UNDERSTANDING THE HIGH-ENERGY PARTICLE RADIATION ENVIRONMENT AT MARS IS IMPORTANT FOR BOTH SCIENTIFIC AND EXPLORATION PURPOSES. SUCH RADIATION CAUSES IONIZATION IN THE LOWER AND MIDDLE MARTIAN ATMOSPHERE ALTERING ATMOSPHERIC CHEMISTRY AND IONOSPHERIC STRUCTURE THEREBY AFFECTING COMMUNICATIONS. IT ALSO POSES A CLEAR HEALTH HAZARD FOR HUMANS IN ORBIT AND ON THE SURFACE. IN THIS PROPOSED STUDY WE FOCUS ON TWO POPULATIONS OF ENERGETIC PROTONS THAT CONTRIBUTE TO THE RADIATION ENVIRONMENT AT MARS: 1) SOLAR ENERGETIC PARTICLES (SEPS) ASSOCIATED WITH SOLAR FLARES AND CORONAL MASS EJECTIONS (CMES) AND 2) GALACTIC COSMIC RAY (GCR) PROTONS THAT ORIGINATE BEYOND THE HELIOSPHERE. MEASUREMENTS FROM MARS MISSIONS WITHIN THE LAST TWO DECADES PROVIDE INFORMATION ON THESE HIGH-ENERGY PARTICLES EITHER AS: 1) ENERGY SPECTRA FROM SOLID-STATE DETECTOR INSTRUMENTS DESIGNED SPECIFICALLY TO MEASURE HIGH-ENERGY CHARGED PARTICLES BOTH IN ORBIT AND ON THE SURFACE OR 2) BACKGROUND COUNT RATES IN ELECTROSTATIC ANALYZERS AND GAMMA RAY/NEUTRON DETECTORS IN ORBIT CAUSED BY PARTICLES PENETRATING INSTRUMENT HOUSING ARE PROXIES FOR FLUXES OF>~30 MEV PROTONS AND>~2 MEV ELECTRONS. WE THUS PROPOSE TO CROSS-CALIBRATE THE DATA SETS FROM FIVE MARS MISSIONS MARS GLOBAL SURVEYOR (MGS) MARS ODYSSEY (ODY) MARS EXPRESS (MEX) MARS ATMOSPHERE AND VOLATILE EVOLUTION (MAVEN) AND MARS SCIENCE LABORATORY (MSL). OUR AIM IS TO ADDRESS THE FOLLOWING OVERARCHING QUESTION: WHAT ARE THE CHARACTERISTICS OF THE HIGH-ENERGY CHARGED PARTICLE RADIATION ENVIRONMENT IN ORBIT AND AT THE SURFACE OF MARS AND HOW DO THEY VARY OVER AND BETWEEN SOLAR CYCLES? TO ADDRESS THIS QUESTION WE IDENTIFY TWO PRIMARY SCIENTIFIC OBJECTIVES: 1) CHARACTERIZE THE HIGH ENERGY CHARGED PARTICLE RADIATION ENVIRONMENT IN NEAR-MARS SPACE AND ITS VARIABILITY WITH SOLAR ACTIVITY OVER THE LAST TWO SOLAR CYCLES. 2) DETERMINE THE RELATIONSHIP BETWEEN THE RADIATION ENVIRONMENT IN MARS ORBIT RELATE AND ON THE MARTIAN SURFACE. THE FIRST OBJECTIVE WILL BE ACHIEVED THROUGH THE FOLLOWING TWO TASKS TO BE PERFORMED SERIALLY: TASK 1: EXTEND THE EFFECTIVE ENERGY RANGE OF THE MAVEN/SEP INSTRUMENT FROM ION ENERGIES OF 6 MEV UP TO ~1 GEV USING SIMULATIONS OF CHARGED PARTICLE INTERACTIONS WITH IT. TASK 2: STARTING FROM THESE NEWLY EXTENDED SPECTRA FROM MAVEN CROSS-CALIBRATE MEASUREMENTS FROM FOUR MARS ORBITER MISSIONS (MAVEN MEX ODY AND MGS) TO A) PRODUCE AN UNPRECEDENTED 20-YEAR CONTINUOUS RECORD OF THE ENERGETIC PARTICLE ENVIRONMENT IN MARS ORBIT AND B) CHARACTERIZE ITS VARIABILITY IN TIME SPACE AND WITH RESPECT TO SOLAR ACTIVITY. THE SECOND OBJECTIVE WILL BE ACHIEVED THROUGH OUR THIRD TASK: TASK 3: CORRELATE CHARGED PARTICLE MEASUREMENTS AT THE SURFACE BY MSL/RAD WITH SIMULTANEOUS ORBITAL MEASUREMENTS BY MAVEN/ SEP AND USE SIMULATIONS OF PARTICLE TRANSPORT THROUGH THE MARTIAN ATMOSPHERE TO PROBE THE PROCESSES LINKING THE TWO REGIONS. THE PROPOSED WORK IS RELEVANT TO THE NASA MARS DATA ANALYSIS PROGRAM CALL AS WE AIM TO ENHANCE THE SCIENTIFIC RETURN FROM MISSION TO MARS . THE ANTICIPATED ANALYSIS AND RESULTS WILL PROVIDE A BETTER UNDERSTANDING OF THE CHARGED PARTICLE RADIATION ENVIRONMENT IN ORBIT AND AT THE SURFACE OF MARS.