Inner Planets: Mercury The first planet that you will visit is the one that is closest to the Sun. Mercury, shown in the picture, is the second-smallest planet. Its surface has many craters. Craters form when meteorites, which are chunks of rock or metal that fall from the sky, strike a planet’s surface. You will read about meteorites later in this section. Because of Mercury’s small size and low gravity, most gases that could form an atmosphere escape into space. The nearly absent atmosphere and the closeness of this planet to the Sun cause great extremes in temperature on Mercury. Its surface temperature can reach 430°C during the day and drop to -180°C at night, making the planet unfit for life.
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