Composition of Mercury. Mercury is a terrestrial planet, like like all of the 4 inner planets: Mercury, Venus, Earth and Mars. It's the smallest of them, and has a diameter of only 4,879 km at its equator.
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Uniquences
Mercury is the planet nearest to the Sun. It has no atmosphere and its solid surface, like that of our Moon, is covered with many craters. Mercury orbits the Sun once in just 88 days and has no moons.