Learn about the planet Mercury

Mercury in astronomy, innermost planet of the solar system. Its average distance from the Sun is about 58 million km (35,960,000 miles). Except for Pluto, it is the smallest of the nine major planets, having a diameter of about 4,870 km (3,050 miles).

Mercury's orbit around the Sun is highly elliptical. Its period of revolution (only 88 Earth days) is the shortest among the planets and its average orbital speed of 48 km per second the highest. The planet is thus aptly named after the god Mercury, Roman counterpart of the Greek Hermes, fleet-footed messenger of the gods. Mercury also has an extremely low rotation velocity. Because it turns about its axis so slowly, one day on Mercury (one complete rotation) is equal to 59 Earth days. A solar day on the planet (the time from one sunrise to the next, for example) is 176 Earth days, however, owing to the revolution of Mercury around the Sun.

Mercury is difficult to observe from Earth because of its proximity to the Sun. Relatively little was known about its topographic details until 1974, when the U.S. Mariner 10 space probe transmitted numerous closeup photographs of the planet.

Much of the Mercurian surface is heavily cratered like that of the Moon, but it differs from the latter in various ways. Because Mercury's surface gravity is considerably greater than the Moon's, ejecta reach only 65 percent of the distance they would reach on the Moon. Vast areas of the planet known as intercrater plains are peppered with small craters. They are, however, devoid of large ones. Some of these plains appear to be as old as the heavily cratered terrain, while others seem to be much younger. Their origin is still unknown. The most impressive feature imaged by Mariner 10 is the 1,300-kilometre Caloris basin, centred in Mercury's northern hemisphere. This large circular structure, formed by a huge meteorite impact, is bounded by a ring of mountains, which stands about 1 to 2 km high. Smooth plains appeared after the Caloris impact. Different in character from the intercrater plains, they resemble the lunar maria. Their volcanic origin, however, has not yet been confirmed. The only surface features younger than the smooth plains are certain impact craters, some of which have extensive rays. Still another kind of topographic feature detected on Mercury is sinuous scarps that extend for hundreds of kilometres. These long, steep cliffs may have originated as a result of crustal compression. Various investigators have hypothesized that Mercury's core once consisted of molten iron, which shrank by several kilometres upon cooling. The resultant settling of the planetary crust produced large-scale wrinkles in the form of the scarps.

That Mercury, like the Earth, has a large iron core has been suggested by the discovery of a magnetic field in its vicinity. The presence of such a core would account for the high mean density of the planet. Although Mercury is considerably smaller than the Earth, its mean density is about 5.44 grams per cubic cm, as compared to 5.5 grams per cubic cm for the Earth.

Mercury's atmosphere is negligible, consisting chiefly of a tenuous layer of hydrogen, helium, sodium, potassium, oxygen, and possibly neon. Trace amounts of argon, carbon dioxide, krypton, and xenon are also present. The planet's gravity is so weak that it cannot retain significant amounts of atmospheric gases. Temperatures at the surface of Mercury fluctuate drastically, ranging from a high of approximately 675 K to a low of about 100 K.