Mercury's exosphere is constantly escaping into space, implying that some process resupplies its constituents. The main source of hydrogen and helium is likely to be the Solar wind. Other atomic and molecular species are thought to originate from the Hermian crust through three main processes: vaporization of surface material by meteoritic impacts; sputtering by energetic charged particles from the Solar wind; and photo- and electron-induced desorption of alkali metal atoms. Mercury's orbit has a significant eccentricity, which causes a strong variation in the Solar light and particle fluxes reaching the planet. Solar flares contribute additional variation. As a result the Hermian exosphere is extremely changeable: its atomic concentrations can fluctuate by orders of magnitude within a few hours
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