OBJECTIVES AND GOALS. OUR OVERALL GOAL IS TO CONDUCT A TARGETED STUDY OF PROCESSES IN MERCURY'S EXOSPHERE THROUGH THREE TASKS. IN TASK 1 WE WILL USE ALL THE ULTRAVIOLET AND VISIBLE SPECTROMETER (UVVS) DATA IN CONJUNCTION WITH AN EXISTING MONTE CARLO CODE TO DETERMINE THE OVERALL "BACKGROUND" STRUCTURE OF THE NA AND MG EXOSPHERES (CA HAS BEEN DONE PREVIOUSLY). IN TASK 2 WE WILL INVESTIGATE THE SHORT-TERM TEMPORAL AND SMALL-SCALE SPATIAL ASPECTS OF THE NA MG AND CA EXOSPHERES. IN TASK 3 WE WILL USE THE MONTE CARLO CODE TO EXPLORE THE PARAMETER SPACE OF THE SOURCE AND LOSS PROCESSES USING THE DATA AS CONSTRAINTS. WE WILL EXAMINE THE ROLES OF THERMAL ACCOMMODATION GAS-SURFACE INTERACTIONS HIGH-ENERGY PROCESSES AND LOSS PROCESSES IN GENERATING THE OBSERVED EMISSIONS NOT JUST IN TERMS OF WHAT THEY SPECIFICALLY DO IN GENERATING MERCURY'S EXOSPHERE BUT ALSO IN TERMS OF THE CONSTRAINTS THAT THE UVVS DATA PLACE ON THESE PROCESSES IN GENERAL. THE FINAL OUTCOME OF THIS STUDY WILL NOT ONLY BE A MORE COMPLETE PICTURE OF WHAT MERCURY'S EXOSPHERE LOOKS LIKE BUT ALSO WHY IT LOOKS THAT WAY AND WHAT IT MEANS FOR OTHER EXOSPHERES. AS REVEALED BY MESSENGER MERCURY HAS PERHAPS THE MOST COMPLEX EXOSPHERE IN THE SOLAR SYSTEM. DURING THE MISSION UVVS MEASURED NA MG AND CA EMISSION DAILY FOR OVER SIXTEEN MERCURY YEARS. THESE SPECIES HAVE BEEN SHOWN TO DIFFER DRAMATICALLY FROM ONE ANOTHER BOTH SPATIALLY AND TEMPORALLY. ANALYSES TO DATE PARTICULARLY FOR NA AND MG HAVE PRIMARILY EMPLOYED SIMPLE CHAMBERLAIN FITS TO INTERPRET A LIMITED SUBSET OF THE OBSERVATIONS. HOWEVER THE NATURE OF THE MESSENGER ORBIT AND THE COMPLICATED GEOMETRY OF THE UVVS OBSERVATIONS REQUIRE THE USE OF A THREE-DIMENSIONAL MONTE CARLO CODE AND THE EXPLORATION OF ALL THE DATA TO FULLY UNDERSTAND THE OBSERVED STRUCTURE. THERE IS STILL MUCH TO BE EXPLOITED IN THE UVVS DATASET AND DOING SO WILL ADVANCE OUR KNOWLEDGE OF THE PROCESSES BEHIND THIS UNIQUE EXOSPHERE AND PROVIDE INSIGHT INTO THE EXOSPHERES OF OTHER AIRLESS SOLAR SYSTEM BODIES AND ROCKY EXOPLANETS. METHODOLOGY. TO ACHIEVE OUR OBJECTIVES WE WILL EXTRACT THE RELEVANT OBSERVATIONS FROM THE PDS AND THEN UTILIZE AN EXISTING MONTE CARLO CODE TO ANALYZE THE FULL UVVS DATASET WITH A FOCUS ON THE PROCESSES AT WORK. WE WILL INCORPORATE SURFACE COMPOSITION MAPS PUBLISHED BY THE MESSENGER X-RAY AND GAMMA RAY TEAMS TO DERIVE ACCURATE SOURCE RATES AND WE WILL USE PUBLISHED SOLAR WIND AND PLANETARY ION FLUXES ONTO THE SURFACE TO ESTIMATE SPUTTERING. WE WILL USE OUR CODE TO TRACK THE FLUX BALANCE BETWEEN THE EXOSPHERE AND SURFACE AND TO MATCH THE SPATIAL DISTRIBUTION OF EACH SPECIES FOR A GIVEN PRESUMED SOURCE AND LOSS PROCESS TO THE DISTRIBUTION DETERMINED FROM THE UVVS DATA. USE OF A MONTE CARLO CODE ALLOWS US TO CALCULATE VELOCITIES ESSENTIAL TO DETERMINATION OF DOPPLERSHIFT DEPENDENT G-VALUES (SCATTERING EFFICIENCY PER ATOM) USED TO CALIBRATE MODEL COLUMN DENSITY TO INTENSITY ALONG THE LINE OF SIGHT FOR ACCURATE COMPARISON TO THE OBSERVED EMISSIONS. WE WILL EXPLORE THE CONTRIBUTIONS OF KNOWN SOURCE PROCESSES (IMPACT VAPORIZATION PHOTON-STIMULATED DESORPTION SPUTTERING ION-STIMULATED DIFFUSION AND SEQUESTRATION) IN DETAIL. ONLY THROUGH THE COMBINATION OF A DETAILED MODEL AND THE FULL UVVS DATASET AS WE PROPOSE CAN A COMPREHENSIVE ANALYSIS OF SOURCE AND LOSS PROCESSES IN THE EXOSPHERE BE ACHIEVED. RELEVANCE. THIS WORK IS RELEVANT TO NASA DDAP BECAUSE IT SUPPORTS INVESTIGATIONS THAT INCREASE THE SCIENTIFIC RETURN FROM DISCOVERY MISSIONS SUCH AS MESSENGER. STUDIES TO DATE HAVE NOT UTILIZED THE FULL RANGE OF EXOSPHERIC OBSERVATIONS ACQUIRED BY MESSENGER LEAVING MANY QUESTIONS UNANSWERED. THE INSIGHT INTO EXOSPHERIC PROCESSES GAINED FROM THIS STUDY WILL FIND APPLICATION TO MORE GENERAL STUDIES OF SOLAR SYSTEM AND EXOPLANET EXOSPHERES MAKING THIS INVESTIGATION OF GREAT VALUE TO THE PLANETARY COMMUNITY. THE PROPOSED ANALYSIS WILL BE A SIGNIFICANT ENHANCEMENT OF THE OVERALL SCIENTIFIC RETURN FROM MESSENGER.