Space and Time
Size: Diameter 4,880km
Mass: 3.302x10(exp23)kg
Composition: Iron rich core
Mean temperature: 350°c by day, -170 °c at night
Distance from Sun: 57.91 million km
Atmosphere/weather: Negligible atmosphere
Moon/satellites: None
Orbital length: 88 days
Length of day: 59 Earth days
Distance from Earth: Max 207.9 million km, nin 92 million km
Mercury is the closest planet to the Sun. It is the second smallest planet in the solar system, after Pluto. It has an elliptical (non-circular) orbit which takes it both close to and far from the Sun. It travels at nearly 50km per second, faster than any other planet.
Until recently it was thought that the same side of Mercury always faced the Sun (like our Moon). In 1965, however, astronomers discovered that Mercury actually rotates three times every two orbits around the Sun. Mercury has almost no atmosphere. What little there is comes from atoms blasted off the surface by the solar winds. It is made up of mainly oxygen, sodium and helium. Because of the extreme surface temperatures, these atoms quickly escape into space and are constantly replenished. Mercury's surface is marked by thousands of impact craters. These have been caused by collisions with meteors. There are some areas of smooth terrain, as well as cliffs, some a mile high, caused by impacts. The Caloris Basin, one of the largest features on the planet is about 1,300km in diameter. It was the result of an asteroid impact early in the solar system's history and is probably the cause of strange surface features on the opposite side of the planet. |
The surface of Mercury, showing the marks of meteor impacts.
Mercury is the second densest body in the solar system after Earth. It has an interior made of a large iron core with a radius of 1,800 to 1,900km. This is nearly 75 percent of the planet's diameter and nearly the size of Earth's Moon. Mercury's outer shell, comparable to Earth's outer shell (called the mantle) is only 500 to 600km thick.
Mercury was visited by the Mariner 10 spacecraft in 1974-5. This discovered that Mercury has a weak magnetic field. More recent radar observations show that there may be ice at the two poles inside deep craters.
In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman equivalent of the Greek god Hermes, the messenger of the gods. The planet probably received this name because it moves so quickly across the sky. Mercury has been known since at least the time of the Sumerians (3rd millennium BC). It was given two names by the Greeks: Apollo for its apparition as a morning star and Hermes as an evening star. Greek astronomers knew, however, that the two names referred to the same body. Heraclitus, an ancient Greek philosopher and astronomer, even believed that Mercury and Venus orbit the Sun, not the Earth.
A mosaic of Mercury from images taken by the Mariner 10 spacecraft.
Images, information and videos courtesy of NASA.