Type of Planet
Mercury is a terrestrial planet, meaning primarily composed of rock and metal.
Mercury is basically a large metal ball of iron with a very thin rocky crust. The iron core makes up 70 percent of its composition, and nearly three quarters of the planet's diameter.
Size (Diameter), Mass, and Density
Mercury is the smallest planet in the Solar System, and the same size as Earth's moon. It is 38 percent the diameter of the Earth.
18 Mercury's would equal the mass of the Earth.
The density of Mercury is 5.43 grams per cubic centimeter, compared to Earth's 5.51.
As Mercury is smaller, and has less mass and density, than Earth, it also has less gravity. Mercury has 38 percent of the Earth's gravity.
Mercury is the only rocky planet that has a global magnetic field like Earth.
Mercury’s magnetic field is 150 times weaker than that of Earth. Due to Mercury's close proximity to the sun, it is more exposed to solar wind which weakens its magnetic field.
Mercury has almost no atmosphere. Mercury's thin atmosphere is constantly being blown away into space by the solar wind.
The planet's small size means that its gravity is too weak to hold down a normal atmosphere. However, gases are also constantly being added to Mercury's atmosphere, that's why it still has an atmosphere at all.
Mercury's temperature varies more than any other planet in the Solar System, ranging from a daytime 430 degrees celsius, to -170 degrees celsius at night. This is because it has almost no atmosphere to retain heat.
Length of a Day (Rotation)
Mercury rotates on its axis slower than Earth, so a day on Mercury lasts 58 Earth days.
Mercury barely experiences any seasons at all. This is because it has almost no axial tilt at only 2.11 degrees.
Length of a Year (Orbit)
Mercury orbits the sun quicker than Earth, so a year on Mercury lasts only 88 Earth days.
Distance from the Sun
Mercury is the closest planet to the Sun at 36 million miles.