1. L. Becker, Space Science Division, National Aeronautics and Space Administration Ames Research Center, Moffett Field, CA 94035, USA, and Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0212, USA.
2. R. J. Poreda, Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA.
3. J. L. Bada, Scripps Institution of Oceanography, University of California at San Diego, La Jolla, CA 92093-0212, USA.