The four inner planets (closest to the Sun – Mercury, Venus, Earth, and Mars) – are referred to as the inner planets. They are similar to Earth. All are solid, dense, and rocky. None of the inner planets have rings. Compared to the outer planets, the inner planets are small. They have shorter orbits around the Sun and they spin more slowly.
Venus spins backwards and spins the slowest of all the planets. All of the inner planets were geologically active at one time. They are all made of cooled igneous rock with inner iron cores. Earth has one big, round moon, while Mars has two very small, irregular moons. Mercury and Venus do not have moons.
Mercury is the smallest planet. It has no moon. The planet is also closest to the Sun. As the Figure below shows, the surface of Mercury is covered with craters, like Earth’s moon. The presence of impact craters that are so old means that Mercury hasn’t changed much geologically for billions of years. With only a trace of an atmosphere, it has no weather to wear down the ancient craters.
Short year, long days
Mercury is named after the Roman messenger god. Mercury was a messenger because he could run extremely fast. The Greeks gave the planet this name because Mercury appears to move very quickly in its orbit around the Sun. Mercury orbits the Sun in just 88 Earth days. Mercury has a very short year (one complete revolution around the sun), but it also has very long days.
Mercury rotates slowly on its axis, turning exactly three times for every two times it orbits the Sun. This combination of rotation and orbital motion results in a solar day (noon to noon) on Mercury that is two Mercury years long.
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