MERCURY

from wikipedia

Mercury is the innermost and smallest planet in the solar system, orbiting the Sun once every 88 days. Mercury is bright when viewed from Earth, ranging from −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 and evening twilight. Comparatively little is known about it; the first of two spacecraft to approach Mercury was Mariner 10 from 1974 to 1975, which mapped only about 45% of the planet’s surface. The second was the MESSENGER spacecraft, which mapped another 30% of the planet during its flyby of January 14, 2008. MESSENGER will make two more passes by Mercury, followed by orbital insertion in 2011, and will survey and map the entire planet. Physically, Mercury is similar in appearance to the Moon. It is heavily cratered, has no natural satellites and no substantial atmosphere. It has a large iron core, which generates a magnetic field about 1% as strong as that of the Earth. It is an exceptionally dense planet due to the large size of its core. The surface temperatures on Mercury range from about 90 to 700 K (−180 to 430 °C), 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 Sumerians in the third millennium BC. Before the 4th century BC, Greek astronomers believed the planet to be two separate objects: one visible only at sunrise, which they called Apollo; the other visible only at sunset, which they called Hermes. The English name for the planet comes from the Romans, who named it after the Roman god Mercury, which they equated with the Greek Hermes. The astronomical symbol for Mercury is a stylized version of Hermes' caduceus.

Internal structure

Mercury is one of the four terrestrial planets that is a rocky body like the Earth. It is the smallest of the four, with a diameter of 4879 km at its equator. Mercury is even smaller—albeit more massive—than the largest natural satellites in the solar system, Ganymede and Titan. Mercury consists of approximately 70% metallic and 30% silicate material. The density of the planet is the second highest in the solar system at 5.43 g/cm³ (water is 1.00 g/cm³), only slightly less than Earth’s density. If the effect of gravitational compression were to be factored out, the materials of which Mercury is made would be denser, with an uncompressed density of 5.3 g/cm³ versus Earth’s 4.4 g/cm³.

1. Crust - 100–200 km thick
2. Mantle - 600 km thick
3. Core - 1,800 km radius

Mercury’s density can be used to infer details of its inner structure. While the Earth’s high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller and its inner regions are not nearly as strongly compressed. Therefore, for it to have such a high density, its core must be large and rich in iron. Geologists estimate that Mercury’s core occupies about 42% of its volume; for Earth this proportion is 17%. Recent research strongly suggests Mercury has a molten core.

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.

Mercury’s crust is believed to be 100–200 km thick. One distinctive feature of Mercury’s surface are numerous narrow ridges, some extending over several hundred kilometers. It is believed that these were formed as Mercury’s core and mantle cooled and contracted at a time when the crust had already solidified.

Mercury has a higher iron content in the core than any other major planet in our solar system, and several theories have been proposed to explain this. The most widely accepted theory is that Mercury originally had a metal-silicate ratio similar to common chondrite meteors, thought to be typical of average solar system rocky matter, and a mass approximately 2.25 times its current mass. However, early in the solar system’s history, Mercury may have been 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 as a relatively major component. A similar process 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 protosunsolar wind. contracted, temperatures near Mercury could have been between 2500 and 3500 K (about 4,000 to 5,800 °F), and possibly even as high as 10000 K (about 17,500 °F). Much of Mercury’s surface rock could have been vaporized at such temperatures, forming an atmosphere of "rock vapor" which could have been carried away by the

A third theory proposes 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.
Atmosphere

Mercury is too small for its gravity to retain any significant atmosphere over long periods of time; however, it does have a tenuous atmosphere containing hydrogen, helium, oxygen, sodium, calcium and potassium. This atmosphere is not stable—atoms are continuously lost and replenished from a variety of sources. 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 present, being brought to Mercury by comets striking its surface.
Magnetic field and magnetosphere

Despite its small size and slow 59-day-long rotation, Mercury has a significant, and apparently global, magnetic field. According to measurements taken by Mariner 10, it is about 1.1% as strong as the Earth’s. Mercury's magnetic field is dipolar and nearly aligned with the planet's spin axis. It is likely that this magnetic field is generated in a manner similar to Earth’s, by a dynamo of circulating liquid core material. A mechanism that has been suggested for keeping it liquid are particularly strong tidal effects during periods of high orbital eccentricity. Mercury’s magnetic field is strong enough to deflect the solar wind around the planet, creating a magnetosphere. It is also strong enough to trap solar wind plasma within the magnetosphere, which contributes to the space weathering of the planet's surface.

Orbit and rotation

The orbit of Mercury is the most eccentric of the planets; its eccentricity is 0.21 with its distance from the Sun ranging from 46,000,000 to 70,000,000 kilometers. It takes 88 days to complete an orbit. The diagram on the left illustrates the effects of the eccentricity, showing Mercury’s orbit overlain with a circular orbit having the same semi-major axis. 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, is used to illustrate the varying heliocentric distance. This varying distance to the Sun, combined with a 3:2 spin-orbit resonance of the planet’s 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 during local noon would never see the Sun more than 1/100 of one degree north or south of the zenith. Conversely, at the poles the Sun never rises more than 0.01° above the horizon.

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 move in a retrograde direction. Four days after perihelion, the Sun’s normal apparent motion resumes at these points.
Advance of perihelion

It was noticed in the 19th century that the slow precession of Mercury’s orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets (notably by the French mathematician Le Verrier). 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 the orbit of Uranus 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 the explanation for the observed precession. The effect is very small: the Mercurian relativistic perihelion advance excess is just 42.98 arcseconds per century, therefore it requires a little over twelve million orbits for a full excess turn. Similar, but much smaller effects, operate for other planets, being 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars, and 10.05 for 1566 Icarus.
Spin–orbit 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—at perihelion, when the solar tide is strongest, the Sun is nearly still in Mercury’s sky. The original reason astronomers thought it was synchronously locked was that 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. Orbital simulations indicate 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.