EVIDENCE FOR FLUVIAL FEATURES AND STANDING LIQUID WATER INDICATE THAT MARS WAS A WARMER AND WETTER PLACE IN ITS PAST HOWEVER CLIMATE MODELS HAVE HISTORICALLY BEEN UNABLE TO PRODUCE CONDITIONS TO YIELD A WARM EARLY MARS UNDER THE FAINT YOUNG SUN. SOME MODELS INVOKE THICK GREENHOUSE ATMOSPHERES TO PRODUCE CONTINUOUSLY WARM CONDITIONS WHILE OTHERS ARGUE THAT IMPACTS DURING THE LATE HEAVY BOMBARDMENT CAUSED TRANSIENT WARMING ON AN OTHERWISE COLD MARS. A THIRD OPTION SUGGESTS THAT EARLY MARS EXPERIENCED CLIMATE LIMIT CYCLES BETWEEN SHORT PERIODS OF WARMTH AND PROLONGED PERIODS OF GLACIATION DUE TO MODULATION OF GREENHOUSE WARMING BY THE CARBONATE-SILICATE CYCLE. ASPECTS OF ALL THESE MODELS MAY HAVE OPERATED DURING MARS' HISTORY: THE NOACHIAN EXPERIENCED HEAVY IMPACTS AND LARGE-SCALE VOLCANIC ACTIVITY WITH THE EARLIEST SIGNS OF FLUVIAL FEATURES WHILE THE HESPERIAN WAS LESS GEOLOGICALLY ACTIVE AND INDICATES EVIDENCE OF STANDING LIQUID WATER. THIS RESEARCH WILL INVESTIGATE THE CONFLUENCE OF AVAILABLE HYPOTHESES FOR WARMING EARLY MARS WITH AN ENSEMBLE MODELING APPROACH TO THE EVOLUTION OF CLIMATE ACROSS THE PLANET S GEOLOGICAL HISTORY. METHODS THIS RESEARCH WILL CALCULATE OBSERVATIONALLY-CONSISTENT TRAJECTORIES FOR THE CLIMATE HISTORY OF MARS USING AN OPTIMIZED ENSEMBLE OF ENERGY BALANCE MODEL (EBM) CLIMATE CALCULATIONS. THIS EBM HAS BEEN USED IN PREVIOUS STUDIES OF PLANETARY HABITABILITY TO IDENTIFY THE CONTRIBUTION OF FACTORS SUCH AS INCIDENT STELLAR FLUX VOLCANIC OUTGASSING RATE PLANETARY OBLIQUITY GREENHOUSE GAS INVENTORY AND ATMOSPHERIC MASS LOSS. THIS STUDY WILL EXAMINE THE CONFLUENCE OF ALL THESE FACTORS TO THE HISTORY OF LIQUID WATER ON MARS BY USING AN OPTIMIZATION ALGORITHM THAT ITERATIVELY IMPROVES AN ENSEMBLE OF EBM SIMULATIONS WITH CONSTRAINTS BASED UPON KEY CHARACTERISTICS DURING THE NOACHIAN HESPERIAN AND AMAZONIAN. CANDIDATE TRAJECTORIES WILL BE EVALUATED AGAINST GENERAL CIRCULATION MODEL (GCM) CALCULATIONS TO ASSESS THE ROBUSTNESS OF THE EBM APPROACH. THE RESULTS OF THIS CLIMATE MODEL OPTIMIZATION WILL PROVIDE SEVERAL TRAJECTORIES FOR HOW THESE QUANTITIES HAVE CHANGED WITH TIME OVER THE HISTORY OF MARS FROM THE NOACHIAN TO THE PRESENT. RELEVANCE THIS PROPOSED RESEARCH IS RELEVANT TO NASA'S HABITABLE WORLDS PROGRAM AS A THEORETICAL STUDY OF PLAUSIBLE CLIMATE TRAJECTORIES TO EXPLAIN THE PAST PRESENCE OF SURFACE LIQUID WATER ON MARS. THE RESULTS OF THIS RESEARCH WILL CONSTRAIN "THE ASTROBIOLOGICAL POTENTIAL OF PAST OR PRESENT ENVIRONMENTS ON OR IN THE MARTIAN SURFACE OR SUBSURFACE" (E.4-1) BY USING AN OPTIMIZED ENSEMBLE OF CLIMATE MODEL SIMULATIONS CONSTRAINED BY OBSERVATIONS. THE SCOPE OF THIS RESEARCH FOCUSES ON THE HABITABILITY OF EARLY MARS SO THIS PROPOSAL IS NOT RELEVANT TO THE EMERGING WORLDS PROGRAM THE EXOBIOLOGY PROGRAM OR ANY EXISTING DATA ANALYSIS PROGRAMS.