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What is Mercury?

Due to Mercury's rotation and highly elliptical orbit, the Sun appears to rise briefly, set, and rise again before it travels westward across the sky. At sunset, the Sun appears to set, rise again briefly, and then set again. Mercury is only about one-third the size of the Earth, yet it has only 5.5 percent of Earth's mass. It is smaller than any other planet except Pluto. Mercury is very close to the Sun (36 million miles away) and has no substantial atmosphere. These factors contribute to the fact that the surface of Mercury has the greatest temperature range of any planet or natural satellite in our solar system. The surface temperature on the side of Mercury closest to the Sun reaches 800F (427C) degrees Fahrenheit, a temperature hot enough to melt tin. On the side facing away from the Sun, or the night side, the temperature drops to -280F (-180C) degrees Fahrenheit. Scientists have detected a magnetic field surrounding Mercury, though it is not as strong as the field around the Earth. Scientists theorize that Mercury's field is due to an iron-bearing core or possibly to the solar winds. Mercury's atmosphere is very thin and is composed of helium and sodium. The surface of Mercury has been shaped by three processes: impact cratering where large objects struck the surface resulting in crater formation, volcanism where lava flooded the surface, and tectonic activity where the planet's crust moved in order to adjust to the planetary cooling and contracting. Mercury does not have any naturally occurring satellites.

How have we been able to gather information about Mercury?

Mariner 10, the first and only probe to visit Mercury, was launched in 1973. That vessel, the final in Mariner series, first approached Venus , then it used that planet's gravitational field to send it like a slingshot around the sun in the direction of Mercury. It came within 470 miles of Mercury and photographed about 40% of the planet's surface. Mariner 10 collected very valuable information about Mercury. It found that the planet's surface is covered with deep craters, separated by plains and huge banks of cliffs. Mercury's most notable feature is an ancient crater called the Coloris Basin, which is about the size of Texas. Then it flew past Mercury twice more until it ran out of gas.

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