Mercury is the closest planet in the solar system to the sun, and since Pluto is no longer considered a planet, it's also the smallest; it's just a little larger than Earth's moon. Because Mercury is so small and so close to the sun, it has a hard time attracting an atmosphere, but it does have one. Its atmosphere is so thin, though, astronomers usually refer to it as an exosphere.
Weak Magnetic Field
Mercury's diameter is a third as large as that of the Earth, but in terms of volume, it is 18 times smaller. The planet is dense, though, and gravitational attraction on its surface is about one-third of what it is on Earth. Although scientists used to believe it had no magnetic field, they now know it has one, but it is only about 1 percent as strong as Earth's. Because Mercury has an iron core that accounts for 42 percent of its volume, the weakness of its magnetic field is puzzling, but the probable cause is the same one that prevents the planet from having a thick atmosphere -- the solar wind.
Compared to Earth's atmosphere, that of Mercury is thin enough to be almost negligible; scientists weren't sure it was there until they received data from the Mariner 10 spacecraft, which conducted multiple flybys of the planet in 1974 and 1975. After analyzing data from the Messenger spacecraft, which launched in 2004 and sends detailed atmospheric readings from Mercury orbit, scientists now know more about the thin atmosphere's composition. It is 95.3 percent carbon dioxide, 2.7 percent nitrogen, 1.6 percent argon and about 0.2 percent oxygen, with trace amounts of carbon monoxide, water vapor and nitric oxide, as well as sulfur, iron, calcium, magnesium, sodium and organic compounds.
Effect of the Solar Wind
Mercury's small size and proximity to the sun are the main reasons it doesn't have much of an atmosphere. Most of it is blown away by the solar wind, which also has a limiting effect on the magnetic field. Because of the size of its iron core, its magnetic field should be 30 times stronger than it is, but the solar wind charges particles in the atmosphere, creating another magnetic field -- or magnetosphere. The magnetic coupling of the magnetosphere and the field generated by the planet's core account for the discrepancy, scientists believe.
Data from Messenger also revealed that Mercury's atmosphere isn't static. It changes from pole to pole, and its composition varies according to the planet's distance from the sun. Mercury has an eccentric orbit. The difference between perihelion and aphelion is about 23 million kilometers (14.3 million miles), and concentrations of calcium, magnesium and sodium vary significantly with distance from the sun. Moreover, the magnetic field does not converge the same at both poles. There is a larger hole at the south pole that allows in more charged particles, which, besides affecting the atmosphere, may cause differences in weathering and erosion to the planet's surface at the poles.
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