly smaller than that of Jupiter, which has the second smallest axial tilt of all planets at 3.1 degrees. Thismeans that to an observer at Mercury's poles, the center of the Sun never rises more than 2.1 arcminutes above

the horizon.[7]

At certain points on Mercury's surface, an observer would be able to see the Sun rise about halfway, thenreverse and set before rising again, all within the same Mercurian day. This is because approximately four Earthdays before perihelion, Mercury's angular orbital velocity equals its angular rotational velocity so that the Sun'sapparent motion ceases; closer to perihelion, Mercury's angular orbital velocity then exceeds the angularrotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a retrograde

direction. Four Earth days after perihelion, the Sun's normal apparent motion resumes.[12]



Orbit of Mercury (yellow). Dates refer

to 2006.

Animation of Mercury's and Earth's

revolution around the Sun

For the same reason, there are two points on Mercury's equator,180 degrees apart in longitude, at either of which, around perihelionin alternate Mercurian years (once a Mercurian day), the Sunpasses overhead, then reverses its apparent motion and passesoverhead again, then reverses a second time and passes overhead athird time, taking a total of about 16 Earth-days for this entireprocess. In the other alternate Mercurian years, the same thinghappens at the other of these two points. The amplitude of theretrograde motion is small, so the overall effect is that, for two orthree weeks, the Sun is almost stationary overhead, and is at itsmost brilliant because Mercury is at perihelion, its closest to theSun. This prolonged exposure to the Sun at its brightest makesthese two points the hottest places on Mercury. Conversely, thereare two other points on the equator, 90 degrees of longitude apartfrom the first ones, where the Sun passes overhead only when theplanet is at aphelion in alternate years, when the apparent motion ofthe Sun in the Mercurian sky is relatively rapid. These points, whichare the ones on the equator where the apparent retrograde motionof the Sun happens when it is crossing the horizon as described inthe preceding paragraph, receive much less solar heat than the firstones described above.

Mercury attains inferior conjunction (near approach to the Earth)

every 116 Earth days on average,[2] but this interval can range from105 days to 129 days due to the planet's eccentric orbit. Mercury

can come as close as 77.3 million km to the Earth,[2] but it will notbe closer to Earth than 80 Gm until AD 28,622. The next approach

to within 82.1 Gm is in 2679, and to within 82 Gm in 4487.[74] Itsperiod of retrograde motion as seen from Earth can vary from 8 to15 days on either side of inferior conjunction. This large range arises

from the planet's high orbital eccentricity.[12]

Spinorbit resonance

For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating once foreach orbit and always keeping the same face directed towards the Sun, in the same way that the same side ofthe Moon always faces the Earth. Radar observations in 1965 proved that the planet has a 3:2 spinorbitresonance, rotating three times for every two revolutions around the Sun; the eccentricity of Mercury's orbitmakes this resonance stableat perihelion, when the solar tide is strongest, the Sun is nearly still in Mercury's

sky.[75]

The original reason astronomers thought it was synchronously locked was that, whenever Mercury was bestplaced for observation, it was always nearly at the same point in its 3:2 resonance, hence showing the sameface. This is because, coincidentally, Mercury's rotation period is almost exactly half of its synodic period withrespect to Earth. Due to Mercury's 3:2 spinorbit resonance, a solar day (the length between two meridian

transits of the Sun) lasts about 176 Earth days.[12] A sidereal day (the period of rotation) lasts about 58.7 Earth

days.[12]

Simulations indicate that the orbital eccentricity of Mercury varies chaotically from nearly zero (circular) to more

than 0.45 over millions of years due to perturbations from the other planets.[12][76] This is thought to explainMercury's 3:2 spinorbit resonance (rather than the more usual 1:1), because this state is more likely to arise



After one orbit, Mercury has rotated

1.5 times, so after two complete

orbits the same hemisphere is again

illuminated.

during a period of high eccentricity.[77] Numerical simulations showthat a future secular orbital resonant perihelion interaction with Jupitermay cause the eccentricity of Mercury's orbit to increase to the pointwhere there is a 1% chance that the planet may collide with Venus

within the next five billion years.[78][79]

Advance of perihelion

Main article: Perihelion precession of Mercury

In 1859, the French mathematician and astronomer Urbain Le Verrierreported that the slow precession of Mercury's orbit around the Suncould not be completely explained by Newtonian mechanics andperturbations by the known planets. He suggested, among possibleexplanations, that another planet (or perhaps instead a series ofsmaller 'corpuscules') might exist in an orbit even closer to the Sun

than that of Mercury, to account for this perturbation.[80] (Otherexplanations considered included a slight oblateness of the Sun.) Thesuccess of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to placefaith in this possible explanation, and the hypothetical planet was named Vulcan, but no such planet was ever

found.[81]

The perihelion precession of Mercury is 5600 arcseconds (1.5556) per century relative to the Earth, or

574.100.65 arcseconds per century[82] relative to the inertial ICFR. Newtonian mechanics, taking into account

all the effects from the other planets, predicts a precession of 5557 arcseconds (1.5436) per century.[82] In theearly 20th century, Albert Einstein's General Theory of Relativity provided the explanation for the observedprecession. The effect is very small: the Mercurian relativistic perihelion advance excess is just 42.98 arcsecondsper century; therefore, it requires a little over twelve million orbits for a full excess turn. Similar, but muchsmaller, effects operate for other planets: 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars,

and 10.05 for 1566 Icarus.[83][84]

Observation

Mercury's apparent magnitude varies between 2.6[9] (brighter than the brightest star Sirius) and about +5.7(approximating the theoretical limit of naked-eye visibility). The extremes occur when Mercury is close to the

Sun in the sky.[9][10] Observation of Mercury is complicated by its proximity to the Sun, as it is lost in the Sun'sglare for much of the time. Mercury can be observed for only a brief period during either morning or evening

twilight.[citation needed]

Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.[85]

Like the Moon and Venus, Mercury exhibits phases as seen from Earth. It is "new" at inferior conjunction and"full" at superior conjunction. The planet is rendered invisible from Earth on both of these occasions because of

its relative nearness to the Sun.[citation needed]



Mosaic image by Mariner,

1974

Mercury is technically brightest as seen from Earth when it is at a full phase.Although the planet is farthest away from Earth when it is full the greaterilluminated area that is visible and the opposition brightness surge more than

compensates for the distance.[9] The opposite is true for Venus, whichappears brightest when it is a crescent, because it is much closer to Earth than

when gibbous.[9][86]

Nonetheless, the brightest (full phase) appearance of Mercury is an essentiallyimpossible time for practical observation, because of the extreme proximity ofthe Sun. Mercury is best observed at the first and last quarter, although theyare phases of lesser brightness. The first and last quarter phases occur atgreatest elongation east and west, respectively. At both of these timesMercury's separation from the Sun ranges anywhere from 17.9 at perihelion

to 27.8 at aphelion.[87][88] At greatest elongation west, Mercury rises at itsearliest before the Sun, and at greatest elongation east, it sets at its latest after

the Sun.[89]

At tropical and subtropical latitudes, Mercury is more easily seen than athigher latitudes. This is the result of two effects: (i) the Sun ascends above thehorizon more steeply at sunrise and descends more steeply at sunset, so thetwilight period is shorter, and (ii) at the right times of year, the ecliptic intersects the horizon at a very steepangle, meaning that Mercury can be relatively high (altitude up to 28) in a fully dark sky. Such conditions canexist, for instance, after sunset near the Spring Equinox, in March/April for the southern USA and inSeptember/October for South Africa and Australasia. Conversely, pre-sunrise viewing is easiest near the

Autumn Equinox.[citation needed][g]

At temperate latitudes, Mercury is more often easily visible from Earth's Southern Hemisphere than from itsNorthern Hemisphere. This is because Mercury's maximum possible elongations west of the Sun always occurwhen it is early autumn in the Southern Hemisphere, whereas its maximum possible eastern elongations happen

during late winter in the Southern Hemisphere.[89] In both of these cases, the angle Mercury strikes with theecliptic is maximized, allowing it to rise several hours before the Sun in the former instance and not set untilseveral hours after sundown in the latter in countries located at southern temperate zone latitudes, such as

Argentina and South Africa.[89] By contrast, at the major population centers of the northern temperate latitudes,

Mercury is never above the horizon of a more-or-less fully dark night sky.[citation needed]

Ground-based telescope observations of Mercury reveal only an illuminated partial disk with limited detail. Thefirst of two spacecraft to visit the planet was Mariner 10, which mapped about 45% of its surface from 1974 to1975. The second is the MESSENGER spacecraft, which after three Mercury flybys between 2008 and 2009,

attained orbit around Mercury on March 17, 2011,[91] to study and map the rest of the planet.[92]

The Hubble Space Telescope cannot observe Mercury at all, due to safety procedures that prevent its pointing

too close to the Sun.[93]

Naked-eye viewing

Mercury is seen most easily when it is close to its greatest elongation, which means that its angular separationfrom the Sun is greatest. It can be near greatest western elongation, which means it is west of the Sun in the sky,so it is visible soon before sunrise, or greatest eastern elongation, which means it is visible soon after sunset.However, the exact dates of the greatest elongations are not the best ones on which to try to see Mercury. Thephase of the planet greatly affects its apparent brightness. At greatest elongation, it is approximately at halfphase. It is brighter when it is gibbous, which means that the best times to see Mercury are a few days before



greatest eastern elongation, in the evening, or a few days after greatest western elongation, in the morning. Theapparent inclination of the ecliptic to the horizon is also important. When the inclination is large, as occurs nearthe spring equinox in the evening, and near the autumnal equinox in the morning (this is true for observers in bothhemispheres), Mercury is higher in the sky when the Sun is just below the horizon, which makes it easier to seethan it other times. The inclination of the ecliptic is also greater for observers at low latitudes than high ones. It ishelpful if Mercury is close to aphelion at the time of observation, because this makes it further from the Sun thanat other times. However, it also makes the planet less brightly illuminated, so the visibility advantage is not great.At present, Mercury is fairly close to aphelion when viewed at greatest western elongation at the Marchequinox, or at greatest eastern elongation at the September equinox. (Over long periods of time, this changes as

Mercury's orbit shifts.)[citation needed]

Putting all these factors together, the best time for an observer in the Southern Hemisphere to see Mercury is inthe morning, near the March equinox, a few days after Mercury is at greatest western elongation, or in theevening, near the September equinox, a few days before greatest eastern elongation. An observer in theNorthern Hemisphere cannot optimize all the factors simultaneously. Usually, the best chances of seeing theplanet are in the evening, near the March equinox, a few days before greatest eastern elongation, or in themorning, near the September equinox, a few days after greatest western elongation. The inclination of the

ecliptic is then large, but Mercury is not close to aphelion.[citation needed]

Mercury's period of revolution around the Sun is 88 days. It therefore makes about 4.15 revolutions around theSun in one Earth-year. In successive years the position of Mercury on its orbit therefore shifts by 0.15revolutions when seen on specific dates, such as the equinoxes. Therefore, if, for example, greatest easternelongation happens on the March equinox of some year, about three years later greatest western elongation willhappen near the March equinox, because the position of Mercury on its orbit at the equinox will have changedby about half (.45) a revolution. Thus, if the timings of elongations and equinoxes are unfavourable for observing

Mercury in some year, they will be fairly favourable within about three years later.[citation needed] Furthermore,since the shift of .15 revolutions in a year makes up a seven-year cycle (0.1571.0), in the seventh yearMercury will follows almost exactly (earlier by 7 days) the sequence of phenomena it showed seven years

before.[87]

When conditions are near optimal, Mercury is easy to see. However, optimal conditions are rare, and many

casual observers search for Mercury without success.[citation needed]

Observation history

Ancient astronomers

The earliest known recorded observations of Mercury are from the Mul.Apin tablets. These observations were

most likely made by an Assyrian astronomer around the 14th century BC.[94] The cuneiform name used to

designate Mercury on the Mul.Apin tablets is transcribed as Udu.Idim.Gu\u4.Ud ("the jumping planet").[h][95]

Babylonian records of Mercury date back to the 1st millennium BC. The Babylonians called the planet Nabu

after the messenger to the gods in their mythology.[96]

The ancient Greeks of Hesiod's time knew the planet as (Stilbon), meaning "the gleaming", and

(Hermaon).[97] Later Greeks called the planet Apollo when it was visible in the morning sky, andHermes when visible in the evening. Around the 4th century BC, Greek astronomers came to understand thatthe two names referred to the same body, Hermes (: Herms), a planetary name that is retained in

modern Greek (: Ermis).[98] The Romans named the planet after the swift-footed Roman messenger god,



Ibn al-Shatir's model for the

appearances of Mercury, showing the

multiplication of epicycles using the

Tusi-couple, thus eliminating the

Ptolemaic eccentrics and equant.

Mercury (Latin Mercurius), which they equated with the Greek Hermes, because it moves across the sky faster

than any other planet.[99][100] The astronomical symbol for Mercury is a stylized version of Hermes'

caduceus.[101]

The Roman-Egyptian astronomer Ptolemy wrote about the possibility of planetary transits across the face of theSun in his work Planetary Hypotheses. He suggested that no transits had been observed either because planets

such as Mercury were too small to see, or because the transits were too infrequent.[102]

In ancient China, Mercury was known as Chen Xing (), theHour Star. It was associated with the direction north and the phase of

water in the Wu Xing.[103] Modern Chinese, Korean, Japanese andVietnamese cultures refer to the planet literally as the "water star" (

), based on the Five elements.[104] Hindu mythology used the nameBudha for Mercury, and this god was thought to preside over

Wednesday.[105] The god Odin (or Woden) of Germanic paganism

was associated with the planet Mercury and Wednesday.[106] TheMaya may have represented Mercury as an owl (or possibly fourowls; two for the morning aspect and two for the evening) that served

as a messenger to the underworld.[107]

The ancient association of Mercury with Wednesday is still visible inthe names of Wednesday in various modern languages of Latindescent, e.g. mercredi in French, mircoles in Spanish, or miercuriin Romanian. The names of the days of the week were, in classicaltimes, all related to the names of the seven bodies that were then

considered to be planets.[citation needed]

In ancient Indian astronomy, the Surya Siddhanta, an Indian astronomical text of the 5th century, estimates thediameter of Mercury as 4,841 kilometres (3,008 mi), an error of less than 1% from the currently accepteddiameter of 4,880 kilometres (3,032 mi). This estimate was based upon an inaccurate guess of the planet's

angular diameter as 3.0 arcminutes (50 millidegrees).[citation needed]

In medieval Islamic astronomy, the Andalusian astronomer Ab Ishq Ibrhm al-Zarql in the 11th centurydescribed the deferent of Mercury's geocentric orbit as being oval, like an egg or a pignon, although this insight

did not influence his astronomical theory or his astronomical calculations.[108][109] In the 12th century, IbnBajjah observed "two planets as black spots on the face of the Sun", which was later suggested as the transit of

Mercury and/or Venus by the Maragha astronomer Qotb al-Din Shirazi in the 13th century.[110] (Note that most

such medieval reports of transits were later taken as observations of sunspots.[111])

In India, the Kerala school astronomer Nilakantha Somayaji in the 15th century developed a partiallyheliocentric planetary model in which Mercury orbits the Sun, which in turn orbits the Earth, similar to the

Tychonic system later proposed by Tycho Brahe in the late 16th century.[112]

Ground-based telescopic research

The first telescopic observations of Mercury were made by Galileo in the early 17th century. Although heobserved phases when he looked at Venus, his telescope was not powerful enough to see the phases ofMercury. In 1631, Pierre Gassendi made the first telescopic observations of the transit of a planet across theSun when he saw a transit of Mercury predicted by Johannes Kepler. In 1639, Giovanni Zupi used a telescopeto discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated



Transit of Mercury. 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.

conclusively that Mercury orbited around the Sun.[12]

A very rare event in astronomy is the passage of one planet in front of another (occultation), 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 John Bevis at the Royal Greenwich Observatory.[113] The next

occultation of Mercury by Venus will be on December 3, 2133.[114]

The difficulties inherent in observing Mercury mean that it has been farless studied than the other planets. In 1800, Johann Schrter madeobservations of surface features, claiming to have observed 20 kmhigh mountains. Friedrich Bessel used Schrter's drawings toerroneously estimate the rotation period as 24 hours and an axial tilt

of 70.[115] In the 1880s, Giovanni Schiaparelli mapped the planetmore accurately, and suggested that Mercury's rotational period was

88 days, the same as its orbital period due to tidal locking.[116] Thisphenomenon is known as synchronous rotation. The effort to map thesurface of Mercury was continued by Eugenios Antoniadi, whopublished a book in 1934 that included both maps and his own

observations.[63] Many of the planet's surface features, particularly the

albedo features, take their names from Antoniadi's map.[117]

In June 1962, Soviet scientists at the Institute of Radio-engineeringand Electronics of the USSR Academy of Sciences led by VladimirKotelnikov became first to bounce radar signal off Mercury and receive it, starting radar observations of the

planet.[118][119][120] Three years later radar observations by Americans Gordon Pettengill and R. Dyce using300-meter Arecibo Observatory radio telescope in Puerto Rico showed conclusively that the planet's rotational

period was about 59 days.[121][122] The theory that Mercury's rotation was synchronous had become widelyheld, and it was a surprise to astronomers when these radio observations were announced. If Mercury weretidally locked, its dark face would be extremely cold, but measurements of radio emission revealed that it wasmuch 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.[123]

Italian astronomer Giuseppe Colombo noted that the rotation value was about two-thirds of Mercury's orbitalperiod, and proposed that the planet's orbital and rotational periods were locked into a 3:2 rather than a 1:1

resonance.[124] Data from Mariner 10 subsequently confirmed this view.[125] This means that Schiaparelli's andAntoniadi's maps were not "wrong". Instead, the astronomers saw the same features during every second orbitand recorded them, but disregarded those seen in the meantime, when Mercury's other face was toward the

Sun, because the orbital geometry meant that these observations were made under poor viewing conditions.[115]

Ground-based optical observations did not shed much further light on the innermost planet, but radioastronomers using interferometry at microwave wavelengths, a technique that enables removal of the solarradiation, were able to discern physical and chemical characteristics of the subsurface layers to a depth of

several meters.[126][127] Not until the first space probe flew past Mercury did many of its most fundamentalmorphological properties become known. Moreover, recent technological advances have led to improvedground-based observations. In 2000, high-resolution lucky imaging observations were conducted by the MountWilson Observatory 1.5 meter Hale telescope. They provided the first views that resolved surface features on

the parts of Mercury that were not imaged in the Mariner mission.[128] Later imaging has shown evidence of ahuge double-ringed impact basin even larger than the Caloris Basin in the non-Mariner-imaged hemisphere. It

has informally been dubbed the Skinakas Basin.[129] Most of the planet has been mapped by the Arecibo radar

telescope, with 5 km resolution, including polar deposits in shadowed craters of what may be water ice.[130]



Mariner 10, the first probe to visit the

innermost planet

Research with space probes

Main article: Exploration of Mercury

Reaching Mercury from Earth poses significant technical challenges, because the planet orbits so much closer tothe Sun than Earth. A Mercury-bound spacecraft launched from Earth must travel over 91 million kilometersinto the Sun's gravitational potential well. Mercury has an orbital speed of 48 km/s, whereas Earth's orbitalspeed is 30 km/s. Thus the spacecraft must make a large change in velocity (delta-v) to enter a Hohmann

transfer orbit that passes near Mercury, as compared to the delta-v required for other planetary missions.[131]

The potential energy liberated by moving down the Sun's potential well becomes kinetic energy; requiringanother large delta-v change to do anything other than rapidly pass by Mercury. To land safely or enter a stableorbit the spacecraft would rely entirely on rocket motors. Aerobraking is ruled out because the planet has verylittle atmosphere. A trip to Mercury requires more rocket fuel than that required to escape the Solar System

completely. As a result, only two space probes have visited the planet so far.[132] A proposed alternative

approach would use a solar sail to attain a Mercury-synchronous orbit around the Sun.[133]

Mariner 10

Main article: Mariner 10

The first spacecraft to visit Mercury was NASA's Mariner 10

(197475).[99] The spacecraft used the gravity of Venus to adjustits orbital velocity so that it could approach Mercury, making it boththe first spacecraft to use this gravitational "slingshot" effect and the

first NASA mission to visit multiple planets.[131] Mariner 10provided the first close-up images of Mercury's surface, whichimmediately showed its heavily cratered nature, and revealed manyother types of geological features, such as the giant scarps whichwere later ascribed to the effect of the planet shrinking slightly as its

iron core cools.[134] Unfortunately, due to the length ofMariner 10's orbital period, the same face of the planet was lit ateach of Mariner 10's close approaches. This made observation of

both sides of the planet impossible,[135] and resulted in the mapping

of less than 45% of the planet's surface.[136]

On March 27, 1974, two days before its first flyby of Mercury, Mariner 10's instruments began registering largeamounts of unexpected ultraviolet radiation near Mercury. This led to the tentative identification of Mercury'smoon. Shortly afterward, the source of the excess UV was identified as the star 31 Crateris, and Mercury's

moon passed into astronomy's history books as a footnote.[citation needed]

The spacecraft made three close approaches to Mercury, the closest of which took it to within 327 km of the

surface.[137] At the first close approach, instruments detected a magnetic field, to the great surprise of planetarygeologistsMercury's rotation was expected to be much too slow to generate a significant dynamo effect. Thesecond close approach was primarily used for imaging, but at the third approach, extensive magnetic data wereobtained. The data revealed that the planet's magnetic field is much like the Earth's, which deflects the solar wind

around the planet. The origin of Mercury's magnetic field is still the subject of several competing theories.[138]



MESSENGER being prepared for

launch

On March 24, 1975, just eight days after its final close approach, Mariner 10 ran out of fuel. Because its orbit

could no longer be accurately controlled, mission controllers instructed the probe to shut down.[139] Mariner 10

is thought to be still orbiting the Sun, passing close to Mercury every few months.[140]

MESSENGER

Main article: MESSENGER

A second NASA mission to Mercury, named MESSENGER(MErcury Surface, Space ENvironment, GEochemistry, andRanging), was launched on August 3, 2004, from the Cape CanaveralAir Force Station aboard a Boeing Delta 2 rocket. It made a fly-by ofthe Earth in August 2005, and of Venus in October 2006 and June2007 to place it onto the correct trajectory to reach an orbit around

Mercury.[141] A first fly-by of Mercury occurred on January 14,

2008, a second on October 6, 2008,[142] and a third on September

29, 2009.[143] Most of the hemisphere not imaged by Mariner 10 hasbeen mapped during these fly-bys. The probe successfully entered anelliptical orbit around the planet on March 18, 2011. The first orbital

image of Mercury was obtained on March 29, 2011. The probe finished a one-year mapping mission,[142] andis now on a one-year extended mission expected to end in 2013. In addition to continued observations and

mapping of Mercury, MESSENGER will observe the 2012 solar maximum.[144]

The mission is designed to clear up six key issues: Mercury's high density, its geological history, the nature of itsmagnetic field, the structure of its core, whether it has ice at its poles, and where its tenuous atmosphere comesfrom. To this end, the probe is carrying imaging devices which will gather much higher resolution images of muchmore of the planet than Mariner 10, assorted spectrometers to determine abundances of elements in the crust,and magnetometers 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.[24]

BepiColombo

Main article: BepiColombo

The European Space Agency is planning a joint mission with Japan called BepiColombo, which will orbit

Mercury with two probes: one to map the planet and the other to study its magnetosphere.[145] Once launched

in 2015, the spacecraft bus is expected to reach Mercury in 2019.[146] The bus will release a magnetometerprobe 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.[145] The mapper probe will carry an array of spectrometerssimilar to those on MESSENGER, and will study the planet at many different wavelengths including infrared,

ultraviolet, X-ray and gamma ray.[147]

See also

Mercury in astrology

Mercury in fictionColonization of Mercury

Exploration of Mercury



Mercury, from Liber

astronomiae, 1550

Notes

a. â b Pluto was considered a planet from its discovery in 1930 to 2006, butafter that it has been classified as a dwarf planet. Pluto's orbital eccentricityis greater than that of Mercury. Pluto is also smaller than Mercury, but wasassumed to be larger until 1976.

b. ^ In astronomy, the words "rotation" and "revolution" have differentmeanings. "Rotation" is the turning of a body about an axis which passesthrough the body, as in "The Earth rotates once a day.". "Revolution" ismotion around a centre which is external to the body, usually in orbit, as in"The Earth takes a year for each revolution around the Sun.". The verbs"rotate" and "revolve" mean doing rotation and revolution, respectively.

c. ^ This is like the rotation of the Moon. Relative to the fixed stars, the Moon rotates once a month. As seenfrom Earth, it appears not to rotate at all. The angular velocity of the rotation in a rotating frame of reference isthe difference between the rotation velocities of the body in question and the frame of reference, relative to thefixed stars. In the case of the Moon, seen in a frame of reference that rotates in the prograde direction onerotation per month, the difference is zero, so the Moon appears not to rotate. In the case of Mercury, seen in aframe of reference that rotates with its orbital motion, the apparent rotational velocity is one rotation (i.e., 3-2)every two orbital revolutions. In the case of Venus, which rotates in the retrograde direction relative to thefixed stars, the apparent rotational speed, seen e.g. from the Sun, is faster than the true rotational speed.

d. ^ Because of the extremely small tilt of Mercury's axis, sunrise and sunset would be seen due east and westfrom anywhere on the planet, except at the poles and places very close to them.

e. ^ Seen from the Sun, Mercury appears (approximately) not to be rotating as it passes through perihelion. Thisis the time when the Sun's tidal effect on Mercury is the strongest, roughly 3.5 times as great as at aphelion.The strong tidal effect locks Mercury's rotation so it is approximately synchronized with its orbital motion atthat time, and so that Mercury always has exactly the same axis pointed toward or away from the Sun as itpasses through perihelion. This causes the 3:2 rotation lock.

f. ^ If the area of Washington is about 177 km2 and 2.5 miles is taken to equal 4 km, Solomon's estimate would

equal about 700 cubic kilometres of ice, which would have a mass of about 600 billion tons (6 1014 kg).

g. ^ Longitude on Mercury increases in the westerly direction. A small crater named Hun Kal provides the

reference point for measuring longitude. The center of Hun Kal is 20 west longitude.[90]

h. ^ Some sources precede the cuneiform transcription with "MUL". "MUL" is a cuneiform sign that was used inthe Sumerian language to designate a star or planet, but it is not considered part of the actual name. The "4" is areference number in the Sumero-Akkadian transliteration system to designate which of several syllables acertain cuneiform sign is most likely designating.

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