Mercury (planet)

Mercury

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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 34 d (0.240 846 9 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 perihelion	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	6.083×1010 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
Adjective	Mercurian
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%

Note: This article contains special characters.

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, and its greatest angular separation from the Sun (greatest elongation) is only 28.3°, meaning it is only seen in twilight. The planet remains comparatively little-known: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only 40–45% of the planet's surface.

Physically, Mercury is similar in appearance to the Moon as it is heavily cratered. It has no natural satellites and no atmosphere. The planet has a large iron core which generates a magnetic field about 1% as strong as that of the Earth. Surface temperatures on Mercury range from about 90-700 K, with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.

The Romans named the planet after the fleet-footed messenger god Mercury, probably for its fast apparent motion in the twilight sky. The astronomical symbol for Mercury ( Unicode: ☿) is a stylized version of the god's head and winged hat atop his caduceus an ancient astrological symbol. Before the 5th century BC, Greek astronomers believed the planet to be two separate objects. The Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet as the water star, 水星, based on the Five Elements.

Historical understanding

Mercury has been known since at least the time of the ancient astrologers, 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. The Hebrews who saw it appeared only near the sun called it Kochav-hama - "the star of the sun". It was given two names by the ancient Greeks: Apollo when visible in the morning sky and Hermes when visible in the evening. In astrological calculations, these two cycles were called Morning Star and Evening Star. However, Greek astronomers came to understand that the two names referred to the same body, with Pythagoras being the first to propose the idea. Heraclitus even believed that Mercury and Venus orbited the Sun, not the Earth.

In 1631, Pierre Gassendi, who viewed the transit of Mercury predicted by Johannes Kepler, became the first person to observe the transit of a planet across the Sun. 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.

Physical characteristics

Temperature and sunlight

The mean surface temperature of Mercury is 452 K (178.8 C /353.9 F), but it ranges from 90(-183.15 C /-297.7 F)to 700 K (426.85 C /800 F); by comparison, the temperature on Earth varies by only about 150 K (150 C /270 F). The sunlight on Mercury's surface is 6.5 times as intense as it is on Earth, with the solar constant having a value of 9.13 kW/m².

Surface features

During and shortly following the formation of Mercury, it was heavily bombarded by comets and asteroids for a period that came to an end 3.8 billion years ago. During this period of intense crater formation, the surface 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.

Apart from craters with diameters in the range of hundreds of meters to hundreds of kilometers, there are others of gigantic proportions such as Caloris, the largest structure on the surface of Mercury with a diameter of 1,300 km. The impact was so powerful that it caused lava eruptions from the crust of the planet and left a concentric ring over 2 km tall surrounding the impact crater. The consequences of Caloris are also impressive; it is widely accepted as the cause for the fractures and leaks on the opposite side of the planet.

The plains of Mercury have two distinct ages: the younger plains are less heavily cratered and probably formed when lava flows buried earlier terrain. One unusual feature of the planet's surface is the numerous compression folds which criss-cross the plains. It is thought that as the planet's interior cooled it contracted, and its surface began to deform. The folds can be seen on top of other features, such as craters and smoother plains, indicating that they are more recent. Mercury's surface is also flexed by 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 given the following designations:

• Craters (see List of craters on Mercury)
• Albedo features — areas of markedly different reflectivity
• Dorsa — ridges (see List of ridges on Mercury)
• Montes — mountains (see List of mountains on Mercury)
• Planitiae — plains (see List of plains on Mercury)
• Rupes — scarps (see List of scarps on Mercury)
• Valles — valleys (see List of valleys on Mercury)

Interior composition

Mercury 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. Despite having so much iron, the reason Mercury has a lower density than Earth is that the latter's mass is about 20 times greater, resulting in a more highly compressed interior with a high density. The iron core fills 42% of the planet's volume (Earth's core only fills 17%).

Surrounding the core is a 600 km mantle. It is thought that early in Mercury's history, a giant impact with a body several hundred kilometres across stripped the planet of much of its original mantle material, resulting in the relatively thin mantle compared to the sizable core.

Rotation

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, in the same way that the same side of the Moon always faces the Earth. However, 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. 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, hence showing the same face. This would be also the case if it was totally locked. 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.

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 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 axial tilt is only 0.01 degrees. This is over 300 times smaller than that of Jupiter, which is 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.

Orbit

The orbit of Mercury has a high eccentricity, with the planet's distance from the Sun ranging from 46,000,000 to 70,000,000 kilometers. Among the major planets, only Pluto has a more eccentric orbit. However, because of the limit of Mercury's orbit, all of the planets except the Earth and Venus have a larger spread between perihelion and aphelion (Mars' is 42.6 Gm to Mercury's 23.8 Gm, for example). There are even several outer planet satellites that beat Mercury's spread: Saturn's S/2004 S 18 (with 30.8 Gm) and Neptune's Psamathe and S/2002 N 4 (42.0 and 47.9 Gm, respectively).

When it was discovered, the slow precession of Mercury's orbit around the Sun could not be completely explained by Newtonian mechanics, and for many years it was hypothesized that another planet might exist in an orbit even closer to the Sun to account for this perturbation (other explanations considered included a slight oblateness of the Sun). The hypothetical planet was even named Vulcan. However, in the early 20th century, Albert Einstein's General Theory of Relativity provided a full explanation for the observed precession. Mercury's precession showed the effects of mass dilation, providing a crucial observational confirmation of one of Einstein's theories. This was a very slight effect: the Mercurian relativistic perihelion advance excess is a mere 43 arcseconds per century. The effect is even smaller for other planets, being 8.6 arcseconds per century for Venus, 3.8 for Earth, and 1.3 for Mars.

Research indicates that the eccentricity of Mercury's orbit varies chaotically from 0 (circular) to a very high 0.47 over millions of years. 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.

Magnetosphere

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. However, scientists are unsure whether Mercury's core could still be liquid, although it could perhaps be kept liquid by tidal effects during periods of high orbital eccentricity. It is also possible that Mercury's magnetic field is a remnant of an earlier dynamo effect that has now ceased, with the magnetic field becoming "frozen" in solidified magnetic materials.

Iron content

Mercury has a higher iron content than any other object in the solar system. Several theories have been proposed to explain Mercury's high metallicity. One theory is 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).

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 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 solar wind.

A third theory suggests 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 of these theories predicts a different surface composition. Hence, one of the aims of the MESSENGER mission to the planet is to take observations that will allow the theories to be tested. Tentative suggestions have been made that Mercury may be a Chthonian planet.

Observing Mercury

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. At most other times, Mercury can be observed for only a brief period during either morning or evening twilight.

Mercury exhibits moon-like phases as seen from Earth, being "new" at inferior conjunction and "full" at superior conjunction. The planet is rendered invisible on both of these occasions by virtue of its rising and setting in concert with the Sun in each case. The 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 it is at aphelion).

Mercury is brightest as seen from Earth when it is at a "gibbous" phase, between half-full and full. Mercury's smaller orbit means it is not much farther away, and the fuller phase more than outweighs its greater distance from Earth.

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 arises from the planet's high degree of orbital eccentricity.

Mercury is more often easily visible from 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. By contrast, at northern temperate latitudes Mercury is never above the horizon of a more-or-less fully dark night sky. Mercury can, like several other planets and the brightest stars, be seen during a total solar eclipse.

The only observed instance of an occultation of Mercury by Venus was by John Bevis at the Royal Greenwich Observatory on May 28, 1737.

See also: Aspects of Mercury

Exploration

Reaching Mercury from Earth poses significant technical challenges since the planet orbits three times closer to the Sun than the Earth. A Mercury-bound spacecraft launched from Earth must travel over 91 million kilometers 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. Hence, 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. This increases 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. The approaching spacecraft cannot use aerobraking to help enter orbit around Mercury and must rely on rocket boosters, since the planet has no atmosphere. Hence, a trip to Mercury requires even more rocket fuel than that required to escape the solar system completely. As a result of these problems, there have not been many missions to Mercury as of 2005.

NASA

The only spacecraft to approach Mercury was NASA's Mariner 10 (1974-1975). The spacecraft used the gravity of Venus to adjust its orbital velocity so that it could approach Mercury. Mariner 10 provided the first close-up images of Mercury's surface. The spacecraft made three close approaches to Mercury, the closest of which took it to within 327 km of the surface. Unfortunately, the same face of the planet was lit at each close approach, resulting in less than 45% of the planet's surface's being mapped. Mariner 10 also found the first evidence for Mercury's magnetic field and measured temperatures across its surface.

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 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, which 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 2011-2012. The probes will reach Mercury about four years later, orbiting and charting its surface and magnetosphere for a year.

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The Solar System
Planets: Mercury - Venus - Earth (Moon) - Mars - Jupiter - Saturn - Uranus - Neptune - Pluto
Other: Sun - Asteroid belt - Kuiper belt - Scattered disc - Oort cloud
See also astronomical objects and the solar system's list of objects, sorted by radius or mass.