Atmosphere of Mercury

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Mercury, the closest planet to the Sun, has the weakest magnetic field and the smallest mass of the four terrestrial planets. It has an extremely tenuous and variable exosphere containing hydrogen, helium, oxygen, sodium, calcium, potassium, and water vapor, with a combined pressure of about 10−14 bar (1 nPa). The exosphere is replenished by particles from the solar wind and material released from Mercury's surface. Solar radiation pushes some of these particles away from the Sun, producing a comet-like tail behind the planet. The existence of an atmosphere on Mercury remained controversial until the 1970s, although by then a consensus had emerged that the planet, like the Moon, lacked a substantial atmosphere. This view was confirmed in 1974 when the Mariner 10 spacecraft detected only a tenuous exosphere. In 2009, the MESSENGER spacecraft obtained more detailed measurements and detected magnesium in Mercury's exosphere.

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