Mercury
Mercury is the innermost and smallest planet in the solar system, orbiting the Sun once every 88 days. It ranges in brightness from about -2.0 to 5.5 in apparent magnitude, but is not easily seen as its greatest angular separation from the Sun (greatest elongation) is only 28.3°. It can only be seen in morning or evening twilight. Comparatively little is known about the planet: the only spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only 40%-45% of the planet№s surface. The period of rotation is 58.6462 days.
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 to 700 K, with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.
Physically, Mercury is similar in appearance to the Moon as it is heavily cratered. It has no natural satellites and no substantial atmosphere. The planet has a large iron core which generates a magnetic field about 0.1% as strong as that of the Earth.
Surface temperatures on Mercury range from about 90 to 700 K (-180 to 430°C, -292 to 806°F), with the subsolar point being the hottest and the bottoms of craters near the poles being the coldest.
Recorded observations of Mercury date back to the time of the Sumerians, in the third millennium BC.
The Romans named the planet after the Roman god Mercurius, equated to the Greek Hermes and the Babylonian Nabu. The astronomical symbol for Mercury is a stylized version of the god№s head and winged hat atop his caduceus, an ancient astrological symbol. The Greeks of Hesiod's time called it Stilbon (the gleaming) and Hermaon.
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. In India, the planet was named Budha, after the son of Chandra (the Moon).
The Chinese, Korean, Japanese, and Vietnamese cultures refer to the planet as the water star, based on the Five Elements.
The Hebrews named it Kokhav Hamah, 'the star of the hot one' ('the hot one' being the Sun).
Mercury is smaller than several of the natural satellites or moons in our solar system (see the list of solar system objects by radius).
Mercury's high density can be used to infer details of its inner structure. While the Earth's high density results partly from compression at the core, Mercury is much smaller and its inner regions are not nearly so compressed. Therefore, for it to have such a high density, its core must be large and iron rich. Geologists estimate that Mercury's core occupies about 42% of its volume. (Earth's core occupies about 17% of its volume.)
Surrounding the core is a 600 km mantle. It is generally thought that early in Mercury's history, a giant impact with a body several hundred kilometers across stripped the planet of much of its original mantle material, resulting in the relatively thin mantle compared to the sizable core, although alternative theories exist and are discussed further below.
Mercury's crust is thought to be about 100200 km thick. One very distinctive feature of Mercury's surface is numerous ridges, some extending over several hundred kilometers. It is believed that these were formed as Mercury's core and mantle cooled and contracted after the crust had solidified.
Mercury has a higher iron content than any other major planet in our solar system. Several theories have been proposed to explain Mercury's high metallicity. The most widely accepted 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. Giant Impact Hypothesis
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 and 3500 K, and possibly even as high as 10000 K. Much of Mercury's surface rock could have vaporized at such temperatures, forming an atmosphere of "rock vapor" which could 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, and two upcoming space missions, MESSENGER and BepiColombo, both aim to take observations that will allow the theories to be tested.
Mercury's surface is very similar in appearance to that of the Moon, showing extensive mare-like plains and heavy cratering, indicating that it has been geologically inactive for billions of years. The small number of unmanned missions to Mercury means that its geology is the least well understood of the terrestrial planets.
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.
Craters on Mercury range in diameter from a few meters to hundreds of kilometers across. The largest known crater is the enormous Caloris Basin, with a diameter of 1300 km. The impact which created the Caloris Basin was so powerful that it caused lava eruptions and left a concentric ring over 2 km tall surrounding the impact crater. At the antipode of the Caloris Basin is a large region of unusual, hilly terrain known as the "Weird Terrain". It is believed that shock waves from the impact traveled around the planet and, when they converged on the antipodal point of the impact, caused extensive fracturing of the surface there.
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 crisscross 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.
Despite the generally extremely high temperature of its surface, observations strongly suggest that ice exists on Mercury. The floors of some deep craters near the poles are never exposed to direct sunlight, and temperatures there remain far lower than the global average. Water ice strongly reflects radar, and observations reveal that there are patches of very high radar reflection near the poles. While ice is not the only possible cause of these reflective regions, astronomers believe it is the most likely.
Atmosphere
Mercury is much too small for its gravity to retain any significant atmosphere over long periods of time, but it does have a very tenuous atmosphere containing hydrogen, helium, oxygen, sodium and potassium. The atmosphere is not stable - atoms are continuously lost and replenished, from a variety of sources. The hydrogen and helium atoms probably come from the solar wind, diffusing into Mercury's magnetosphere before later escaping back into space. Radioactive decay of elements within Mercury's crust is another source of helium, as well as sodium and potassium. Water vapor is probably also present, water being brought to Mercury by comets impacting on its surface.
Magetic Field
Despite its slow rotation, Mercury has a relatively strong magnetic field, 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.
Mercury's magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere inside which the solar wind does not penetrate. This is in contrast to the situation on the Moon, which has a magnetic field too weak to stop the solar wind impacting on its surface and so lacks a magnetosphere.
Orbit and Rotation
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. It takes 88 days to complete the orbit. Among the major planets, only Pluto has a more eccentric orbit. The higher velocity of the planet when it is near perihelion is clear from the greater distance it covers in each 5-day interval. The size of the spheres, inversely proportional to their distance from the Sun, illustrates the varying heliocentric distance. The varying the distance to the Sun combined with a unique 2:3 resonance of the planet rotation around its axis, result in complex variations of the surface temperature.
Mercury's orbit is inclined by 7° to the plane of Earth's orbit (the ecliptic), as shown in the diagram on the left. As a result, transits of Mercury across the face of the Sun can only occur when the planet is crossing the plane of the ecliptic at the time it lies between the Earth and the Sun. This occurs about every seven years on average.
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.
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. Four days after perihelion, the Sun's normal apparent motion resumes.
Advance of Perihelion
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 success of the search for Neptune based on its perturbations of Uranus' orbit led astronomers to place great faith in this explanation, and 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 - Mercury is slightly heavier at perihelion than it is at aphelion because it is moving faster, and so it slightly "overshoots" the perihelion position predicted by Newtonian gravity. The effect is very small: the Mercurian relativistic perihelion advance excess is just 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.
Orbital resonance
For many years it was thought that Mercury was synchronously 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 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 transits of the Sun) lasts about 176 Earth days. A sidereal day (the period of rotation) lasts about 58.7 Earth days.
Observation
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 moonlike 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 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.
Mercury is brightest as seen from Earth when it is at a gibbous phase, between half full and full. Although the planet is further away from Earth when it is gibbous than when it is a crescent, the greater illuminated area visible more than compensates for the greater distance. The opposite is true for Venus, which appears brightest when it is a thin crescent.