Facts about Planet Mercury
* Diameter: 4,878km (3,032 miles) at its equator, which is about two-fifths of Earth's diameter.
* Temperature: ?
* Orbit: 57,910,000 km (0.38 AU) from Sun. Orbiting the Sun once every 88 days.
* Average Distance: About 58 million km (36 million miles)
* Time to Rotate: 58.6 days
* Mass: 3.30e23 kg (5.5% of Earth's)
* Moons: 0
* Period of Rotation: 58.6462 days.
Orbit
Mercury has a very elliptical (oval-shaped) orbit. At perihelion (at its closest point) it is about 46 million km (28.58 million miles) from the Sun, but at aphelion (at its farthest point) it is 70 million km. Mercury is about 77.3 million km (48 million miles) from Earth at its closest approach. Mercury is not easily seen from Earth due to its small angular separation from the Sun. Mercury moves around the sun faster than any other planet. Mercury travels about 48 km (30 miles) per second and it takes 88 Earth days to orbit the sun. The Earth goes around the sun once every 365 days (one year).
Rotation
The planet rotates once about every 59 Earth days, a rotation slower than that of any other planet except Venus. As a result of the planet's slow rotation on its axis and rapid movement around the sun, a day on Mercury lasts 176 Earth days (interval between one sunrise and the next).
Composition
Mercury is the second densest major body in the solar system after Planet Earth and its density is slightly less than the Earths. Mercury's smaller mass makes its force of gravity only about a third as strong as that of the Earth. An object that weighs 100 pounds on the Earth would weigh only about 38 pounds on Mercury.
Mercury has a large iron core which is most likely at least partially molten and generates a magnetic field about 1% as strong as that of Earth's. Mercury's interior appears to resemble that of the Earth. Both planets have a rocky layer called a mantle beneath their crust and both planets have an iron core.
Surface
The surface of Mercury consists of cratered terrain and smooth plains and many deep craters similar to those on the moon. The craters formed when meteors or small comets crashed into the planet. The largest known crater is Caloris Basin, with a diameter of 1300 km (800 miles).
Like the other terrestrial planets (Venus, Earth and Mars) Mercury is made mostly of rock and metal. Mercury's surface appears to be much like that of the moon. It reflects approximately 6 percent of the sunlight it receives, about the same as the moon's surface reflects. Like the moon, Mercury is covered by a thin layer of minerals called silicates in the form of tiny particles.
Water
Scans of Mercury made by Earth-based radar indicate that craters at Mercury's poles contain water ice. The floors of the craters are permanently shielded from sunlight, so the temperature never gets high enough to melt the ice.
Temperature
Mercury is a planet of extreme temperature variations. It is hotter on Venus, but with less fluctuations. The temperature on the planet may reach 450 degrees C (840 degrees F) during the day. But at night, the temperature may drop as low as -170 degrees C (-275 degrees F). The sunlight on Mercury’s surface is 6.5 times as intense as it is on Earth due its closeness to the sun.
Atmosphere
Mercury is dry, extremely hot and almost airless. Planet Mercury is too small for its gravity to retain any significant atmosphere over long periods of time. The weak atmosphere contains hydrogen, helium, oxygen, sodium, calcium and potassium.
Due to the heat of the planet, the very thin atmosphere is blasted off its surface by the solar wind and quickly escapes into space. Mercury's atmosphere is constantly being replenished.
Mercury does not have enough atmosphere to slow down meteoroids and burn them up by friction. The sun's rays are approximately seven times as strong on Mercury as they are on the Earth. The sun also appears about 2 1/2 times as large in Mercury's sky as in the Earth's.
Name
The Greeks gave it two names: Apollo for when it appeared as a morning star and Hermes when it came as an evening star.
In Roman mythology Mercury is the god of commerce, travel and thievery, the Roman 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
Life
The plant and animal life of the Earth could not live on Mercury because of the lack of oxygen and the intense heat. Scientists doubt that the planet has any form of life.
Viewing
Because of Mercury's size and nearness to the sun, the planet is often hard to see from the Earth without a telescope. At certain times of the year, Mercury can be seen low in the western sky just after sunset. At other times, it can be seen low in the eastern sky just before sunrise.
Phases
When viewed through a telescope, Mercury can be seen going through ‘changes’ in shape and size. These apparent changes are called phases and resemble those of the moon. They result from different parts of Mercury's sunlit side being visible from the Earth at different times.
History
Mercury has been known since ancient times. Until the mid-1960's, astronomers believed that Mercury rotated once every 88 Earth days, the same time the planet takes to go around the sun. If Mercury did this, one side of the planet would always face the sun and the other side would always be dark. However, radar studies conducted in 1965 showed that the planet rotates once in about 59 days.
The only spacecraft to come close to Mercury was Mariner 10 from 1974 to 1975, which was only able to map 40%–45% of the planet's surface.
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