Introductory Astronomy: The Planet MERCURY

Basic Facts:
Distance from Sun: 0.31 - 0.47 AU
Period of revolution about Sun: 88 days
Period of rotation: 59 days
Radius: 0.38 Earth radii
Mass: 0.055 Earth masses
Average Density: 5.4 g/cm^3
Magnetic Field: Weak (1/100 of the Earth's)
Surface features: Heavily cratered, scarps, ringed basin
Atmosphere: no permanent atmosphere
Temperature: 800 to - 280 degrees Fahrenheit

Mercury is the closest planet to the Sun. It is one of the smallest of the planets, second only to Pluto. It is much too small to retain a true atmosphere, although it "borrows" gases from the solar wind to form a tenuous atmosphere less dense than any vacuum on Earth. This means that the planet suffers great temperature extremes, with the side facing the Sun enduring high temperatures and the side facing away from the Sun suffering very low temperatures.

Mercury is a rocky planet. In the past, lava flows and cratering shaped the surface of the planet. Now, however, there are no active lava flows nor erosion (since there is basically no atmosphere). Mercury's surface has changed very little during the past 4 billion years. Its core is thought to be iron, which explains why it is so dense and why it has a weak magnetic field. Mercury also has ice that forms near its north rotational pole. This ice is probably in the form of terrafrost (water frozen just under the surface of the soil). Ice can exist here because Mercury rotates around an axis perpendicular to its orbit, so sunlight strikes the pole at a great angle.

To learn more about Mercury, click here.