OBJECTIVES: IF LIFE EVER EMERGED ON MARS IT MIGHT HAVE BEEN SIMILAR TO CHEMOTROPHIC BACTERIA WHICH CREATE MINERAL DEPOSITS IN KINETICALLY UNFAVORABLE CONDITIONS FOR THE EQUIVALENT ABIOTIC REACTIONS. HEMATITE-JAROSITE DEPOSITS AT MERIDIANI PLANUM REQUIRE THAT FERROUS FE FROM WEATHERING OF MARTIAN BASALT WAS OXIDIZED IN ACIDIC CONDITIONS (PH 2-4). AT SUCH LOW PH FE OXIDATION BY O2 IS VERY SLOW AND THE FORMATION OF SIMILAR DEPOSITS ON EARTH IS USUALLY MEDIATED BY BACTERIA. TIME LIMITS FOR FE OXIDATION ON MARS ARE IMPOSED BY THE LIFETIME OF STANDING WATER BODIES AND IF ABIOTIC FE OXIDATION RATES IN ACID WATERS ARE FOUND TO BE INADEQUATE TO ACCOUNT FOR HEMATITEJAROSITE DEPOSITS THIS COULD MAKE A COMPELLING CASE THAT BIOLOGY PLAYED A ROLE IN ACCELERATING OXIDATION. HOWEVER FE OXIDATION CAN ALSO BE DRIVEN BY UV PHOTONS. EARLIER STUDIES SUGGEST THIS MAY BE A VIABLE PATHWAY FOR HEMATITE-JAROSITE FORMATION BUT THE KINETICS ARE INADEQUATELY CHARACTERIZED IN THE CHEMICAL AND RADIATIVE ENVIRONMENT OF MERIDIANI PLANUM. THE DEPENDENCE OF THE QUANTUM YIELD (ATOMS OXIDIZED PER PHOTON ABSORBED) ON FE SPECIATION AND UV WAVELENGTH AND HAS NOT BEEN DETERMINED WHICH LIMITS EXTRAPOLATION OF EXPERIMENTAL PHOTO-OXIDATION RATES TO ACIDIC WATERS UNDER MARTIAN SURFACE CONDITIONS. CHARACTERIZING PHOTO-OXIDATION RATES ON MARS IS NECESSARY IN ASSESSING WHETHER IRON OXIDATION COULD HAVE BEEN AN ENERGY SOURCE FOR LIFE AT MERIDIANI PLANUM. WE WILL CONDUCT NEW PHOTO-OXIDATION EXPERIMENTS TO DEFINE THE QUANTUM YIELD AS A FUNCTION OF WAVELENGTH AND FE SPECIATION AND DETERMINE RATES OF OXIDATION UNDER A UV SPECTRUM RELEVANT TO THE INCIDENT RADIATION AT THE MARTIAN SURFACE. OUR EXPERIMENTAL RESULTS WILL FORM THE BASIS OF THE HOLISTIC GEOCHEMICAL MODELS THAT WE WILL BUILD TO DETERMINE THE RELATIVE EFFICIENCY OF COMPETING BIOTIC AND ABIOTIC PATHWAYS TO FE OXIDATION IN ACID WATERS.