Introduction

Mercury is a small, bleak planet, and the closest object to our sun. Mercury is the second smallest planet in the solar system; only Pluto is smaller. Mercury's diameter is a little over one-third the Earth's, yet it has just 5.5 percent of Earth's mass. On average, Mercury is 36 million miles (58 million kilometres) from the sun. One effect of the sun's intense gravitational field is to tilt Mercury's orbit and to stretch it into a long ellipse (oval). Mercury orbits the sun so quickly, in just eighty-eight days. In contrast to its short year, Mercury has an extremely long day. It takes the planet the equivalent of fifty-nine Earth days to complete one rotation.

How visible is Mercury from Earth?

Because of the sun's intense glare, it is difficult to observe Mercury from Earth. Mercury is visible only periodically, just above the horizon, for about one hour before sunrise and one hour after sunset. For these reasons, many people have never seen Mercury.

How have been able to gather information about Mercury?

Little was known about Mercury until the space probe Mariner 10 photographed the planet in 1975. Mariner first approached the planet Venus in February 1974, and then used that planet's gravitational field to send it around like a slingshot in the direction of Mercury. The second leg of the journey to Mercury took seven weeks. On its first flight past Mercury, Mariner 10 came within 470 miles (756 kilometres) of the planet and photographed about 40 percent of the its surface. The probe then went into orbit around the sun and flew past Mercury twice more in the next year before running out of fuel.

What did the space probe Mariner 10 find out about Mercury?

Mariner 10 collected much valuable information about Mercury. It found that the planet's surface is covered with deep craters, separated by plains and huge banks of cliffs. Mercury's most notable feature is an ancient crater called the Calories Basin, which is about the size of Texas.

What forms Mercury's core?

The space probe Mariner 10 gathered information about Mercury's core, which is nearly solid metal and is composed primarily of iron and nickel. This core, the densest of any in the solar system, accounts for about four-fifths of Mercury's diameter. It may also be responsible for creating the magnetic field that protects Mercury from the sun's harsh particle wind.

What are the climatic conditions like on Mercury?

Mercury's very thin atmosphere is made of sodium, potassium, helium, and hydrogen. Temperatures on Mercury reach 800 degrees Fahrenheit (427 degrees Celsius) during its long day and -280 degrees Fahrenheit (-173 degrees Celsius) during its long night, when heat escapes through the negligible atmosphere.

How did Mercury's topography evolve?
Astronomers believe that Mercury, like the moon, was originally made of liquid rock, and that the rock solidified as the planet cooled. Some meteorites hit the planet during the cooling stage and formed craters. Other meteorites, however, were able to break through the cooling crust. The impact caused lava to flow up to the surface and cover older craters forming plains.

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