INTRODUCTION: THE LONG DURATION EXPOSURE FACILITY (LDEF) WAS LAUNCHED INTO LOW EARTH ORBIT IN 1984 AND RETRIEVED IN 1990 BY COLUMBIA. THE NOMINAL ORBIT WAS ~480 KM BUT OVER THE COURSE OF THE EXTENDED MISSION THE ORBIT DECAYED TO 330 KM. A SUITE OF TARGETS WERE PLACED AT VARIOUS LOCATIONS ON LDEF TO ASSESS AND COLLECT IMPACTING IDPS. IN ADDITION TO COLLECTING IDPS TARGETS ON THE LEADING EDGE OF THE SPACECRAFT ALSO IMPACTED EARTHS UPPER ATMOSPHERE AT AN ORBITAL VELOCITY ~ 8 KM/S. HERE WE PROPOSE TO USE DEPTHPROFILING BY ION MICROPROBE TO MEASURE THE OXYGEN ISOTOPE COMPOSITION OF EARTHS UPPER ATMOSPHERE. THE OXYGEN COMPOSITION OF THE UPPER ATMOSPHERE IS PRIMARILY O ATOMS AND O+ IONS. THESE HIGHLY REACTIVE SPECIES BECOME IMPLANTED IN THE TARGET SURFACE AND POSSIBLY REPLACE O IN ANY TARGET OXIDE LAYER PRESENT AT LAUNCH. PRELIMINARY NANOSIMS DATA OBTAINED AT WASHINGTON UNIVERSITY FOR AN LDEF AL TARGET (FROM A REGION OF THE TARGET WITH NO IDP IMPACTS) REVEAL DELTA18O ~ -200 TO -300 RELATIVE TO SMOW. SUCH A LARGEDEPLETION IN 18O IS CONSISTENT WITH DIFFUSIVE SEPARATION OF O ISOTOPES ABOVE THE TERRESTRIAL HOMOPAUSE (~100 KM). THE NOISE LEVEL OF THESE MEASUREMENTS WAS HIGH DUE TO THE LOW COUNT RATES. WE EXPECT THAT THE HIGHER COUNT RATES FROM THE ION MICROPROBE WILL YIELD BETTER SIGNAL TO NOISE. IN ADDITION TO DETERMINING THE OXYGEN ISOTOPE COMPOSITION OF EARTHS UPPER ATMOSPHERE THIS WORK MAY HAVE IMPLICATIONS FOR ESCAPE OF OXYGEN FROM MARS. THE NASA MAVEN MISSION IS CURRENTLY IN ORBIT AT MARS AND WILL IN THE NEAR FUTUREMAKE MEASUREMENTS OF OXYGEN (AND OTHER) ISOTOPES IN THE UPPER ATMOSPHERE. THE ANALYSES PROPOSED HERE OFFER THE POSSIBILITY OF A DIRECT COMPARISON OF UPPER ATMOSPHERIC OXYGEN ISOTOPES FOR THE TWO PLANETS.PROPOSED RESEARCH:1. ION MICROPROBE MEASUREMENTS OF O ISOTOPES FROM LDEF AL TARGETS. PRELIMINARY NANOSIMS DATA ON ONE AL TARGET ALREADYDEMONSTRATES THAT A LARGE ISOTOPE FRACTIONATION IS MEASURABLE IN LDEF TARGETS. OUR FIRST OBJECTIVE WILL BE TO ANALYZE ~ 10 AL TARGETS ALL FROM THE LEADING EDGE OF THE SPACECRAFT. WE PRESENTLY HAVE 3 TARGETS AND WILL REQUEST 7 MORE FORM NASA JSC.2. ION MICROPROBE MEASUREMENTS ON OTHER LDEF TARGETS. WE ALSO HAVE ~ 10 GE TARGETS AND ONE AU TARGET. WE WILL ANALYZE THESE FOR O ISOTOPE COMPOSITION. MORE AU TARGETS WILL BE REQUESTED FORM JSC IF WE SEE AN INTERESTING ISOTOPE SIGNATURE. THE LACK OF AN OXIDE LAYER IN THE AU TARGETS MAY MAKE ANALYSIS OF THEM DIFFICULT OR IMPOSSIBLE SINCE THE PENETRATION DEPTH OF 8 KM/S O ATOMS IS EXPECTED TO BEVERY SHALLOW.3. MODELING OF RESULTS AND IMPLICATIONS FOR MARS. ALTHOUGH THE FOCUS OF THIS PROPOSAL IS ON THE MEASUREMENTS FROM THE LDEF TARGETS WE WILL INTERPRET THE RESULTS USING AN ATMOSPHERIC MODEL THAT INCLUDES DIFFUSIVE SEPARATION AND WE WILL APPLY THE RESULTS TO A SIMILAR MODEL FOR MARS FOR COMPARISON WITH EVENTUAL MAVEN DATA.