Because of its proximity to the Sun, Mercury is always observed within 27 degrees of the Sun in the cast before sunrise or in the west after sunset. The closest planet to the Sun and the second smallest of the nine planets, it has been observed from Earth since prehistoric times. However, because of its size it is fainter than Venus, Mars, Jupiter and Saturn?the other planets visible to the naked eye.
Mercury has a sidereal period of just three months, the shortest of any planet. Because of this, it has the appearance from Earth of moving faster than the others, a characteristic which led the Greeks to name it Hermes, after the messenger of the gods. The Romans. in turn, called the planet Mercury after their own deities’ wing-footed messenger.
Johann Hieronymus Schroeter (1745-1816) became the first astronomer to record his observations of Mercury’s surface detail, but his drawings, like those of Giovanni Schiaparelli (1835-1910) more than a century later. were ill-defined and turned out to be inaccurate. The great American astronomer Percival Lowell (1855-1916) reported that he had observed streaks on Mercury’s surface similar to those that he and Schiaparelli had both observed on Mars. Schiaparelli had called these Martian features canali( (channels) and Lowell decided they were canals, built by intelligent life. Both astronomers agreed, however, that the streaks on Mercury’s surface were of natural origin. The major milestone in the observation of Mercury came in March 1974 when the American spacecraft Mariner 10 began a series of three flybys at a distance of about 12.000 miles, in which it was able to photograph, in great detail, objects as small as 325 feet across.
The Mariner 10 photos revealed a planet whose surface features could easily be mistaken for those on the Earth’s Moon. Like the Lunar surface, that of Mercury is pocked by thousands of craters. With the exception of the relatively smooth Caloris Basin, Mercury’s surface is characterized almost exclusively by craters, overlapping craters and craters within craters. The Lunar surface, by contrast, has more larger open areas known as maria, or seas. The Caloris Basin. which is itself pocked by hundreds of relatively smaller craters, is the only major open plain comparable to the Moon’s maria. Unlike the Lunar seas, which arc ancient lava flows, it is believed that the Caloris Basin was created by a massive, ancient impact. as is indicated by-the presence of mountains and ridges around its periphery, possibly caused by seismic waves. Other ridges and escarpments are to be seen on the surface, and are possibly due to the expansion and contraction of Mercury’s core as it cooled and shrank. Some of the cliffs produced by this effect rise as much as 6300 feet above the adjacent valley floors. There is some evidence of ancient volcanic activity on Mercury, but less than that of the Moon.
Because of Mercury’s overall density., its core is thought to be largely (70 percent) composed of iron, with the surface crust being silica rock. like that of the Earth or Lunar surfaces. Due perhaps to its slow. rotation. Mercury has a relatively weak magnetic field. despite it’ being composed mostly of iron.
Unlike the other three inner terrestrial planets. Mercury has virtually no atmosphere. Faint traces of gaseous helium form 98 percent of the ‘atmosphere,’ with the remainder being composed mostly of hydrogen. with minute traces of argon and neon also being present. The helium was probably captured from the Sun. because any gasses emanating from the interior of the planet would have long ago dissipated into space.
Mercury’s surface temperatures vary widely. The midday temperature on the side facing the Sun can be as hot as 610 degrees Fahrenheit. while at night temperatures can plummet to-346 degrees Fahrenheit because there is no atmosphere to hold the heat.