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Planets In The Solar System
Mercury
Mercury is the innermost planet in the Solar System. It is also the smallest, and its orbit is the most eccentric (that is, the least perfectly circular) of the eight planets. It orbits the Sun once in about 88 Earth days, completing three rotations about its axis for every two orbits. The planet is named after the Roman god Mercury, the messenger to the gods.
Mercury's surface is heavily cratered and similar in appearance to Earth's Moon,
indicating that it has been geologically inactive for billions of
years. Due to its near lack of an atmosphere to retain heat, Mercury's
surface experiences the steepest temperature gradient of all the
planets, ranging from a very cold 100 K at night to a very hot 700 K during the day. Mercury's axis has the smallest tilt of any of the Solar System's planets, but Mercury's orbital eccentricity
is the largest. The seasons on the planet's surface are caused by the
variation of its distance from the Sun rather than by the axial tilt,
which is the main cause of seasons on Earth and other planets. At perihelion, the intensity of sunlight on Mercury's surface is more than twice the intensity at aphelion. Mercury and Venus can each make appearances in Earth's sky both as a morning star and an evening star (because they are closer to the Sun than the Earth),
and at times Mercury can technically be regarded as a very bright
object when viewed from Earth; however, its proximity in the sky to the
Sun makes it more difficult to see than Venus .
Internal structure
Names for features on Mercury come from a variety of sources. Names coming from people are limited to the deceased. Craters are named for artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field. Ridges, or dorsa, are named for scientists who have contributed to the study of Mercury. Depressions or 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 rupēs are named for ships of scientific expeditions. Valleys or valles are named for radio telescope facilities.
Albedo features are areas of markedly different reflectivity, as seen by telescopic observation. Mercury possesses dorsa (also called "wrinkle-ridges"), Moon-like highlands, montes (mountains), planitiae, or plains, rupes (escarpments), and valles (valleys).
Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the late heavy bombardment that came to an end 3.8 billion years ago. During this period of intense crater formation, the planet received impacts over its entire surface, facilitated by the lack of any atmosphere to slow impactors down. During this time the planet was volcanically active; basins such as the Caloris Basin were filled by magma from within the planet, which produced smooth plains similar to the maria found on the Moon.
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 the Moon's, both of which contain significant stretches of similar geology, such as maria and plateaus.
The largest known crater is Caloris Basin, with a diameter of 1,550 km. The impact that created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric ring over 2 km tall surrounding the impact crater. At the antipode
of the Caloris Basin is a large region of unusual, hilly terrain known
as the "Weird Terrain". One hypothesis for its origin is that shock
waves generated during the Caloris impact traveled around the planet,
converging at the basin’s antipode (180 degrees away). The resulting
high stresses fractured the surface.
Alternatively, it has been suggested that this terrain formed as a
result of the convergence of ejecta at this basin’s antipode.
Overall, about 15 impact basins have been identified on the imaged part of Mercury. A notable basin is the 400 km wide, multi-ring Tolstoj Basin which has an ejecta blanket extending up to 500 km from its rim and a floor that has been filled by smooth plains materials. Beethoven Basin has a similar-sized ejecta blanket and a 625 km diameter rim. Like the Moon, the surface of Mercury has likely incurred the effects of space weathering processes, including Solar wind and micrometeorite impacts.
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. 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. 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.
One unusual feature of the planet’s surface is the numerous compression folds, or rupes, 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. Mercury’s surface is flexed by significant tidal bulges raised by the Sun—the Sun’s tides on Mercury are about 17 times stronger than the Moon’s on Earth.
Surface conditions and "atmosphere" (exosphere)
Main article: Atmosphere of Mercury
The surface temperature of Mercury ranges from 100 K to 700 K due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. The subsolar point reaches about 700 K during perihelion then drops to 550 K at aphelion. On the dark side of the planet, temperatures average 110 K. The intensity of sunlight on Mercury’s surface ranges between 4.59 and 10.61 times the solar constant (1,370 W·m−2).
Although the daylight temperature at the surface of Mercury is generally extremely high, observations strongly suggest that ice (frozen water) exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K; far lower than the global average. Water ice strongly reflects radar, and observations by the 70 m Goldstone telescope and the VLA in the early 1990s revealed that there are patches of very high radar reflection near the poles. While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.
The icy regions are believed to contain about 1014–1015 kg of ice, and may be covered by a layer of regolith that inhibits sublimation. By comparison, the Antarctic ice sheet on Earth has a mass of about 4×1018 kg, and Mars' south polar cap contains about 1016 kg of water. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by impacts of comets.
Mercury is too small and hot for its gravity to retain any significant atmosphere over long periods of time; it does have a "tenuous surface-bounded exosphere" containing hydrogen, helium, oxygen, sodium, calcium, potassium
and others. This exosphere is not stable—atoms are continuously lost
and replenished from a variety of sources. Hydrogen and helium atoms
probably come from the solar wind, diffusing into Mercury’s magnetosphere before later escaping back into space. Radioactive decay of elements within Mercury’s crust 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 is present, released by a combination of processes such as: comets striking its surface, sputtering creating water out of hydrogen from the solar wind
and oxygen from rock, and sublimation from reservoirs of water ice in
the permanently shadowed polar craters. The detection of high amounts of
water-related ions like O+, OH-, and H2O+ was a surprise.
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.
Sodium, potassium and calcium were discovered in the atmosphere during the 1980–1990s, and are believed to result primarily from the vaporization of surface rock struck by micrometeorite impacts. In 2008 magnesium was discovered by MESSENGER probe. Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic poles. This would indicate an interaction between the magnetosphere and the planet's surface.
Main article: Mercury's magnetic field
Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by Mariner 10, it is about 1.1% as strong as the Earth’s. The magnetic field strength at the Mercurian equator is about 300 nT. Like that of Earth, Mercury's magnetic field is dipolar. Unlike Earth, Mercury's poles are nearly aligned with the planet's spin axis. Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.
It is likely that this magnetic field is generated by way of a dynamo effect, in a manner similar to the magnetic field of Earth. 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.
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, is strong enough to trap solar wind plasma. This contributes to the space weathering of the planet's surface. Observations taken by the 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.
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 800 km wide orflux transfer events, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface.
The process of linking interplanetary and planetary magnetic fields, called magnetic reconnection, is common throughout the cosmos. It occurs in Earth's magnetic field, where it generates magnetic tornadoes as well. 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.
Mercury has the most eccentric
orbit of all the planets; its eccentricity is 0.21 with its distance
from the Sun ranging from 46,000,000 to 70,000,000 km (29,000,000 to
43,000,000 mi). It takes 87.969 earth 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 semi-major axis.
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 spin-orbit resonance of the planet's rotation around its axis, result in complex variations of the surface temperature. This resonance makes a single day on Mercury last exactly two Mercury years, or about 176 Earth days.
Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (the ecliptic), as shown in the diagram on the right. As a result, transits 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.
Mercury's axial tilt is almost zero, with the best measured value as low as 0.027 degrees. This is significantly smaller than that of Jupiter, which has 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 arcminutes above the horizon.
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 Mercurian day. This is because approximately four Earth days before perihelion, Mercury's angular orbital velocity exactly equals its angular rotational velocity so that the Sun's apparent motion ceases; at perihelion, Mercury's angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.
Mercury attains inferior conjunction (near approach to the Earth) every 116 Earth days on average, 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 million km to the Earth, but it will not be 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. Its period of retrograde motion 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 orbital eccentricity.
Spin–orbit resonance
For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating
once for each orbit and always keeping the same face directed towards
the Sun, in the same way that the same side of the Moon always faces the
Earth. Radar
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.
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 solar day (the length between two meridian transits of the Sun) lasts about 176 Earth days. A sidereal day (the period of rotation) lasts about 58.7 Earth days.
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. 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. Numerical simulations show that a future secular orbital resonant perihelion interaction with Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the planet may collide with Venus within the next five billion years.
In 1859, the French mathematician and astronomer Urbain Le Verrier reported that the slow precession of Mercury’s orbit around the Sun could not be completely explained by Newtonian mechanics 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. (Other explanations considered included a slight oblateness of the Sun.) The success of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to place faith in this possible explanation, and the hypothetical planet was named Vulcan, but no such planet was ever found.
The perihelion precession of Mercury is 5600 arc seconds (1.5556°) per century relative to the Earth, or 574.10±0.65 arc-seconds per century relative to the inertial ICFR. Newtonian mechanics, taking into account all the effects from the other planets, predicts a precession of 5557 seconds of arc (1.5436°) per century. In the early 20th century, Albert Einstein’s General Theory of Relativity provided the explanation for the observed precession. The effect is very small: the Mercurian relativistic perihelion advance excess is just 42.98 arcseconds 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: 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for 1566 Icarus.
Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.
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.
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 greater illuminated area that is visible and the opposition brightness surge more than compensates for the distance. The opposite is true for Venus, which appears brightest when it is a crescent, because it is much closer to Earth than when gibbous.
Nonetheless, the brightest (full phase) appearance of Mercury is an
essentially impossible time for practical observation, because of the
extreme proximity of the Sun. Mercury is best observed at the first and
last quarter, although they are phases of lesser brightness. The first
and last quarter phases occur at greatest elongation east and west, respectively. At both of these times Mercury's separation from the Sun ranges anywhere from 17.9° at perihelion to 27.8° at aphelion.
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.
At tropical and subtropical latitudes, Mercury is more easily seen than at higher latitudes. This is the result of two effects: (i) the Sun ascends above the horizon more steeply at sunrise and descends more steeply at sunset, so the twilight period is shorter, and (ii) at the right times of year, the ecliptic intersects the horizon at a very steep angle, meaning that Mercury can be relatively high (altitude up to 28°) in a fully dark sky. Such conditions can exist, for instance, after sunset near the Spring Equinox, in March/April for the southern USA and in September/October for South Africa and Australasia. Conversely, pre-sunrise viewing is easiest near the Autumn Equinox.
At temperate latitudes, Mercury is more often easily visible from Earth’s Southern Hemisphere than from its Northern Hemisphere. This is because Mercury's 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. In both of these cases, the angle Mercury strikes with the ecliptic 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 Argentina and South Africa. 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.
Ground-based telescope observations of Mercury reveal only an illuminated partial disk with limited detail. The first of two spacecraft to visit the planet was Mariner 10, which mapped about 45% of its surface from 1974 to 1975. The second is the MESSENGER spacecraft, which after three Mercury flybys between 2008 and 2009, attained orbit around Mercury on March 17, 2011, to study and map the rest of the planet.
The Hubble Space Telescope cannot observe Mercury at all, due to safety procedures which prevent its pointing too close to the Sun.
Naked-eye viewing
Mercury is seen most easily when it is close to greatest elongation,
which means that its angular separation from 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. The phase of the planet greatly
affects its apparent brightness. At greatest elongation, it is
approximately at half phase. 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. The apparent inclination of
the ecliptic to the horizon is also important. When the inclination is
large, as occurs near the spring equinox in the evening, and near the
autumnal equinox in the morning (this is true for observers in both
hemispheres), Mercury is higher in the sky when the Sun is just below
the horizon, which makes it easier to see than it other times. The
inclination of the ecliptic is also greater for observers at low
latitudes than high ones. It is helpful if Mercury is close to aphelion
at the time of observation, since this makes it further from the Sun
than at 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 March equinox, or at greatest eastern elongation at
the September equinox. (Over long periods of time, this changes as
Mercury's orbit shifts.)
Putting all these factors together, the best time for an observer in the Southern Hemisphere to see Mercury is in the morning, near the March equinox, a few days after Mercury is at greatest western elongation, or in the evening, near the September equinox, a few days before greatest eastern elongation. An observer in the Northern Hemisphere cannot optimize all the factors simultaneously. Usually, the best chances of seeing the planet are in the evening, near the March equinox, a few days before greatest eastern elongation, or in the morning, 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.
Mercury's period of revolution around the Sun is 88 days. It therefore makes about 4.15 revolutions around the Sun in one Earth-year. In successive years the position of Mercury on its orbit shifts by 0.15 revolutions when seen on specific dates, such as the equinoxes. Therefore, if, for example, greatest eastern elongation happens on the March equinox of some year, about three years later greatest western elongation will happen near the March equinox, since the position of Mercury on its orbit at the equinox will have changed by about half 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.
When conditions are near optimal, Mercury is easy to see. However, optimal conditions are rare, and many casual observers search for Mercury without success.
The ancient Greeks of Hesiod's time knew the planet as Στίλβων (Stilbon), meaning "the gleaming", and Ἑρμάων (Hermaon). Later Greeks called the planet Apollo when it was visible in the morning sky, and Hermes when visible in the evening. Around the 4th century BC, Greek astronomers came to understand that the two names referred to the same body, Hermes (Ερμής: Ermis), a planetary name which is retained in modern Greek. The Romans named the planet after the swift-footed Roman messenger god, Mercury (Latin Mercurius), which they equated with the Greek Hermes, because it moves across the sky faster than any other planet. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.
The Roman-Egyptian astronomer Ptolemy wrote about the possibility of planetary transits across the face of the Sun 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.
In ancient China, Mercury was known as Chen Xing (辰星), the Hour Star. It was associated with the direction north and the phase of water in the Wu Xing. Modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet literally as the "water star" (水星), based on the Five elements. Hindu mythology used the name Budha for Mercury, and this god was thought to preside over Wednesday. The god Odin (or Woden) of Germanic paganism was associated with the planet Mercury and Wednesday. The Maya 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 underworld.
The ancient association of Mercury with Wednesday is still visible in the names of Wednesday in various modern languages of Latin descent, e.g. mercredi in French, miércoles in Spanish, or miercuri in Romanian. The names of the days of the week were, in classical times, all related to the names of the seven bodies that were then considered to be planets.
In ancient Indian astronomy, the Surya Siddhanta, an Indian astronomical text of the 5th century, estimates the diameter of Mercury as 3,008 miles, an error of less than 1% from the currently accepted diameter of 3,032 miles (4,880 km). This estimate was based upon an inaccurate guess of the planet's angular diameter as 3.0 arcminutes.
In medieval Islamic astronomy, the Andalusian astronomer Abū Ishāq Ibrāhīm al-Zarqālī in the 11th century described 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. In the 12th century, Ibn Bajjah 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. (Note that most such medieval reports of transits were later taken as observations of sunspots.)
In India, the Kerala school astronomer Nilakantha Somayaji in the 15th century developed a partially heliocentric 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.
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. The next occultation of Mercury by Venus will be on December 3, 2133.
The difficulties inherent in observing Mercury mean that it has been far less studied than the other planets. In 1800 Johann Schröter made observations of surface features, claiming to have observed 20 km high mountains. Friedrich Bessel used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°. In the 1880s Giovanni Schiaparelli mapped the planet more accurately, and suggested that Mercury’s rotational period was 88 days, the same as its orbital period due to tidal locking. This phenomenon is known as synchronous rotation and is shown by Earth’s Moon. The effort to map the surface of Mercury was continued by Eugenios Antoniadi, who published a book in 1934 that included both maps and his own observations. Many of the planet's surface features, particularly the albedo features, take their names from Antoniadi's map.
In June 1962 Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences led by Vladimir Kotelnikov became first to bounce radar signal off Mercury and receive it, starting radar observations of the planet. Three years later radar observations by Americans Gordon Pettengill and R. Dyce using 300-meter Arecibo Observatory radio telescope in Puerto Rico showed conclusively that the planet’s rotational period was about 59 days. 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.
Italian astronomer Giuseppe Colombo noted that the rotation value was about two-thirds of Mercury's orbital period, and proposed that the planet's orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance. Data from Mariner 10 subsequently confirmed this view. 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
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.
Ground-based optical observations did not shed much further light on
the innermost planet, but radio astronomers using interferometry at
microwave wavelengths, a technique that enables removal of the solar
radiation, were able to discern physical and chemical characteristics of
the subsurface layers to a depth of several meters.
Not until the first space probe flew past Mercury did many of its most
fundamental morphological properties become known. Moreover, recent
technological advances have led to improved ground-based observations.
In 2000, high-resolution lucky imaging observations were conducted by the Mount Wilson Observatory
1.5 meter Hale telescope. They provided the first views that resolved
surface features on the parts of Mercury which were not imaged in the
Mariner mission. Later imaging has shown evidence of a huge double-ringed impact basin even larger than the Caloris Basin in the non-Mariner-imaged hemisphere. It has informally been dubbed the Skinakas Basin.
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.
Internal structure
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| Terrestrial planets: Mercury, Venus, Earth, and Mars (to scale) |
Mercury is one of four terrestrial planets in the Solar System, and is a rocky body like the Earth. It is the smallest planet in the Solar System, with an equatorial radius of 2,439.7 km. Mercury is even smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material. 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. If the effect of gravitational compression 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.
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| Internal structure of Mercury: 1. Crust: 100–300 km thick 2. Mantle: 600 km thick 3. Core: 1,800 km radius |
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 core,
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.
Geologists estimate that Mercury’s core occupies about 42% of its
volume; for Earth this proportion is 17%. Recent research strongly
suggests that Mercury has a molten core. Surrounding the core is a 500–700 km mantle consisting of silicates. Based on data from the Mariner 10 mission and Earth-based observation, Mercury’s crust is believed to be 100–300 km thick.
One distinctive feature of Mercury’s surface is the presence of
numerous narrow ridges, extending up to several hundred kilometers in
length. 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.
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 chondrite meteorites, thought to be typical of the Solar System's rocky matter, and a mass approximately 2.25 times its current mass. 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.
The impact would have stripped away much of the original crust and
mantle, leaving the core behind as a relatively major component. A similar process, known as the giant impact hypothesis, has been proposed to explain the formation of Earth’s Moon.
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 (Celsius equivalents about 273 degrees less), and possibly even
as high as 10,000 K.
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 solar wind.
A third hypothesis proposes that the solar nebula caused drag on the particles from which Mercury was accreting, which meant that lighter particles were lost from the accreting material. Each hypothesis predicts a different surface composition, and two upcoming space missions, MESSENGER and BepiColombo, both aim to make observations to test them. MESSENGER
has found higher-than-expected potassium and sulfur levels on the
surface, suggesting that the giant impact hypothesis and vaporization of
the crust and mantle did not occur since potassium and sulfur would
have been driven off by the extreme heat of these events. The findings
seem to favor the third hypothesis in which many lighter planetary
materials were driven off leaving behind higher metal concentrations.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 and heavy cratering, indicating that it has been geologically
inactive for billions of years. Since our knowledge of Mercury's geology has been based on the 1975 Mariner flyby and terrestrial observations, it is the least understood of the terrestrial planets. As data from the recent MESSENGER
flyby is processed this knowledge will increase. For example, an
unusual crater with radiating troughs has been discovered which
scientists called "the spider." It later received the name ApollodorusNames for features on Mercury come from a variety of sources. Names coming from people are limited to the deceased. Craters are named for artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field. Ridges, or dorsa, are named for scientists who have contributed to the study of Mercury. Depressions or 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 rupēs are named for ships of scientific expeditions. Valleys or valles are named for radio telescope facilities.
Albedo features are areas of markedly different reflectivity, as seen by telescopic observation. Mercury possesses dorsa (also called "wrinkle-ridges"), Moon-like highlands, montes (mountains), planitiae, or plains, rupes (escarpments), and valles (valleys).
Mercury was heavily bombarded by comets and asteroids during and shortly following its formation 4.6 billion years ago, as well as during a possibly separate subsequent episode called the late heavy bombardment that came to an end 3.8 billion years ago. During this period of intense crater formation, the planet received impacts over its entire surface, facilitated by the lack of any atmosphere to slow impactors down. During this time the planet was volcanically active; basins such as the Caloris Basin were filled by magma from within the planet, which produced smooth plains similar to the maria found on the Moon.
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 the Moon's, both of which contain significant stretches of similar geology, such as maria and plateaus.
Impact basins and craters
Craters on Mercury range in diameter from small bowl-shaped cavities to multi-ringed impact basins 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. | |
| The so-called "Weird Terrain" was formed at the point antipodal to the Caloris Basin Impact |
Overall, about 15 impact basins have been identified on the imaged part of Mercury. A notable basin is the 400 km wide, multi-ring Tolstoj Basin which has an ejecta blanket extending up to 500 km from its rim and a floor that has been filled by smooth plains materials. Beethoven Basin has a similar-sized ejecta blanket and a 625 km diameter rim. Like the Moon, the surface of Mercury has likely incurred the effects of space weathering processes, including Solar wind and micrometeorite impacts.
Plains
There are two geologically distinct plains regions on Mercury. Gently rolling, hilly plains in the regions between craters are Mercury's oldest visible surfaces, 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. It is not clear whether they are of volcanic or impact origin. 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. 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. 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.
One unusual feature of the planet’s surface is the numerous compression folds, or rupes, 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. Mercury’s surface is flexed by significant tidal bulges raised by the Sun—the Sun’s tides on Mercury are about 17 times stronger than the Moon’s on Earth.
Surface conditions and "atmosphere" (exosphere)
Main article: Atmosphere of Mercury The surface temperature of Mercury ranges from 100 K to 700 K due to the absence of an atmosphere and a steep temperature gradient between the equator and the poles. The subsolar point reaches about 700 K during perihelion then drops to 550 K at aphelion. On the dark side of the planet, temperatures average 110 K. The intensity of sunlight on Mercury’s surface ranges between 4.59 and 10.61 times the solar constant (1,370 W·m−2).
Although the daylight temperature at the surface of Mercury is generally extremely high, observations strongly suggest that ice (frozen water) exists on Mercury. The floors of deep craters at the poles are never exposed to direct sunlight, and temperatures there remain below 102 K; far lower than the global average. Water ice strongly reflects radar, and observations by the 70 m Goldstone telescope and the VLA in the early 1990s revealed that there are patches of very high radar reflection near the poles. While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.
The icy regions are believed to contain about 1014–1015 kg of ice, and may be covered by a layer of regolith that inhibits sublimation. By comparison, the Antarctic ice sheet on Earth has a mass of about 4×1018 kg, and Mars' south polar cap contains about 1016 kg of water. The origin of the ice on Mercury is not yet known, but the two most likely sources are from outgassing of water from the planet’s interior or deposition by impacts of comets.
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| Radar image of Mercury's north pole |
Sodium, potassium and calcium were discovered in the atmosphere during the 1980–1990s, and are believed to result primarily from the vaporization of surface rock struck by micrometeorite impacts. In 2008 magnesium was discovered by MESSENGER probe. Studies indicate that, at times, sodium emissions are localized at points that correspond to the planet's magnetic poles. This would indicate an interaction between the magnetosphere and the planet's surface.
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, magnetic field. According to measurements taken by Mariner 10, it is about 1.1% as strong as the Earth’s. The magnetic field strength at the Mercurian equator is about 300 nT. Like that of Earth, Mercury's magnetic field is dipolar. Unlike Earth, Mercury's poles are nearly aligned with the planet's spin axis. Measurements from both the Mariner 10 and MESSENGER space probes have indicated that the strength and shape of the magnetic field are stable.
It is likely that this magnetic field is generated by way of a dynamo effect, in a manner similar to the magnetic field of Earth. 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.
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| Graph showing relative strength of Mercury's magnetic field |
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 800 km wide orflux transfer events, form open windows in the planet's magnetic shield through which the solar wind may enter and directly impact Mercury's surface.
The process of linking interplanetary and planetary magnetic fields, called magnetic reconnection, is common throughout the cosmos. It occurs in Earth's magnetic field, where it generates magnetic tornadoes as well. 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.
Orbit and rotation
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| Animation of Mercury's and Earth's revolution around the Sun |
Mercury's orbit is inclined by 7 degrees to the plane of Earth's orbit (the ecliptic), as shown in the diagram on the right. As a result, transits 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.
Mercury's axial tilt is almost zero, with the best measured value as low as 0.027 degrees. This is significantly smaller than that of Jupiter, which has 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 arcminutes above the horizon.
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 Mercurian day. This is because approximately four Earth days before perihelion, Mercury's angular orbital velocity exactly equals its angular rotational velocity so that the Sun's apparent motion ceases; at perihelion, Mercury's angular orbital velocity then exceeds the angular rotational velocity. Thus, to a hypothetical observer on Mercury, the Sun appears to move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.
Mercury attains inferior conjunction (near approach to the Earth) every 116 Earth days on average, 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 million km to the Earth, but it will not be 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. Its period of retrograde motion 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 orbital eccentricity.
Spin–orbit resonance
For many years it was thought that Mercury was synchronously tidally locked with the Sun, rotating
once for each orbit and always keeping the same face directed towards
the Sun, in the same way that the same side of the Moon always faces the
Earth. Radar
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. |
| After one orbit, Mercury has rotated 1.5 times, so after two complete orbits the same hemisphere is again illuminated. |
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. 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. Numerical simulations show that a future secular orbital resonant perihelion interaction with Jupiter may cause the eccentricity of Mercury's orbit to increase to the point where there is a 1% chance that the planet may collide with Venus within the next five billion years.
Advance of perihelion
Main article: Perihelion precession of MercuryIn 1859, the French mathematician and astronomer Urbain Le Verrier reported that the slow precession of Mercury’s orbit around the Sun could not be completely explained by Newtonian mechanics 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. (Other explanations considered included a slight oblateness of the Sun.) The success of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to place faith in this possible explanation, and the hypothetical planet was named Vulcan, but no such planet was ever found.
The perihelion precession of Mercury is 5600 arc seconds (1.5556°) per century relative to the Earth, or 574.10±0.65 arc-seconds per century relative to the inertial ICFR. Newtonian mechanics, taking into account all the effects from the other planets, predicts a precession of 5557 seconds of arc (1.5436°) per century. In the early 20th century, Albert Einstein’s General Theory of Relativity provided the explanation for the observed precession. The effect is very small: the Mercurian relativistic perihelion advance excess is just 42.98 arcseconds 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: 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for 1566 Icarus.
Observation
Mercury’s apparent magnitude varies between −2.6 (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. 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.Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.
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.
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 greater illuminated area that is visible and the opposition brightness surge more than compensates for the distance. The opposite is true for Venus, which appears brightest when it is a crescent, because it is much closer to Earth than when gibbous.
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| Mosaic image by Mariner, 1974 |
At tropical and subtropical latitudes, Mercury is more easily seen than at higher latitudes. This is the result of two effects: (i) the Sun ascends above the horizon more steeply at sunrise and descends more steeply at sunset, so the twilight period is shorter, and (ii) at the right times of year, the ecliptic intersects the horizon at a very steep angle, meaning that Mercury can be relatively high (altitude up to 28°) in a fully dark sky. Such conditions can exist, for instance, after sunset near the Spring Equinox, in March/April for the southern USA and in September/October for South Africa and Australasia. Conversely, pre-sunrise viewing is easiest near the Autumn Equinox.
At temperate latitudes, Mercury is more often easily visible from Earth’s Southern Hemisphere than from its Northern Hemisphere. This is because Mercury's 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. In both of these cases, the angle Mercury strikes with the ecliptic 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 Argentina and South Africa. 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.
Ground-based telescope observations of Mercury reveal only an illuminated partial disk with limited detail. The first of two spacecraft to visit the planet was Mariner 10, which mapped about 45% of its surface from 1974 to 1975. The second is the MESSENGER spacecraft, which after three Mercury flybys between 2008 and 2009, attained orbit around Mercury on March 17, 2011, to study and map the rest of the planet.
The Hubble Space Telescope cannot observe Mercury at all, due to safety procedures which prevent its pointing too close to the Sun.
Naked-eye viewing
Mercury is seen most easily when it is close to greatest elongation,
which means that its angular separation from 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. The phase of the planet greatly
affects its apparent brightness. At greatest elongation, it is
approximately at half phase. 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. The apparent inclination of
the ecliptic to the horizon is also important. When the inclination is
large, as occurs near the spring equinox in the evening, and near the
autumnal equinox in the morning (this is true for observers in both
hemispheres), Mercury is higher in the sky when the Sun is just below
the horizon, which makes it easier to see than it other times. The
inclination of the ecliptic is also greater for observers at low
latitudes than high ones. It is helpful if Mercury is close to aphelion
at the time of observation, since this makes it further from the Sun
than at 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 March equinox, or at greatest eastern elongation at
the September equinox. (Over long periods of time, this changes as
Mercury's orbit shifts.)Putting all these factors together, the best time for an observer in the Southern Hemisphere to see Mercury is in the morning, near the March equinox, a few days after Mercury is at greatest western elongation, or in the evening, near the September equinox, a few days before greatest eastern elongation. An observer in the Northern Hemisphere cannot optimize all the factors simultaneously. Usually, the best chances of seeing the planet are in the evening, near the March equinox, a few days before greatest eastern elongation, or in the morning, 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.
Mercury's period of revolution around the Sun is 88 days. It therefore makes about 4.15 revolutions around the Sun in one Earth-year. In successive years the position of Mercury on its orbit shifts by 0.15 revolutions when seen on specific dates, such as the equinoxes. Therefore, if, for example, greatest eastern elongation happens on the March equinox of some year, about three years later greatest western elongation will happen near the March equinox, since the position of Mercury on its orbit at the equinox will have changed by about half 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.
When conditions are near optimal, Mercury is easy to see. However, optimal conditions are rare, and many casual observers search for Mercury without success.
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. The cuneiform name used to designate Mercury on the Mul.Apin tablets is transcribed as Udu.Idim.Gu\u4.Ud ("the jumping planet"). 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.The ancient Greeks of Hesiod's time knew the planet as Στίλβων (Stilbon), meaning "the gleaming", and Ἑρμάων (Hermaon). Later Greeks called the planet Apollo when it was visible in the morning sky, and Hermes when visible in the evening. Around the 4th century BC, Greek astronomers came to understand that the two names referred to the same body, Hermes (Ερμής: Ermis), a planetary name which is retained in modern Greek. The Romans named the planet after the swift-footed Roman messenger god, Mercury (Latin Mercurius), which they equated with the Greek Hermes, because it moves across the sky faster than any other planet. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.
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| 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. |
In ancient China, Mercury was known as Chen Xing (辰星), the Hour Star. It was associated with the direction north and the phase of water in the Wu Xing. Modern Chinese, Korean, Japanese and Vietnamese cultures refer to the planet literally as the "water star" (水星), based on the Five elements. Hindu mythology used the name Budha for Mercury, and this god was thought to preside over Wednesday. The god Odin (or Woden) of Germanic paganism was associated with the planet Mercury and Wednesday. The Maya 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 underworld.
The ancient association of Mercury with Wednesday is still visible in the names of Wednesday in various modern languages of Latin descent, e.g. mercredi in French, miércoles in Spanish, or miercuri in Romanian. The names of the days of the week were, in classical times, all related to the names of the seven bodies that were then considered to be planets.
In ancient Indian astronomy, the Surya Siddhanta, an Indian astronomical text of the 5th century, estimates the diameter of Mercury as 3,008 miles, an error of less than 1% from the currently accepted diameter of 3,032 miles (4,880 km). This estimate was based upon an inaccurate guess of the planet's angular diameter as 3.0 arcminutes.
In medieval Islamic astronomy, the Andalusian astronomer Abū Ishāq Ibrāhīm al-Zarqālī in the 11th century described 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. In the 12th century, Ibn Bajjah 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. (Note that most such medieval reports of transits were later taken as observations of sunspots.)
In India, the Kerala school astronomer Nilakantha Somayaji in the 15th century developed a partially heliocentric 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.
Ground-based telescopic research
The first telescopic observations of Mercury were made by Galileo in the early 17th century. Although he observed phases when he looked at Venus, his telescope was not powerful enough to see the phases of Mercury. In 1631 Pierre Gassendi made the first telescopic observations of the transit of a planet across the Sun when he saw a transit of Mercury predicted by Johannes Kepler. In 1639 Giovanni Zupi used a telescope to discover that the planet had orbital phases similar to Venus and the Moon. The observation demonstrated conclusively that Mercury orbited around the Sun.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. The next occultation of Mercury by Venus will be on December 3, 2133.
The difficulties inherent in observing Mercury mean that it has been far less studied than the other planets. In 1800 Johann Schröter made observations of surface features, claiming to have observed 20 km high mountains. Friedrich Bessel used Schröter's drawings to erroneously estimate the rotation period as 24 hours and an axial tilt of 70°. In the 1880s Giovanni Schiaparelli mapped the planet more accurately, and suggested that Mercury’s rotational period was 88 days, the same as its orbital period due to tidal locking. This phenomenon is known as synchronous rotation and is shown by Earth’s Moon. The effort to map the surface of Mercury was continued by Eugenios Antoniadi, who published a book in 1934 that included both maps and his own observations. Many of the planet's surface features, particularly the albedo features, take their names from Antoniadi's map.
In June 1962 Soviet scientists at the Institute of Radio-engineering and Electronics of the USSR Academy of Sciences led by Vladimir Kotelnikov became first to bounce radar signal off Mercury and receive it, starting radar observations of the planet. Three years later radar observations by Americans Gordon Pettengill and R. Dyce using 300-meter Arecibo Observatory radio telescope in Puerto Rico showed conclusively that the planet’s rotational period was about 59 days. 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.
Italian astronomer Giuseppe Colombo noted that the rotation value was about two-thirds of Mercury's orbital period, and proposed that the planet's orbital and rotational periods were locked into a 3:2 rather than a 1:1 resonance. Data from Mariner 10 subsequently confirmed this view. 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
