THE PROPOSED WORK SEEKS A BETTER UNDERSTANDING OF HOW PROCESSES IN MERCURY'S SURFACE AND MAGNETOSPHERE AFFECT THE PLANET'S ATMOSPHERE. WE WILL ADDRESS THREE KEY SCIENCE QUESTIONS IN OUR FIELD'S CURRENT UNDERSTANDING: (1) WHAT ARE THE RELATIVE CONTRIBUTIONS OF SOURCES DRIVING MERCURY'S ATMOSPHERE AND ARE THEY DISTINCTLY SEPARATE OR MUTUALLY INTERACTING? (2) HOW DO ALKALI LOSS RATES VARY WITH SEASON? (3) CAN THE ATMOSPHERE REDISTRIBUTE THE SURFACE COMPOSITION? METHODS: OUR APPROACH INVOLVES ANALYSIS OF GROUND-BASED DATA AND THE INTERCONNECTION OF MAGNETOSPHERE SURFACE AND ATMOSPHERE MODELS USING THEIR DERIVED CONSTRAINTS. OUR STUDY FOCUSES ON THE MAIN ATMOSPHERIC CONSTITUENT THAT ALSO HAS THE BEST EMPIRICAL CONSTRAINTS SODIUM AND SUBSEQUENTLY EXTENDS ITS SCOPE TO INCLUDE POTASSIUM A SPECIES WITH SIMILAR BEHAVIOR BUT FAR FEWER OBSERVATIONAL CONSTRAINTS. EXISTING GROUND-BASED DATA ANALYZED WILL DESCRIBE POLAR ENHANCEMENTS DISCERN THEIR SHORT-TERM VARIABILITY FROM THE MAGNETOSPHERE QUANTIFY IONIZATION RATES AND CHARACTERIZE THE ATMOSPHERIC LOSS. THESE INCLUDE OBSERVATIONS DURING SOLAR TRANSIT AS WELL AS LONG-TERM STUDIES USING AN ECHELLE IMAGE SLICER AND CORONAGRAPHIC IMAGER. A 3D TIME-DEPENDENT MODEL IS NEEDED TO STITCH THESE ELEMENTS TOGETHER. IT MUST INCLUDE A KINETIC TREATMENT OF MAGNETOSPHERIC IONS A THERMAL MODEL OF THE LITHOSPHERE AND A CAREFUL TREATMENT OF GAS-SURFACE INTERACTIONS. IN THIS WAY NUMERICAL MODELING CAN FULLY CHARACTERIZE THE BUDGETARY BALANCE THAT GOVERNS THE ATMOSPHERE'S MASS AND ENERGY WHICH INCLUDES THE TRANSPORT OF MATERIAL THROUGH THE PLANET'S SURFACE AND MAGNETOSPHERE. WE PROPOSE TO ASSEMBLE A COMPREHENSIVE MODEL OF THE MERCURY ATMOSPHERE THAT INCLUDES ALL THESE REQUISITE CONSTRAINTS. SIGNIFICANCE: WE WILL ANALYZE THE HIGHEST SPATIAL AND TEMPORAL RESOLUTION IMAGES OF THE SODIUM DISTRIBUTED ABOVE MERCURY'S DISK ASSOCIATED WITH THE PRECIPITATION THROUGH CUSP REGIONS AND SYNTHESIZE DATA OF THE ESCAPING EXOSPHERE SIMULTANEOUS WITH LIMB SCANS OF THE BOUND ATMOSPHERE. THESE OBSERVATIONS COMBINED WITH COMPREHENSIVE SYSTEM-WIDE MODELING CONFIRM OR REFUTE THE ATMOSPHERE'S ABILITY TO RE-DISTRIBUTE SURFACE COMPOSITION WHEN TIME-EXTRAPOLATED. THEY WILL ALSO ENABLE A MEASUREMENT OF THE FRACTIONAL AND ABSOLUTE ESCAPE RATES OF THE SODIUM ATMOSPHERE WHICH IS THE DOMINANT SPECIES AND GIVE A FIRM EMPIRICAL CONSTRAINT ON THIS ATOM'S LIFETIME THROUGHOUT THE SOLAR SYSTEM. BOTH THESE RESULTS WILL HELP TO FINALLY DISENTANGLE THE DOMINANT EXOSPHERIC SOURCE PROCESSES AT MERCURY. RELEVANCE: PROPOSED WORK IS RELEVANT TO SSW IN THAT IT COMBINES DATA SYNTHESIS AND THEORETICAL MODELING TO STUDY THE PHYSICAL PROCESSES GOVERNING THE GENERATION OF MERCURY'S ALKALI EXOSPHERE. AS THE SSW PROGRAM ELEMENT SUPPORTS BOTH "ANALYSIS AND INTERPRETATION OF EXISTING OBSERVATIONS OF SOLAR SYSTEM OBJECTS AND NUMERICAL MODELING OF PHYSICAL OR CHEMICAL PROCESSES " THE PROPOSED STUDY MEETS PRIMARY RELEVANCE CRITERIA. IN ADDITION THE RESEARCH ADDRESSES NASA STRATEGIC PLAN OBJECTIVES 1.4 TO ("UNDERSTAND THE SUN AND ITS INTERACTIONS WITH EARTH AND THE SOLAR SYSTEM INCLUDING SPACE WEATHER") AND 1.5 TO ("ASCERTAIN THE CONTENT ORIGIN AND EVOLUTION OF THE SOLAR SYSTEM"). DUE TO THE BROAD NATURE OF THE SSW PROGRAM WE SHOULD ALSO JUSTIFY THAT THIS WORK IS BEST SUITED FOR SSW AND NOT TO OTHER RELATED NASA PROGRAM ELEMENTS. WHILE THE PROPOSED WORK MAKES USE OF ARCHIVED MESSENGER DATA IT DOES SO ONLY AS A CONSTRAINT ON MODEL RESULTS. THIS EFFORT DOES NOT INVOLVE ANY NEW ANALYSIS OF MESSENGER DATASETS AND THE SMALL USE MESSENGER DATA PRODUCTS USED WILL DERIVE FROM PUBLISHED LITERATURE. IT WOULD NOT "MAKE SIGNIFICANT USE OF OR GREATLY ENHANCE THE USE OF DATA RETURNED BY" THIS MISSION AND SO IT IS NOT RELEVANT TO DDAP. SIMILARLY WHILE WE WILL MAKE USE OF GROUND-BASED OBSERVATIONS IN-HAND WE DO NOT PROPOSE TO OBTAIN ANY NEW OBSERVATIONS AND SO THIS WORK IS NOT RELEVANT TO SSO/PAST.