Mercury is an extreme planet in several respects. Because of its nearness to the Sun—its average orbital distance is 58 million km (36 million miles)—it has the shortest year (a revolution period of 88
days) and receives the most intense solar radiation of all the planets. With a radius of about 2,440 km (1,516 miles), Mercury is the smallest major planet, smaller even than Jupiter's largest moon, Ganymede, or Saturn's largest moon, Titan. In addition, Mercury is unusually dense. Although its mean density is roughly that of Earth's, it has less mass and so is less compressed by its own gravity; when corrected for self-compression, Mercury's density is the highest of any planet. Nearly two-thirds of Mercury's mass is contained in its largely iron core, which extends from the planet's centre to a radius of about 1,800 km (1,100 miles), or three-quarters of the way to its surface. The planet's rocky outer shell—its surface crust and underlying mantle—is only some 600 km (400 miles) thick.
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