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 (1974–75); only 40–45% of the planet has been mapped.

The planet was named 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 representation of the god's 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—in honor of the Roman god of the Sun when it appeared in the morning. Pythagoras is credited for pointing out that they were one and the same.

Physical characteristics

Atmosphere

Mercury has only trace amounts of an atmosphere. The atmosphere of Mercury is extremely thin; indeed, gas molecules in Mercury's atmosphere collide with the surface of the planet more frequently than they collide with each other; for most purposes Mercury should be considered as being airless. The "atmosphere" is primarily composed of oxygen, potassium, and sodium.

The atoms that compose Mercury's atmosphere are continually being lost to space, with the average "lifespan" of a potassium or sodium atom being approximately 3 hours (during the Mercurian day—and only half that at perihelion). The lost atmosphere is continually replenished by several mechanisms; solar wind captured by the planetary magnetic field, vapor produced by micrometeor impacts, direct thermal evaporation of the polar ice, and/or outgassing.

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 ~11 K (with respect only to solar radiation; not climate or season). The sunlight on Mercury's surface is 8.9 times more intense than that on Earth, a total irradiance of 9126.6 W/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.

Terrain

Mercury's cratered surface appears very similar to the Moon. Mercury's most distinctive surface feature (of what has been photographed) is Caloris Basin, a impact crater ~1350km 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.

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

Craters - see List of craters on Mercury
Albedo features (areas of markedly different reflectivity)
Dorsa, i.e. ridges - see List of ridges on Mercury
Montes, i.e. mountains
Planitiae, i.e. plains - see List of plains on Mercury
Rupes, i.e. scarps - see List of scarps on Mercury
Valles, i.e. valleys - see List of valleys on Mercury

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, with so much iron, has less density than Earth is that the overall mass of Earth compresses the planet and creates a high density. Mercury only has 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 600km mantle.

Rotation

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. Mercury rotates 59 times slower than Earth. Because of Mercury's 3:2 spin-orbit resonance, although a sidereal day (the period of rotation) lasts ~58.7 Earth days, a solar day (the length between two meridian transits of the Sun) lasts ~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.

Orbit

The orbit of Mercury is eccentric, ranging from 46–70 Gm 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.

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; 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.

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). 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.

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.

Getting to Mercury

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. For this reason, 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.

NASA

The only spacecraft to approach Mercury has been the NASA Mariner 10 mission (1974–75).

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 2011–12. 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, and it would thus always be dark at the bottom of even a shallow crater at the planet's pole. Human activities would 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, however. Facilities could be buried under several meters of loose-packed regolith, which in a vacuum would serve as effective thermal insulation as well as providing radiation shielding. 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, or 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 fiction

Mercury is a popular setting for science fiction writers. Recurring themes include the dangers of being exposed to solar radiation; the possibility of escaping excessive radiation by staying within the planet's slow-moving terminator (the boundary between day and night); and autocratic governments (perhaps because of an association of Mercury with hot-temperedness).

Eric Rucker Eddison's series of fantasy novels starting with The Worm Ouroboros (1922) is set on Mercury, but the name is used purely for its exotic value, without regard to facts known about it at the time.
H. P. Lovecraft's The Shadow Out of Time briefly mentions the planet: "Later, as the Earth's span closed, the transferred minds [of the Great Race of Yith] would again migrate through time and space —to another stopping place in the bodies of the bulbous vegetable entities of Mercury."
Only a little more realistic is Kurt Vonnegut's, novel The Sirens of Titan (1959), in which mindless creatures called symphoniums inhabit the caves of Mercury.
Isaac Asimov's short story 'Runaround' in the collection I, Robot (1950) takes place on Mercury and involves a robot specially designed to cope with the intense solar radiation on the planet.
Asimov's juvenile novel Lucky Starr and the Big Sun of Mercury (1956) also takes place there.
A short story by Asimov, 'The Dying Night', is a murder mystery in which astronomers from Mercury, the Moon, and a fictitious space station are implicated in a murder. The dynamics and living conditions of each of these locations is key to discovering which astronomer is guilty.
Arthur C. Clarke's Islands in the Sky (1952) includes a description of a terrifying creature that survives on Mercury by keeping pace with the planet's terminator as it moves around the planet.
In Arthur C. Clarke's novel Rendezvous with Rama (1973), Mercury is ruled by a hot-tempered government of metal miners that tries to destroy the alien spacecraft Rama. The novel shares its background of a colonised Solar System with several others, especially Imperial Earth.
In several of the novels and short stories of Kim Stanley Robinson, especially 'Mercurial' in The Planet on the Table (1986) and Blue Mars (1996), Mercury is the home of a vast city called Terminator. The city rolls around the planet's equator on tracks keeping pace with the planet's rotation, so that the Sun never rises fully above the horizon and the city can avoid the dangerous solar radiation; the motive power comes from solar heat expanding the rails on the day side. The city is ruled by an autocratic dictator called the Lion of Mercury.
Alan E. Nourse's short story Brightside Crossing is a narrative of survivor of one such attempt which had become the ultimate sporting feat.

References

Discovering the Essential Universe by Neil F. Comins (2001)

External links

NASA's Mercury fact sheet (http://nssdc.gsfc.nasa.gov/planetary/factsheet/mercuryfact.html)
'BepiColumbo', ESA's Mercury Mission (http://www.esa.int/export/esaSC/120391_index_0_m.html)
'Messenger', NASA's Mercury Mission (http://messenger.jhuapl.edu/)
SolarViews.Com (http://www.solarviews.com/eng/mercury.htm)
Atlas of Mercury - NASA (http://history.nasa.gov/SP-423/sp423.htm)

The Solar System

See also astronomical objects and the solar system's list of objects, sorted by radius or mass

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