CALORIS BASIN - AN ENHANCED SOURCE FOR POTASSIUM IN MERCURYS ATMOSPHERE

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 × 108 K atoms/cm2. During the observing period in October 1987, when Caloris Basin was in view, the typical K column was ∼2.7 × 10 9 K atoms/cm2. Another large value (2.1 × 10 9 K atoms/cm2) 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.