The authors wish to thank Robert Livezey, Hisashi Nakamura, and Walter Robinson for discussions of aspects of this work. We also appreciate the carefully considered anonymous reviews of the manuscript, which contributed significantly to its improvement. Yochanan Kushnir has been particularly generous with his ideas. TPM wishes to thank William K.-M. Lau for his support of this project. This research was supported in part by the California Space Institute Grant CS-88-92.
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