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hottest planet, terrestrial planets, diameter of Earth, Ganymede, surface gravity

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Mercury (planet), first planet in distance from the Sun in the solar system. The smallest of the rocky or terrestrial planets that include Venus, Earth, and Mars, Mercury has a global magnetic field, but only a trace of an atmosphere and no moons of its own. It is the second hottest planet after Venus. Mercury circles the Sun every 88 Earth days at an average distance of 58 million km (36 million mi) and takes 59 days to turns on its axis. It retains an ancient cratered surface that has changed little since the formation of the solar system, making the planet of special interest to planetary scientists. Mercury was named for the fleet-footed messenger of the gods in Roman mythology.

Mercury’s diameter is 4,879 km (3,032 mi), about 40 percent the diameter of Earth or about 40 percent wider than the Moon. Mercury’s volume and mass are about one-eighteenth that of Earth. Mercury’s mean density, 5.4 g/cm?, is nearly as great as that of Earth and is higher than that of any of the other planets. The force of gravity on the planet’s surface is about one-third of that on Earth’s surface or about twice the surface gravity on the Moon and about the same as the surface gravity on Mars, which is larger than Mercury but less dense. Two moons in the solar system—Jupiter’s Ganymede and Saturn’s Titan—are also larger than Mercury but are much less dense and hence have lower gravity (about the same as the Moon).


Spudis, Paul D., B.A., Sc.M., Ph.D.

Principal Professor Staff, Johns Hopkins University, Applied Physics Laboratory. Author of "The Once and Future Moon"; "Geology of the Multi-Ring Basins: The Moon and Other Planets"; coauthor of "The Clementine Atlas of the Moon"; "Moonwake: The Lunar Frontier".

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