A NASA Discovery mission to conduct the first orbital study
of the innermost planet
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Mercury: The Key to Terrestrial Planet Evolution

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Question 6: What volatiles are important at Mercury?

Mercury Sodium D2 Maps

During MESSENGER's first flyby of Mercury, the distribution of neutral sodium in the "tail" of Mercury's exosphere was measured.

Mercury is surrounded by an extremely thin envelope of gas. It is so thin that, unlike the atmospheres of Venus, Earth, and Mars, the molecules surrounding Mercury don't collide with each other and instead bounce from place to place on the surface like many rubber balls. This tenuous atmosphere is called an "exosphere."

Seven elements are known to exist in Mercury's exosphere: hydrogen, helium, oxygen, sodium, potassium, calcium, and, as discovered by MESSENGER, magnesium. The observed exosphere is not stable on timescales comparable to the age of Mercury, and so there must be sources for each of these elements. High abundances of hydrogen and helium are present in the solar wind, the stream of hot, ionized gas emitted by the Sun. The other elements are likely from material impacting Mercury, such as micrometeoroids or comets, or directly from Mercury's surface rocks. Several different processes may have put these elements into the exosphere, and each process yields a different mix of the elements: vaporization of rocks by impacts, evaporation of elements from surface rocks in sunlight, sputtering by solar wind or magnetospheric ions, or diffusion from the planet's interior. Strong variability in the composition of Mercury's exosphere has been observed, suggesting an interaction of several of these processes.

MESSENGER will determine the composition of Mercury's exosphere using its ultraviolet spectrometer and energetic particle and plasma spectrometer. The exosphere composition measured by these instruments will be compared with the composition of surface rocks measured by the X-ray, gamma-ray, and neutron spectrometers. As MESSENGER orbits Mercury, variations in the exosphere's composition will be monitored. The combination of these measurements will elucidate the nature of Mercury's exosphere and the processes that contribute to it.

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