John P. Bradley^a,b, Hope A. Ishii^a,b, Jeffrey J. Gillis-Davis^b, James Ciston^c, Michael H. Nielsen^d,e, Hans A. Bechtel^f, and Michael C. Martin^f

^aInstitute of Geophysics and Planetary Physics, Lawrence Livermore National Laboratory, Livermore, CA 94550;
^bHawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822;
^cNational Center for Electron Microscopy,
^dMaterials Science Division, and
^eAdvanced Light Source Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720; and
^fDepartment of Materials Science and Engineering, University of California, Berkeley, CA 94720

The solar wind (SW), composed of predominantly ~1-keV H⁺ ions, produces amorphous rims up to ∼150 nm thick on the surfaces of minerals exposed in space. Silicates with amorphous rims are observed on interplanetary dust particles and on lunar and asteroid soil regolith grains. Implanted H⁺ may react with oxygen in the minerals to form trace amounts of hydroxyl (−OH) and/or water (H₂O). Previous studies have detected hydroxyl in lunar soils, but its chemical state, physical location in the soils, and source(s) are debated. If −OH or H₂O is generated in rims on silicate grains, there are important implications for the origins of water in the solar system and other astrophysical environments. By exploiting the high spatial resolution of transmission electron microscopy and valence electron energy-loss spectroscopy, we detect water sealed in vesicles within amorphous rims produced by SW irradiation of silicate mineral grains on the exterior surfaces of interplanetary dust particles. Our findings establish that water is a byproduct of SW space weathering. We conclude, on the basis of the pervasiveness of the SW and silicate materials, that the production of radiolytic SW water on airless bodies is a ubiquitous process throughout the solar system.

Reference
Bradley JP, Ishii HA, Gillis-Davis JJ, Ciston J, Nielsen MH, Bechtel HA and Martin MC (in press) Detection of solar wind-produced water in irradiated rims on silicate minerals. PNAS 111:1732–1735.
[doi:10.1073/pnas.1320115111]

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