Mercury is the smallest planet in the solar system. (Pluto used to hold the title, but it was downgraded to a dwarf planet.) Although its surface resembles our moon, the tiny planet has a density that rivals Earth itself.
Radius, diameter and circumference
Mercury's diameter is 3,030 miles (4,878 km), comparable to the size of the continental United States. This makes it about two-fifths the size of Earth. It is smaller than Jupiter's moon Ganymede and Saturn's moon Titan.
The planet has a mean radius of 1,516 miles (2,440 km), and its circumference at the equator is 9,525 miles (15,329 km). Some planets, such as Earth, bulge slightly at the equator due to their rapid rotation. However, Mercury turns so slowly on its axis that astronomers once thought that the planet was tidally locked, with one side constantly facing the nearby sun. In fact, the planet spins on its axis once every 58.65 Earth days. Mercury orbits once every 87.97 Earth days, so it rotates only three times every two Mercury years. The slow spin keeps the planet's radius at the poles and the equator equal.
Density, mass and volume
Mercury has a mass of 330 x 1023 kilograms. This mass is contained in a volume of 14.6 trillion cubic miles (60.8 trillion cubic km). The mass and volume of Mercury is only about 0.055 times that of Earth.
But because Mercury's small mass is enclosed inside of a tiny body, the planet is the second densest in the solar system, weighing in at 5.427 grams per cubic centimeter, or 98 percent of the density of our planet. Only Earth is denser. This high density in a planet that otherwise resembles the moon raises interesting questions about the composition of the planet's interior.
Mercury's small size makes it too weak to hold onto a significant atmosphere, especially with the constant bombardment it receives from the sun. The planet has a thin atmosphere, but it is constantly blasted into space by the solar wind. Without an atmosphere to help stabilize the incoming heat from the sun, the planet boasts some of the most varying temperature swings in the solar system.
Mercury's surface greatly resembles that of Earth's moon, with craters left over from the heavy bombardment early in the life of the solar system and during the planet's formation. Images taken by the Mariner 10 spacecraft show craters ranging from 328 feet (100 m) to 808 miles (1,300 km) across. The planet hosts numerous escarpments, steep cliffs or slopes created by faults. Some are as high as 1.86 miles (3 km).
— Nola Taylor Redd, SPACE.com Contributor