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Mercury has almost no atmosphere. The planet's small size means that its gravity is too weak to hold down a normal atmosphere. There is a very thin atmosphere around the planet. Mercury's thin atmosphere is constantly being "blown away" into space by the pressure of sunlight and by the solar wind.
If Earth were the size of a nickel, Mercury would be about as big as a blueberry. From an average distance of 36 million miles (58 million kilometers), Mercury is 0.4 astronomical units away from the Sun. One astronomical unit (abbreviated as AU), is the distance from the Sun to Earth
The mass and volume of Mercury is only about 0.055 times that of Earth. But because Mercury's small mass is enclosed inside of a tiny body, the planet is the second densest in the solar system, weighing in at 5.427 grams per cubic centimeter, or 98 percent of the density of our planet.
Because the planet has no atmosphere to retain that heat, nighttime temperatures on the surface can drop to minus 290 degrees Fahrenheit (minus 180 degrees Celsius). Mercury may have water ice at its north and south poles inside deep craters, but only in regions of permanent shadow.
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