Mercury, the closest planet to the Sun

Mercury is the innermost planet in our solar system. Because of its small size – a diameter of 3030 miles – it is not the easiest of planets to observe. German scientist Johannes Hevelius (1611 - 1687) was among the first to observe a Mercurial transit and discovered the Mercurial phases. Most of our information has been collected by the Mariner 10 space probe that made three successful passes of the planet (March 29 1974, September 21 1974, and March 16 1975) before losing contact with Earth. Averaging an orbital velocity of 47.87 km per second, Mercury is the fastest-moving planet in our solar system.

Because of its low escape velocity, Mercury hasn't much of an atmosphere. Based on information from Mariner 10, its surface pressure is ten billionths of a millibar. This barely extant atmosphere includes trace amounts of hydrogen and helium from the solar wind. In 1991, very powerful radio telescopes noticed large sheets of ice contained on the poles, areas unseen by Mariner 10. Due to its 3600 km iron core, Mercury has the greatest density in our Solar System apart from Earth. Composed of 70% iron and 30% rock, Mercury has a molten core, a 600-km thick mantle, and a crust of silicates. Its largest known surface feature is the Caloris Basin with a 1350 km diameter. Mariner 10 also detected a magnetosphere with a 1% surface value of Earth's. Mercury's magnetic field is inclined at 11% to its rotational axis and has the same polarity as Earth's – just enough to protect it from the full force of the solar wind. Mercury's 0.06 albedo is due to a dark-coloured, rough, porous rock that does not reflect much light; its climate indicates a slim chance life could exist there.

Due to the small difference between Mercury's orbital and rotational periods, the interval between night and day on the planet itself is 88 Earth days. This leads to a few peculiarities. As a result of orbital eccentricity, the temperature as measured at perihelion is two and a half times the heat received at aphelion. To an observer situated on the planet's hot pole at aphelion during a sunrise, the Sun would approach the zenith (getting larger as it does so), but since the orbital angular velocity is greater than the constant-spin angular velocity for a time, the Sun appears to stop, retreat backward for 8 Earth days, stop again, and then resume its original course.

 The $427 million Mercury Messenger probe, launched August 2004 aboard a Boeing Delta 2 rocket, will arrive at Mercury in 2011 to explore the planet's atmospheric and magnetic environments, crust structure and composition, geological history, polar areas, and core. Of all the inner rocky planets, Mercury is the least explored with only 45% of its surface photographed by Mariner 10. Though it is two thirds iron, the actual reason for this is a mystery for Messenger to solve. Some scientists believe it was originally a rocky planet that somehow lost its exterior. Messenger will also help scientists find out why Mercury and Earth are the only rocky planets with global magnetic fields. Equatorial temperatures on Mercury can reach 450°C and though signatures of ice craters have been detected at high latitudes with temperatures reaching below -184°C, some scientists believe the 'ice' might just be super-frozen silicon. Coated with heat-resistant ceramic fabric, the room-temperature probe carries a total of seven scientific instruments. Weighing 1.2 tons, Messenger will only briefly encounter Mercury's hotter regions.