The oxygen isotope composition of Earth's heterosphere from ion probe analysis of LDEF targets

The Long Duration Exposure Facility (LDEF), which was placed in low Earth orbit at an altitude of 475 km in 1984 and was retrieved in 1990 at 330 km, returned samples of Earth's heterosphere. Isotopic analyses of several aluminum LDEF targets are presented here to quantitatively constrain isotope fractionation in the heterosphere, but also to provide an analog environment to that which MAVEN is presently sampling around Mars. The gravity-gradient stabilized orbit of LDEF maintained the same face of the spacecraft into the velocity vector. As a result, a high fluence of 1021 atoms cm-2 of atmospheric oxygen was implanted into the oxide layer of the Al targets, which were located along the leading edge of the spacecraft. These O atoms were implanted at the orbital velocity speed of 8 km s-1, and so should be at depths less than about 10 nm within the exiting oxide. We performed O isotope analyses on 3 Al LDEF targets using the CAMECA IMS-1290 ion microprobe at UCLA. A 1 nA ion beam was used, with rastering over a 100 x 100 micron area, and with 16O on Faraday cup detector, and 17O and 18O on electron multiplier detectors. A substantial oxide layer was seen on all 3 LDEF samples, with a depth of 1 micron. High count rates were obtained, 5x107 cps for 16O, and 2x104 for 17O, to depths far in excess of the expected implantation depth, suggesting that knock-on of O atoms by the Cs+ ion probe beam occurred during analysis. As expected in the terrestrial heterosphere, which is dominated by molecular diffusion up to the exobase, the heavy isotopes are highly depleted, with a minimum d18O -280 permil relative to a laboratory aluminum disk standard. These LDEF targets are some of the isotopically lightest O samples from Earth. An analysis of 17O will be presented at the meeting, and its potential implications for MAVEN will be discussed.