Mercury is the closest planet to the Sun, and almost the smallest, with a diameter of only
Mercury takes its name from the Roman god of travel, the counterpart of the Greek god Hermes, the messenger of the Gods. The planet probably received this name because it moves so quickly across the sky. It moves so fast because it is very close to the sun, compared to the other planets ... about a third as close as Earth is.
Mercury was first visited by the spacecraft Mariner 10, which flew by the planet three times in 1974 and 1975. Only 45% of the surface was mapped. Mercury is too close to the Sun to be easily viewed by the Hubble Space Telescope.
Mercury is a unique planet in many ways. Its orbit is highly eccentric ... its distance from the sun varies from 46 million to 70 million kilometres, and its point of closest approach moves slowly around the sun with each orbit. This effect is called precession, and astronomers in the early 20th century tried to explain it by supposing that there was a similar planet on the other side of the sun in the same orbit, which they named Vulcan. The actual cause of this movement, however, was explained by Albert Einstein in his General Theory of Relativity in 1917.
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Mercury was also once thought to always keep one face pointing towards the sun, as the moon does with Earth. This would make Mercury's 'day' the same length as its year. However, in 1965 radar measurements showed that Mercury is in fact rotating three times for every two orbits about the sun, so all sides of Mercury face the sun at some point. To an observer on the surface of Mercury, over the course of its orbit around the sun, the sun would appear on the horizon and slowly grow larger as it rose in the sky; at 'noon', the sun would start to set, and grow smaller as it dropped in the sky many months later. A day on Mercury (sun rise to sun rise) is 176 Earth days long. The sky would always be black, because Mercury has almost no atmosphere to cause scattering of light. 
Temperature changes on the surface of Mercury are the most extreme in the solar system. They range from -180°C as the surface faces away from the sun, to Mercury can sometimes be seen by eye, or with binoculars, in the evening sky just after the sun sets, or in the early morning just before sunrise. Telescopes that are able to filter the light of the sun can also spot Mercury, and occasionally get a glimpse of it as it transits in front of the sun, as shown in the photo just above. The Mariner 10 spacecraft in the early 1970's made three passes by Mercury, one of which came as close as 700 kilometres from the surface. Over 2,700 pictures were taken, covering 45% of Mercury's surface. The pictures taken by Mariner 10 show that the surface of Mercury is very similar to that of the Moon. It is covered with craters, large flat basins, and lava flows. Just as on the Moon, some craters are relatively new, and we can see the material that was ejected onto the surface after the impacts that caused the craters. Other craters appear to be old, and worn down by billions of years of micrometeorite impacts. The largest crater on Mercury is the Caloris basin, shown in the picture at the left below. The Caloris basin is 1,300 kilometres across, and was probably caused billions of years ago by a collision by an object at least 100 km in diameter; the impact pushed up rings of mountains, and scattered debris as far as 800 km away. After the impact the crater was partially filled with lava flows.   The photo at the right above, which covers about 100 kilometres (the two pictures are not to the same scale) illustrates the highly irregular terrain on Mercury, which includes craters, cliffs, huge cracks, and plains. In some areas, patches of smooth lava can be seen filling craters. It is believed that Mercury's interior includes a relatively large iron core, perhaps partially molten, and an outer shell corresponding to Earth's mantle and crust. There may be some indication of volcanic activity, but this remains undecided. Mercury has no real atmosphere; rather, it is constantly surrounded by a haze of molecules that have been 'blasted' off its surface by the intense radiation from the sun. Mercury isn't massive enough to keep these molecules from drifting off into space, so Mercury's 'atmosphere' is much like the dust surrounding a construction site, constantly being blown away and replaced by more. 
Mercury has no known moons, and only a tiny magnetic field (1% that of Earth's, possibly due to a small molten core). Remarkably, radio wave mapping of Mercury's surface from Earth has shown that it does seem to have some water ice, trapped in the shadows of crater walls near the north pole. These were areas not well mapped by Mariner, as shown by the blank spots on the composite map at the right. Here is a useful table of facts about Mercury, which may be printed. In 2005 the Messenger spacecraft was launched, and became the only spacecraft to orbit Mercury. Find out more about Messenger here, including a spectacular view of Mercury's hidden side. |