A tail of atoms escaping from Mercury's gravity due to irradiation by the Sun. Its composition and intensity varies by season. Comprised mostly of Sodium, but Calcium has also been detected. Its length was much diminished during Messenger's third flyby as Mercury was traveling at a different speed along its orbit than it was during the first two flybys. Earthbased observations show that the tail extends more than a thousand planetary radii.
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Magnesium Detected in Mercury's Atmosphere (May 2009)