The mid-day surface temperature on Mercury rises to about 700K (803°F; 428°C), while the mid-nighttime temperature falls to 100K (-279.4°F; -173°C). This temperature variation, the largest experienced by any planet in the solar system, is due to the fact that Mercury has essentially no insulating atmosphere.
The main reason that Mercury does not have a distinctive atmosphere is that it is small and because it is close to the Sun. Mercury's small radius indicates that it has a low escape velocity, just 2.5 mi (4.2 km)/sec. Mariner 10 did detect a very thin atmosphere of hydrogen and helium on Mercury. It is believed, however, that Mercury's wispy atmosphere is composed of atoms that have been temporarily captured from the solar wind. Ground-based observations have found that a sodium and potassium atmosphere exists on the daylight side of Mercury. These atoms are probably released through the interaction of ultraviolet radiation with surface rocks.
See also Doppler effect.
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Martin Beech David T. King, Jr.