SCIENCE GOALS AND OBJECTIVES: THE NORTH POLAR BASAL UNIT (BU) IS A SEDIMENTARY DEPOSIT OF WATER ICE AND LITHIC FINES BURIED WITHIN THE PLANUM BOREUM OF MARS. ITS LARGE VOLUME AND VOLATILE CONTENT MAKE IT ONE OF THE LARGEST WATER ICE RESERVOIRS ON THE PLANET LIKELY THE THIRD LARGEST AFTER THE TWO POLAR LAYERED DEPOSITS. IT LIES STRATIGRAPHICALLY BETWEEN ANCIENT (~3.45 GA) AND VERY YOUNG (&LT;4 MA) UNITS AND THUS PROVIDES THE POTENTIAL FOR A UNIQUE RECORD OF POLAR GEOLOGIC PROCESSES AND CLIMATE EVENTS SPANNING LARGE PORTIONS OF THE LATE HESPERIAN AND AMAZONIAN PERIODS. HOWEVER SEVERAL KEY QUESTIONS REMAIN UNANSWERED REGARDING THE STRATIGRAPHIC ARCHITECTURE COMPOSITION AND THE NATURE AND TIMING OF EVENTS THAT ARE RECORDED IN THE STRATA OF THE BU. THE OBJECTIVE OF THIS PROPOSAL IS TO ANSWER THESE QUESTIONS IN ORDER TO RECONSTRUCT THE EVOLUTION OF THE BU THROUGH THE LATE HESPERIAN AND AMAZONIAN PERIODS. THE BU CAN BE DIVIDED INTO TWO SUBUNITS RUP.S AND CAVI BASED ON THEIR STRATIGRAPHY AND AGE. WE WILL REVEAL THE THREE-DIMENSIONAL SPATIAL DISTRIBUTION STRATIGRAPHY AND COMPOSITION OF THESE UNITS AND DETERMINE THE TIMING OF MAJOR ACCUMULATION AND EROSIONAL EVENTS THAT SHAPED THEM. WE DELINEATE THREE SETS OF SCIENCE QUESTIONS THAT DIRECTLY ADDRESS SEVERAL NASA RESEARCH PRIORITIES DELINEATED IN MEPAG SCIENCE GOALS II-B1 III-A1-3 (AS SOLICITED IN ROSES C.9-4 3.2): (A) WHAT ARE THE EXTENT AND INTERNAL STRATIGRAPHIC STRUCTURES OF THE RUP.S AND CAVI UNITS? (B) WHAT IS THE RUP.S UNIT COMPOSED OF? IS THERE ANY SPATIAL VARIABILITY IN COMPOSITION? (C) WHAT WAS THE ACCUMULATION AND EROSION HISTORY OF THE BU? METHODOLOGY: OUR PROPOSED RESEARCH WORK WILL TAKE ADVANTAGE OF A RICH AND DIVERSE SET OF MARS MISSION DATA INCLUDING SUBSURFACE RADAR SOUNDING AT MULTIPLE FREQUENCIES FROM TWO DISTINCT RADAR SOUNDERS HIGH-RESOLUTION VISIBLE IMAGERY THERMAL IMAGERY AND TOPOGRAPHY WHICH WILL BE ANALYZED IN AN INTEGRATED APPROACH BY THREE MAIN RESEARCH TASKS TO ADDRESS EACH SCIENTIFIC QUESTION. OVER 2 000 RADAR PROFILES ACQUIRED BY THE MARS ADVANCED RADAR FOR SUBSURFACE AND IONOSPHERE SOUNDING (MARSIS) REVEAL THE INTERNAL STRUCTURE OF THE ENTIRE BASAL UNIT INCLUDING THE CONTACT BETWEEN THE RUP.S AND CAVI UNITS ALLOWING US TO DETERMINE THEIR SPATIAL EXTENT AND RELATIVE VOLUMES. THIS EXTENSIVE DATA SET IS COMPLEMENTED BY A SHALLOW RADAR (SHARAD) 3D DATA VOLUME WHICH ENABLES AN EVEN MORE DETAILED RECONSTRUCTION OF THE MORPHOLOGY OF THE TWO UNITS AND THE CONTACT BETWEEN THEM. THANKS TO THE MARSIS SIMULTANEOUS OPERATION ON MULTIPLE FREQUENCIES WE CAN EMPLOY STATE OF THE ART ANALYTICAL TECHNIQUES IN MULTIPLE INDEPENDENT APPROACHES TO DETERMINE THE FRACTION OF WATER ICE AND NATURE OF THE LITHIC FINES CONTAINED WITHIN THE RUP.S UNIT WHICH ARE STILL LARGELY UNKNOWN. FINALLY WE WILL TEST THE HYPOTHESIS THAT THE RUP.S UNIT ACCUMULATED AS A DOME OF MATERIAL GROWING TOWARDS LOWER LATITUDES AND WAS THEN PROGRESSIVELY ERODED IN THE NORTHWARD DIRECTION BY EXAMINING THE IMPACT CRATER RECORD ON THE ADJACENT PLAINS IN HIGH-RESOLUTION VISIBLE IMAGES. EVEN IN THE CASE OF A NULL RESULT THIS EXERCISE WILL CONSTRAIN THE ABSOLUTE AGES OF THE RUP.S UNIT MAIN ACCUMULATION AND EROSION EVENTS THUS PROVIDING INSIGHT TO THE TIMING OF VOLATILE AND LITHIC TRANSPORT ON AT LEAST A REGIONAL SCALE DURING THE LATE HESPERIAN AND EARLY AND MIDDLE AMAZONIAN PERIODS. RELEVANCE: THIS PROJECT DIRECTLY ADDRESSES THE MDAP OBJECTIVE OF ENHANCING THE SCIENTIFIC RETURN FROM MISSIONS TO MARS CONDUCTED BY NASA AND OTHER SPACE AGENCIES. THIS STUDY WILL ANALYZE MISSION DATA FROM MARS GLOBAL SURVEYOR MARS ODYSSEY MARS EXPRESS AND MARS RECONNAISSANCE ORBITER AVAILABLE TO THE PUBLIC IN THE PDS AT LEAST 30 DAYS PRIOR TO THE PROPOSAL DEADLINE.