Mercury (planet)

Mercury is the closest planet to the Sun, and the second-smallest planet in the Solar System. Mercury ranges from −0.4 to 5.5 in apparent magnitude; Mercury is sufficiently "close" to the Sun that telescopes rarely examine it (the greatest elongation is 28.3?). Mercury has no natural satellites. The only spacecraft to approach Mercury was Mariner 10 (197475); only 40–45% of the planet has been mapped.

The Romans named the planet after the Roman god Mercury. The astronomical symbol for Mercury is a circle on top of a short vertical line with a cross below and a semicircle above the circle (Unicode: .It is a stylized version of the god's head and winged hat atop his caduceus. Before the 5th century BC, the planet Mercury actually had two names, as it was not realized it could alternately appear on one side of the Sun and then the other. It was called Hermes when in the evening sky, but was known as Apollo when it appeared in the morning, being closely associated with the greek sun god Helios. Pythagoras is credited for pointing out that they were one and the same.

The Chinese and Japanese cultures refer to the planet as the Water Star, based on the Five Elements.


Physical characteristics

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Temperature and sunlight

The mean surface temperature of Mercury is 452 K, but it ranges from 90–700 K; by comparison, the temperature on Earth varies by only about 11 K (with respect only to solar radiation; not climate or season). The sunlight on Mercury's surface is 6.5 times as intense as it is on Earth, a total irradiance of 9.13 kW/m².

Surprisingly, radar observations taken in 1992 indicated that there is frozen water ice at Mercury's north pole. Such ice is believed to exist at the bottom of permanently shaded craters, where it has been deposited by comet impacts and/or gases arising from the planetary interior.


Main article: Geology of Mercury

Mercury's cratered surface appears very similar to the Moon. Mercury's most distinctive surface feature (of what has been photographed) is Caloris Basin, an impact crater ~1350 km in diameter. The planet is marked with scarps, which apparently formed billions of years ago as Mercury's core cooled and shrank causing the crust to wrinkle. The majority of Mercury's surface is covered with plains of two distinct ages; the younger plains are less heavily cratered and probably formed when lava flows buried earlier terrain. In addition, Mercury has "significant" tidal bulges, raised by the Sun (the Sun's tides on Mercury are about 17% stronger than the Moon's on Earth).

Mercury's terrain features are officially listed as the following:

Interior composition

The planet has a relatively large iron core (even when compared to Earth). Mercury's composition is approximately 70% metallic and 30% silicate. The average density is 5430 kg/m³; which is slightly less than Earth's density. The reason that Mercury, despite having so much iron, has less density than Earth is that the overall mass of Earth compresses the planet and creates a high density. Mercury has only 5.5% of Earth's mass. The iron core fills 42% of the planetary volume (Earth's core only fills 17%). Surrounding the core is a 600 km mantle.


Until radar observations in 1965 proved otherwise it was thought that Mercury was tidally locked with the Sun, rotating once for each orbit and keeping the same face directed towards the Sun at all times. Instead, Mercury 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. The original reason astronomers thought it was tidally locked was because whenever Mercury was best placed for observation, it was always at the same point in its 3:2 resonance, so showing the same face, which would be also the case if it was totally locked. Because of Mercury's 3:2 spin-orbit resonance, although a sidereal day (the period of rotation) lasts about 58.7 Earth days, a solar day (the length between two meridian transits of the Sun) lasts about 176 Earth days.

At certain points on Mercury's surface, an observer would be able to see the Sun rise about halfway, then reverse and set, then rise again, all within the same Mercurian day. This is because approximately four days prior to perihelion, Mercury's orbital velocity exactly equals its rotational velocity, so that the Sun's apparent motion ceases; at perihelion, Mercury's orbital velocity then exceeds the rotational velocity; thus, the Sun appears to be retrograde. Four days after perihelion, the Sun's normal apparent motion resumes.

Mercury's rotation is also remarkable in that it has an axial tilt of only 0.01 degrees, which is over 300 times smaller than Jupiter's axial tilt (the second smallest axial tilt of all planets at 3.1 degrees). This means an observer at Mercury's equator never sees the sun more than 1/100 of one degree north or south of the zenith.


Click image for description

Orbital characteristics (Epoch J2000)
Semimajor axis 57,909,176 km
0.387 098 93 AU
Orbital circumference 0.360 Tm
(2.406 AU)
Eccentricity 0.205 630 69
Perihelion 46,001,272 km
0.307 499 51 AU
Aphelion 69,817,079 km
0.466 698 35 AU
Orbital period 87.969 35 d
(0.240 847 0 a)
Synodic period 115.8776 d
Avg. Orbital Speed 47.36 km/s
Max. Orbital Speed 58.98 km/s
Min. Orbital Speed 38.86 km/s
Inclination 7.004 87°
(3.38? to Sun's equator)
Longitude of the
ascending node
48.331 67°
Argument of the
29.124 78°
Number of satellites 0
Physical characteristics
Equatorial diameter 4879.4 km
(0.383 Earths)
Surface area 7.5 × 107 km²
(0.147 Earths)
Volume 6.1 × 1010 km³
(0.056 Earths)
Mass 3.302×1023 kg
(0.055 Earths)
Mean density 5.427 g/cm³
Equatorial gravity 3.701 m/s²
(0.377 gee)
Escape velocity 4.435 km/s
Rotation period 58.6462 d (58 d 15.5088 h)
Rotation velocity 10.892 km/h (at the equator)
Axial tilt ~0.01°
Right ascension
of North pole
281.01° (18 h 44 min 2 s) 1 (
Declination 61.45°
Albedo 0.10-0.12
Avg. Surface temp.: Day 623 K
Avg. Surface temp.: Night 103 K
Surface temp.
min mean max
90 K 440 K 700 K
Atmospheric characteristics
Atmospheric pressure trace
Potassium 31.7%
Sodium 24.9%
Atomic Oxygen 9.5%
Argon 7.0%
Helium 5.9%
Molecular Oxygen 5.6%
Nitrogen 5.2%
Carbon dioxide 3.6%
Water 3.4%
Hydrogen 3.2%


The orbit of Mercury is eccentric, ranging from 46 million–70 million kilometres in radius; only Pluto among all planets has a more eccentric orbit. The slow precession of this orbit around the Sun could not be completely explained by Newtonian Classical Mechanics, and for some time it was thought that another planet (sometimes referred to as Vulcan) might be present in an orbit even closer to the Sun to account for this perturbation. Einstein's General Theory of Relativity instead provided the explanation for this small discrepancy, however.

Research indicates that the eccentricity of Mercury's orbit varies chaotically from 0 (circular) to a very high 0.45 over millions of years. (Nature, June 24 2004) This is thought to explain Mercury's 3:2 spin-orbit resonance (rather than 1:1), since this state is more likely to arise during a period of high eccentricity.


Despite its slow rotation, Mercury has a relatively strong magnetosphere, with 1% of the magnetic field strength generated by Earth. It is possible that this magnetic field is generated in a manner similar to Earth's, by a dynamo of circulating liquid core material; current estimates suggest that Mercury's core is not hot enough to liquefy nickel-iron, but it is possible that materials with a lower melting point such as sulfur may be responsible. It is also possible that Mercury's magnetic field is a remnant of an earlier dynamo effect that has now ceased, the magnetic field becoming "frozen" in solidified magnetic materials.

Iron content

Mercury has a higher iron percentage than any other object within the system. Several theories have been proposed to explain Mercury's high metallicity.

One theory suggests that Mercury originally had a metal-silicate ratio similar to common chondrite meteors and a mass approximately 2.25 times its current mass, but that early in the solar system's history Mercury was struck by a planetesimal of approximately 1/6 that mass. The impact would have stripped away much of the original crust and mantle; leaving the core behind. A similar theory has been proposed to explain the formation of Earth's Moon, see giant impact theory. Alternately, Mercury may have formed very early in the history of the solar nebula, before the Sun's energy output had stabilized. Mercury starts out with approximately twice its current mass in this theory; but, as the protostar contracted, temperatures near Mercury could have been between 2500–3500 K; and possibly even as high as 10000 K. Much of Mercury's surface rock would have vaporized at such temperatures, forming an atmosphere of "rock vapor" which would have been carried away by the nebular wind. A third theory, similar to the second, argues that the outer layers of Mercury were "eroded" by the solar wind over a longer period of time.

Exploration of Mercury

Early Astronomers

Mercury has been known since at least the time of the Sumerians (3rd millennium BC), who called it Ubu-idim-gud-ud. The earliest recorded detailed observations were made by the Babylonians, who called it gu-ad or gu-utu. It was given two names by the ancient Greeks, Apollo when visible in the morning sky and Hermes when visible in the evening, but Greek astronomers knew that the two names referred to the same body. Heraclitus even believed that Mercury and Venus orbited the Sun, not the Earth.

In 1639, Giovanni Battista used a telescope to discover that the planet had orbital phases just like Venus and the Moon. This demonstrated conclusively that Mercury orbited around the Sun.

Observation of Mercury

Observation of Mercury is severely complicated by its proximity to the Sun, as it is lost in the Sun's glare at least half the time, and at most other times can be observed for only a brief period during either morning or evening twilight.

Like Venus, Mercury exhibits moon-like phases as seen from Earth, being "new" at inferior conjunction and "full" at superior conjunction, rendered invisible on both of these occasions by virtue of its rising and setting in concert with the Sun in each case. The half-moon phase occurs at greatest elongation, when Mercury rises earliest before the Sun when at greatest elongation west, and setting latest after the Sun when at greatest elongation east (its separation from the Sun ranging from 18.5° if it is at perihelion at the time of the greatest elongation to 28.3° if at aphelion). Unlike Venus, however, Mercury is brightest as seen from Earth when it is at a "gibbous" phase; that is to say, between half full and full (by contrast, Venus is brightest when it is between new and half full), the reason for this being that at its maximum possible distance from the Earth (when it would be at "full" phase) Mercury is not even three times as far away from the latter as when it is closest (and also "new"), while for Venus this ratio is almost 6½ to 1. Mercury attains inferior conjunction every 116 days on average, but this interval can range from 111 days to 121 days due to the planet's eccentric orbit. 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 also arising from the planet's high degree of orbital eccentricity.

Curiously, Mercury is more often easily visible from the Earth's Southern Hemisphere than from its Northern Hemisphere; this is due to the fact that its maximum possible elongations west of the Sun always occur when it is early autumn in the Southern Hemisphere, while its maximum possible eastern elongations happen when the Southern Hemisphere is having its late winter season. 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 South Temperate Zone latitudes, such as Argentina and New Zealand. At North Temperate latitudes, by contrast, Mercury is never above the horizon of a more-or-less fully dark night sky (conversely, on occasions when Mercury is concomitantly at perihelion and greatest elongation and the relative angle with the ecliptic is unfavourable from a Southern Hemisphere prospective, it may not be visible at all from that side of the Earth — a scenario which never arises in the Northern Hemisphere).

Mercury can also be seen during a total solar eclipse but this is far too brief to make observations.

Going to Mercury

Reaching Mercury from Earth poses significant technical challenges. Mercury orbits three times closer to the Sun than does Earth, so a Mercury-bound spacecraft launched from Earth must travel over 91 million kilometers down into the Sun's gravitational potential well. From a stationary start, a spacecraft would require no delta-v or energy to fall towards the Sun; however, starting from the Earth, with an orbital speed of 30 km/s, the spacecraft's significant angular momentum resists sunward motion, so the spacecraft must change its velocity considerably to enter into a Hohmann transfer orbit that passes near Mercury.

In addition, the potential energy liberated by moving down the Sun's potential well becomes kinetic energy, increasing the velocity of the spacecraft. Without correcting for this, the spacecraft would be moving too quickly by the time it reached the vicinity of Mercury to land safely or enter a stable orbit. If one imagines driving along a road atop a steep cliff with another road at the bottom, then the journey from Earth to Mercury is rather like swerving off the cliff, freefalling for some time, and then trying to land softly and merge with traffic on the lower road. Clearly, the spacecraft must alter its velocity quite radically to match orbits with Mercury. Furthermore, the approaching spacecraft cannot use aerobraking to help enter orbit around Mercury (since it has no atmosphere) and must use rockets instead. For these reasons, such a trip requires even more rocket fuel than to escape the solar system completely (though reaching the outer planets requires still more fuel to match orbits with the destination planet).

As a result of these problems, there have not been many missions to Mercury to date, and those missions use more efficient gravitational slingshots rather than a direct transfer orbit.


The only spacecraft to approach Mercury has been the NASA Mariner 10 mission (197475).

A second NASA mission to Mercury, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), was launched on August 3, 2004 from the Cape Canaveral Air Force Station in Florida, USA, aboard a Boeing Delta 2 rocket. The MESSENGER spacecraft will make three flybys of Mercury in 2008 and 2009 before entering a year-long orbit of the planet in March 2011. It will explore the planet's atmosphere, composition and structure.

Japan and the ESA

Japan is planning a joint mission with the European Space Agency called BepiColombo that will orbit Mercury with two probes, one to map the planet, and the other to study its magnetosphere. An original plan to include a lander has been shelved. Russian Soyuz rockets will launch the probes, starting in 201112. The probes will reach Mercury about four years later, orbiting and charting its surface and magnetosphere for a year.

Potential for human colonization

A crater at the North or South pole of Mercury might prove to be one of the best locations for an off-Earth colony, as the temperature would remain almost constant (at around minus 200 degrees Celsius). This is because Mercury has negligible axial tilt and essentially no atmosphere to carry heat from its sunlit portion. It would thus always be dark at the bottom of a crater at the planet's pole, even a shallow one. Human activities could warm the colony to a comfortable temperature, and the low ambient temperature would make waste heat disposal easier than most locations off Earth.

A base elsewhere would have to be able to deal with many weeks of continuous intense solar heating followed by many weeks without any external heating at all. This would not necessarily be as difficult as it may first seem. Facilities could be buried under several meters of loose-packed regolith, which in a vacuum would serve as effective thermal insulation as well as a radiation shield. Similar approaches have been proposed for bases on Earth's Moon, which has two-week-long days followed by two-week-long nights. Due to the lack of atmosphere to conduct heat, a thermal radiator hidden in the shadow of a sun screen would be able to reject heat into space even at the height of the Mercurian day. Alternatively, the base could use a heat sink during the day to store up heat for disposal during the night. Protecting mobile vehicles or robots against solar heating might prove much more difficult, however, which may limit the amount of surface activity that could be performed during the day.

Mercury in astrology

Main article: Planets in astrology#Mercury

See also


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Our Solar System
Sun | Mercury | Venus | Earth (Moon) | Mars | Asteroid belts
Jupiter | Saturn | Uranus | Neptune | Pluto | Kuiper belt | Oort cloud
See also astronomical objects and the solar system's list of objects, sorted by radius or mass

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