WE PROPOSE A JOINT LABORATORY AND MODELING STUDY OF PHOTOCHEMICAL PROCESSES IMPORTANT IN THE MARTIAN WATER CYCLE AND HYDROGEN ESCAPE. WE SEEK TO EXAMINE THE ROLE OF KEY REACTIONS IN THE ODD HYDROGEN (H OH HO2 H2O2 HOCO) RADICAL CHEMISTRY ON SEASONAL VARIATIONS OF MARTIAN HYDROGEN ESCAPE AND D/H RATIO IN THE HYDROGEN CORONA.WE PROPOSE EXPERIMENTS TO EXAMINE SOME KEY PHOTOCHEMICAL PROCESSES IN THE HYDROGEN CHEMISTRY OF THE MARTIAN ATMOSPHERE. 1) EXPERIMENTS WILL BE PERFORMED TO MEASURE THE REACTION RATES DEUTERIUM ISOTOPE EFFECT AND PRODUCT YIELDS OF THE REACTION OF HOCO WITH H-ATOMS OVER A RANGE OF TEMPERATURES RELEVANT TO THE MARS ATMOSPHERE. THIS IS ONE OF THE FEW REACTIONS THAT COULD PRODUCE SIGNIFICANT YIELDS OF H2 THROUGHOUT THE ATMOSPHERE. INTERPRETATION OF THE TIME-RESOLVED DATA WILL REQUIRE EXPERIMENTALLY DETERMINING RATES OF A NUMBER OF OTHER HOCO REACTIONS WITH OH O(3P) AND O3 IMPORTANT ON MARS REACTIONS THAT HAVE BEEN PREDICTED TO BE FAST BUT NOT PREVIOUSLY MEASURED. 2) THE VACUUM ULTRA-VIOLET (VUV) ABSORPTION SPECTRUM OF HDO PREVIOUSLY UNMEASURED WILL BE RECORDED. 3) ISOTOPE FRACTIONATION IN THE PHOTOLYSIS OF WATER WILL BE MEASURED AT SELECT VUV WAVELENGTHS. EXPERIMENTS WILL BE PERFORMED BOTH SYNCHROTRON RADIATION AT THE ADVANCED LIGHT SOURCE USER FACILITY (LAWRENCE BERKELEY LABORATORY) USING PHOTIONIZATION MASS SPECTROMETRY COUPLED TO REACTOR FLOW CELLS. ADDITIONAL KINETICS AND BRANCHING RATIO MEASUREMENTS WILL BE DONE IN FLOW CELL EXPERIMENTS USING CAVITY RINGDOWN LASER SPECTROSCOPY. RESULTS OF THE EXPERIMENTS WILL BE INCORPORATED INTO THE CALTECH-JPL CHEMICAL TRANSPORT MODEL TO ASSESS THE IMPACT OF NEW FINDINGS ON THE HYDROGEN AND WATER CHEMISTRY IN MARS AND THE PREDICTED H/D RATIO. RECENT OBSERVATIONS BY SPACE TELESCOPES AND MARS ORBITER REVEALED STRONG SEASONAL DEPENDENCE IN HYDROGEN ESCAPE FROM THE MARS ATMOSPHERE WHILE NASA S MAVEN FOUND A SEASONAL DEPENDENCE IN THE D/H RATIO OF THE OBSERVED HYDROGEN CORONA. THESE VARIATIONS ARE INDICATORS OF A SEASONALLY-VARYING WATER ESCAPE MECHANISM WHICH IS CRUCIAL TO THE UNDERSTANDING OF MARS ATMOSPHERE AND CLIMATE. ALTHOUGH ATMOSPHERIC MODELS HAVE SHOWN THAT THIS COULD BE FROM SEASONAL CHANGES IN WATER VAPOR CONTENT AT HIGH ALTITUDES REASONS FOR THIS CORRELATION ARE STILL UNKNOWN. ONE POSSIBLE EXPLANATION IS THE TEMPERATURE AND PRESSURE DEPENDENCE IN THE REACTION RATE AND ISOTOPE FRACTIONATION OF THE MOLECULAR HYDROGEN FORMATION CHANNEL OF THE ODD HYDROGEN CYCLE IN THE MARTIAN ATMOSPHERE BUT THESE ARE NOT KNOWN. THIS STUDY WILL DIRECTLY ADDRESS NASA S MARS EXPLORATION SCIENTIFIC GOAL OF UNDERSTANDING THE HISTORY OF MARS ATMOSPHERE AND CLIMATE BY PROVIDING LABORATORY STUDIES OF KEY REACTIONS THAT INFLUENCE THE WATER ESCAPE MECHANISM ULTIMATELY INCORPORATING THEM INTO THE CALTECH/JPL 1-D PHOTOCHEMICAL MODEL FOR MARTIAN ATMOSPHERE. RESULTS FROM THIS WORK WILL ENHANCE THE INTERPRETATION OF DATA FROM SEVERAL MARS-RELATED MISSIONS INCLUDING MAVEN MSL MARS EXPRESS AND MEASUREMENTS USING THE HUBBLE SPACE TELESCOPE (HST).