Mercury

Information

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 is a stylized version of the god's head and winged hat atop his caduceus. 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.

Physical Characteristics

Temperature & 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 150 K. 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/m�.

Surface Features

During and shortly following the formation of Mercury, it was heavily bombarded by comets and asteroids for a period of about 8000 million years. 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
• Albedo features � areas of markedly different reflectivity
• Dorsa � ridges
• Montes � mountains
• Planitiae � plains
• Rupes � scarps
• Valles � valleys

Of all the terrestrial planets in the Solar System, the geology of Mercury is the least understood. Reasons for this include Mercury's proximity to the Sun and the resultant dangers to spacecraft of intense solar radiation and high surface temperatures. Also, Mercury's period of rotation is a slow 58 Earth days, so that when NASA's Mariner 10 space probe flew past Mercury three times during 1974 and 1975, it was only able to observe the side facing the Sun during each pass. It is hoped that NASA's MESSENGER probe, launched in August 2004, will greatly contribute to our understanding when it enters orbit around Mercury in March 2011.

With a surface density of 5.44 g/cm3, the surface of Mercury is highly cratered, and those craters are uniformly distributed over the surface. The present surface exhibits such an abundant quantity of craters because Mercury's atmosphere is so thin that meteorites can easily reach the surface without disintegrating. The surface of Mercury has accumulated meteoric impacts since its creation over 4 billion years ago, and for this reason its surface, like those of the Moon and Mars registers the importance such impacts have in determining the duration of this period of craterization, which was very intense for about 3 billion years.

Apart from craters of 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 surrounding the impact crater over 2 km tall. 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.

These kinds of craters which have been filled with lava are known as seas in lunar geology.

As on the Moon, craters in Mercury show the typical characteristics of an impact: the ejected debris forms banks around the crater shaped as linear extensions known as radii (or rays), the brightness of which is stronger because the terrain is relatively younger than the surrounding surface.

Other escarpments have also been seen crossing the planet's surface, in both the heavily cratered and flat areas. They are attributed to the cooling that Mercury has undergone since its formation, shrinking and causing the crust to realign itself.

The planet's high density (5.44 g/cm3) indicates that it has a core of 65% iron, accounting for somewhere about 75% of its diameter. This core is then surrounding by a 600 km thick mantle. As the cooling of the planet caused the core and mantle to shrink after its initial formation, Mercury' lost an estimated 2 to 4 km of its radius, which created the network of fissures visible on its surface.

Mercury's Geological History

Like the Moon and Mars, Mercury's geologic history is divided up into eras. From oldest to youngest, these are: the pre-Tolstojan, Tolstojan, Calorian, Mansurian, and Kuiperian. These ages are based on relative dating only.

After the formation of Mercury over four billion years ago, it received heavy bombardment of comets and asteroids that came to an end 3.8 billion years ago. During this period of intense crater formation, the surface received many impacts. Other massifs, such as the one that formed the Caloris Basin, were filled by magma from within the planet, which produced smooth intercrater plains similar to the maria found on the Moon. As the planet cooled and contracted, its surface began to crack; these surface cracks can be seen on top of other features, such as the craters and smoother plains � a clear indication that they are more recent. Mercury's period of vulcanism ended when the planet's mantle had contracted enough to prevent further lava from breaking through to the surface. This probably occurred at some point during its first 700 or 800 million years of history.

Since then, the only impacts have been the result of the planet being struck by stray asteroids and comets.

Interior Compensation

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/m�, 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 formerly 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 million to 70 million kilometres. Among the major planets, only Pluto has a more eccentric orbit. However, because of the smallness 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 Einstein's predictions. 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 the remaining planets, being 8.6 arcseconds per century for Venus, 3.8 for the 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.

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.[5] Tentative suggestions have been made that Mercury may be a Chthonian planet.

Origin of the Planet's High Luminosity and the Presence of Ice

The first radar observations of Mercury were carried out by the radiotelescopes at Arecibo (Puerto Rico) and Goldstone (California, United States), with assistance from the U.S. National Radio Astronomy Observatory Very Large Array (VLA) facility in New Mexico. The transmissions sent from the NASA Deep Space Network site at Goldstone were at a power level of 460 kW at 8.51 GHz; the signals received by the VLA multi-dish array detected points of radar reflectivity (radar luminosity) with depolarized waves from Mercury's north pole.

Radar maps of the surface of the planet were made using the Arecibo radiotelescope. The survey was conducted with 420 kW UHF band (2.4 GHz) radio waves which allowed for a 15 km resolution. This study not only confirmed the existence of the zones of high reflectivity and depolarization, but also found a number of new areas (bringing the total to 20) and was even able to survey the poles. It has been postulated that surface ice may be responsible for these phenomena.

The belief that Mercury has surface ice may seem absurd at first, given its proximity to the Sun and its subsequently high temperature (420�C by day and 180�C at night). Regardless, it could very well be ice that is responsible for the high luminosity levels, as the silicate rocks that compose most of the surface of Mercury have exactly the opposite effect on luminosity. The presence of ice may be explained by another discovery of the radar surveys from Earth: craters at Mercury's higher latitudes may be deep enough to shield the ice from direct sunlight.

At the South Pole, the location of a large zone of high reflectivity coincides with the location of the Chao Meng-Fu crater, and other small craters containing reflective areas have also been identified.

At the North Pole, the situation is more complicated; no one can correlate the radar images with the data from Mariner 10, due to minor differences in the images. It should also be emphasized that there are areas of high reflectivity that do not correspond to any known craters.

The radar reflection of ice on Mercury is minor compared to that which would occur with pure ice. This may be due to powder deposition that does not cover the surface of the crater completely.

Origin of Ice

Mercury is not unique in having craters that stand in permanent shadow; at the south pole of Earth's Moon there is a large crater (Aitken) where ice is possibly thought to exist. Ice on both Mercury and the Moon must have originated from external sources: comets in the case of the Moon, or meteorites in the case of Mercury. The existence of ice on certain meteorites has been proven; it is therefore conceivable for meteorite impacts to have deposited water in the permanent-shadow craters, where it would have remained for millions or billions of years.

Another hypothesis, which has not been confirmed, is that Mercury has an important flow of water from its interior. It has also not been proved that any mechanism, such as photodissociation, erosion due to solar wind, or small meteorite impact, causes the loss of ice in on the surface.

The behavior of ice on other celestial bodies has its peculiarities. The high temperatures on the surface of Mercury, near 420�C, the emptiness of space (the atmosphere of Mercury is almost imperceptible), and solar rays contribute to the sublimation of ice into vapor and its escape into space.

Nevertheless, it is not thought that such behavior occurs with ice on Mercury because the location of ice at high latitudes makes it so that the temperature is very low. Inside the craters, where there is no solar light, temperatures fall to -171�C; on the polar plains, the temperature does not rise above -106�C.

The evidence for ice on Mercury has not been irrefutably corroborated, but is simply scientific speculation provoked by images of areas of high reflectivity and the coincidence of the existence of large craters in polar zones. It must be made clear, however, that this anomalous reflection could also be due to the existence of metallic sulfates or other materials with the same capacity for reflection.

Mercury's Atmosphere

The existence of an atmosphere on a planet is of great geological relevance, since the erosion caused by wind, changes in temperature levels, moisture levels, etc contribute to the modification of the landscape and the deterioration of materials.

Mercury's atmosphere dissipated shortly after the planet's formation over four billion years ago because of the low level of gravity on the planet and, mainly, the effects of the solar wind. However, there are still traces of a very thin atmosphere with a pressure level of 10-15 bar (which can be considered negligible). The existence of an atmosphere would also keep temperatures more or less stable despite the variations in sunlight levels between night and day; consequently, temperature variations in bodies without atmospheres (or with extremely weak atmospheres) are more pronounced. For example, during the day Mercury's surface reaches a temperature of 420�C, while at night it dips to �180�C.

Due to the abrupt changes in the temperature, the type of interaction over the surface is related to the thermal agitation produced on the materials.

Specifications

Orbital Characteristics

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 days (0.240 846 9 a)
Synodic Period   115.8776 days
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 km� (0.147 Earths)
Volume   6.1�10¹⁰ km� (0.056 Earths)
Mass   3.302�10²³ kg (0.055 Earths)
Mean Density   5.427 g/cm�
Equatorial Gravity   3.701 m/s� (0.377 gee)
Escape velocity   4.435 km/s
Rotation Period   58.6462 days (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)
Declination   61.45�
Albedo   0.10-0.12
Avg. Surface Temp.   Day 623 K
Avg. Surface Temp.   Night 103 K
Adjective   Mercurian
Min. Surface Temp.   90 K
Mean Surface Temp.   440 K
Max. Surface Temp.   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%

** Information provided by Wikipedia, the free online encyclopedia. **

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled "GNU
Free Documentation License".