Caloris Basin: An Enhanced Source for Potassium in Mercury's Atmosphere

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Abstract
Enhanced abundances of neutral potassium (K) in the atmosphere of Mercury have been found above the longitude range containing Caloris Basin. Results of a large data set including six elongations of the planet between June 1986 and January 1988 show typical K column abundances of ∼5.4 × 10 8 K atoms/cm 2 . During the observing period in October 1987, when Caloris Basin was in view, the typical K column was ∼2.7 × 10 9 K atoms/cm 2 . Another large value (2.1 × 10 9 K atoms/cm 2 ) was seen over the Caloris antipode in January 1988. This enhancement is consistent with an increased source of K from the well-fractured crust and regolith associated with this large impact basin. The phenomenon is localized because at most solar angles, thermal alkali atoms cannot move more than a few hundred kilometers from their source before being lost to ionization by solar ultraviolet radiation.