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Mercury (planet)
*** Shopping-Tip: Mercury (planet)
see
Mercury_(planet)
{{Planet Infobox/Mercury}}
{{SpecialCharsNote}}
Image:Mercury inside Lmb.png thumb|right|200px|Diagram showing Mercury's large core
Image:Terrestrial_planet_size_comparisons.jpg thumb|right|300px|Size comparison of terrestrial planets (left to right): Mercury, Venus, Earth, and Mars.
'''Mercury''' is the closest
planet to the
Sun, and the second-smallest planet{{ref|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
impact crater cratered. It has no
natural satellites and no
Celestial body atmosphere 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
Kelvin K, with the subsolar point being the hottest and the bottoms of craters near the
Geographical pole poles being the coldest.
The
Ancient Rome Romans named the planet after the fleet-footed messenger
Roman mythology god Mercury (mythology) Mercury, probably for its fast apparent motion in the twilight sky. The
astronomical symbol for Mercury (
Unicode: {{Unicode.html">caduceus an ancient
astrological symbol. Before the
5th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, the other only at sunset. The
Chinese.html">China|Chinese,_
Korea 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,
Sumer Sumerians (3rd millennium BC), who called it ''Ubu-idim-gud-ud''. The earliest recorded detailed observations were made by the
Babylon 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
Greece Greeks:
Apollo (god) 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 (planet) 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 Battista Zupi 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 K (-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
{{main|Geology of Mercury}}
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
Celestial body atmosphere atmosphere to slow impactors down. During this time, the planet was
volcano 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.
Mercury is the second smallest planet in the solar system
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 Basin 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.{{ref|Van-Hoolst}}
Mercury's terrain features are officially given the following designations:
*
Impact crater Craters (''see
List of craters on Mercury'')
*
Albedo features — areas of markedly different reflectivity
*
Dorsum Dorsa —
ridges (''see
List of ridges on Mercury'')
*
Montes —
mountains (''see
List_of_mountains_on_Mercury#Mountains List of mountains on Mercury'')
*
Planitia Planitiae —
plains (''see
List of plains on Mercury'')
*
Rupes —
escarpment scarps (''see
List of scarps on Mercury'')
*
Vallis Valles —
valleys (''see
List of valleys on Mercury'')
Interior composition
Mercury has a relatively large
iron planetary core 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 (geology) 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.{{ref|Benz}}
Rotation
It was thought that Mercury was synchronously
tidal locking tidally locked with the Sun,
rotation 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 synchronously 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. Due to Mercury's 3:2 spin-orbit resonance, a
solar day (the length between two
meridian (astronomy) meridian Astronomical transit 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 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, the Sun appears to be
retrograde motion 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 would never see the sun more than 1/100 of one degree north or south of the
zenith.
Orbit
The orbit of Mercury has a high
eccentricity (orbit) 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 (planet) 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
gigametre Gm to Mercury's 23.8 Gm, for example). There are even several outer planet satellites that beat Mercury's spread:
Saturn (planet) Saturn's
S/2004 S 18 (with 30.8 Gm) and
Neptune (planet) Neptune's
Psamathe (moon) 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 (planet) Vulcan. However, in the early
20th century,
Albert Einstein's
General relativity General Theory of Relativity provided a full explanation for the observed precession. Mercury's precession showed the effects of
relativistic mass 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
arcsecond 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
chaos theory 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.{{ref|Correia}}
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,{{ref.html">dynamo theory
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 (geology) crust and
mantle (geology) 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 (physics) drag on the particles from which Mercury was
accretion (science) 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.{{ref|Messenger}} Tentative suggestions have been made that Mercury may be a
Chthonian planet.
Observing Mercury
Image:Mercury Caloris Basin2.jpg thumb|right|Mercury's [[Caloris Basin is one of the largest impact features in the Solar System]]
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 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
gravity 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 velocity escape the solar system completely. As a result of these problems, there have not been many missions to Mercury
as of 2005.
NASA
Image:Mariner10.gif thumb|200px|right|The Mariner 10 probe, the only probe yet to visit the innermost planet
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.{{ref|nssdc.gsfc}}
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 launch vehicle 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.
See also
*
Colonization of Mercury
*
Geology of Mercury
*
Mercury in fiction
Notes
#{{note|planet}} The discovery of
2003UB313 2003UB313 has sparked a debate on the
definition of planet definition of the term "planet". If 2003UB
313 is accepted as the tenth planet, it would make Mercury the third smallest in the solar system rather than the second.
#{{note|Van-Hoolst}} Van Hoolst, T., Jacobs, C. (2003), ''Mercury's tides and interior structure'', Journal of Geophysical Research, v. 108, p. 7.
#{{note|Benz}} Benz, W., Slattery, W. L., Cameron, A. G. W. (1988), ''Collisional stripping of Mercury's mantle'', Icarus, v. 74, p. 516-528.
#{{note|Correia}} Correia, A. C. M., Laskar, J. (2004), ''Mercury's capture into the 3/2 spin-orbit resonance as a result of its chaotic dynamics'', Nature, v. 429, p. 848-850.
#{{note|Spohn}} Spohn, T., Breuer, D. (2005), ''Core Composition and the Magnetic Field of Mercury'', American Geophysical Union, Spring Meeting 2005
#{{note|Messenger}}[http://messenger.jhuapl.edu/why_mercury/ Mercury: The Key to Terrestrial Planet Evolution (2005)]. ''JHU/APL - Messenger''.
#{{note|nssdc.gsfc}}[http://nssdc.gsfc.nasa.gov/nmc/tmp/1973-085A.html Mariner 10 (
October 20 2005)]. ''NSSDC Master Catalog Display: Spacecraft''.
References
*Shchuko, O. B. (2004). ''Mercury: can any ice exist at subpolar regions?'', Advances in Space Research, v. 33, p. 2156-2160
*Comins, Neil F. (2001). ''Discovering the Essential Universe''.
*Zuber, Maria T. (2004). [http://www.nasa.gov/worldbook/mercury_worldbook.html Mercury]. ''World Book Online Reference Center''. World Book, Inc. Accessed at nasa.gov.
External links
-
Atlas of Mercury - NASA
-
NASA's Mercury fact sheet
-
'BepiColombo', ESA's Mercury Mission
-
'Messenger', NASA's Mercury Mission
-
SolarViews.com - Mercury
-
Planets - Mercury A kid's guide to Mercury.
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