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This figure was generated from Earth-based measurements and shows the uneven distribution of sodium in Mercury's exosphere at the time of the observations. The outline of the surface of Mercury is shown as a white circle in the images. Figure courtesy of A. E. Potter (Potter et al., Icarus, 2007). |
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 is called an "exosphere."
Six elements are known to exist in Mercury's exosphere: (1) hydrogen, (2) helium, (3) oxygen, (4) sodium, (5) potassium, and (6) calcium. 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. Hydrogen and helium are present in high abundances 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 the rocks due to sunlight, sputtering by solar wind ions, or diffusion from the planet's interior. Variability of the composition of Mercury's exosphere has been observed, suggesting the interaction of several of these processes.
MESSENGER will determine the composition of Mercury's exosphere using its ultraviolet spectrometer and energetic particle 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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