Mercury (planet)

Mercury is the closest planet to the Sun. This is the second-smallest planet within Earth's solar system. Upon the celestial sphere, Mercury ranges from -0.4 to 5.5, in apparent magnitude; Mercury is sufficiently "close" to the Sun that telescopes rarely examine it. 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 astronomical symbol for Mercury is a circle on top of a short vertical line with a cross below and a semicircle above the circle.

Table of contents

1 Physical characteristics 1.1 Atmosphere 1.2 Temperature and sunlight 1.3 Terrain 1.4 Interior composition 1.5 Mercurian orbit 1.6 Mercurian rotation 1.7 The Mercurian magnetosphere 1.8 Why Mercury has so much iron 1.9 Ice on Mercury 2 The Exploration of Mercury 3 Japan and the ESA 4 External links 5 References

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 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 452K, but it ranges from 90-700K; by comparison, the temperature on Earth varies by only ~11K (with respect only to solar radiation; not climate or season). The sunlight on Mercury's surface is 6.3 times more intense than that on Earth, a total irradiance of 3566W/m².

Terrain

Mercury's cratered surface appears very similar to Luna. 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.

See also: List of craters on Mercury

Interior composition

The planet has a relatively large iron core (even when compared to Earth) and is, therefore, much denser; Mercury's composition is approximately 70% metallic and 30% silicate. The average density is 5430kg/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.

Mercurian orbit

The orbit of Mercury is eccentric, ranging from 46M-70Mkm in radius. 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 might be present in an orbit even closer to the sun (sometimes referred to as Vulcan) to account for this perturbation. Einstein's General Theory of Relativity provided the explanation for this small discrepancy instead, however.

Mercurian rotation

At certain points on Mercury's surface, an observer (standing upon one of the tidal bulges) 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; such that the Sun's apparent motion ceases; and, at perihelion, Mercury's orbital velocity exceeds the rotational velocity; thus, the Sun appears to retrograde. Four days after perihelion, the Sun's normal apparent motion resumes.

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-coupling, rotating three times for every two revolutions around the Sun; the eccentrity 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-coupling; although a sidereal day (the period of rotation) lasts ~58.7 Earth days, a solar day (the length between two meridian transitss of the Sun) lasts ~176 Earth days.

The Mercurian 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 eariler dynamo effect that has now ceased, the magnetic field becoming "frozen" in solidified magnetic materials.

Why Mercury has so much iron

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 2500-3500K; and possibly even as high as 10000K. 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.

Ice on Mercury

Radar observations first indicated, in 1992, that there is frozen water ice at Mercury's north pole. Such water is believed to exist at the permanently shaded bottoms of craters, where it is deposited by comets and/or gases from the planetary core.

The Exploration of Mercury

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 is severely complicated by its proximity to the Sun; it is only visible from Earth at sunrise or sunset.

The only spacecraft to approach Mercury was Mariner 10 (1974-75)

A mission to Mercury has been approved by NASA, named MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging), which will launch in 2004 and reach Mercury in 2009.

Japan and the ESA

Japan is planning a joint mission with the European Space Agency that would be the first to land a probe on Mercury. The mission entails three probes, two that would orbit and one that would land, to map the topography and study the origins of the planet. Russian Soyuz rockets would launch the probes starting in 2010. The probes would reach Mercury about four years later, with one of them landing on the planet, and the other two orbiting and charting its surface for a year.

External links

• NASA's Mercury Exploration Program
• ESA's Mission to Mercury
• SolarViews.Com

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

simple:Mercury (planet)