Thanks are due to D. A. Bazylinski, J. L. Kirschvink, and J. W. Schopf for critical reading of the manuscript; to H. Jaksch and J. Greiser (LEO GmbH, Oberkochen, Germany) for help and cooperation in taking micrographs in Figs. 1 A, B, D-L, and 4A; to S. Douglass (Jet Propulsion Laboratory, Pasadena, CA) for Fig. 1C; to J. F. Almagro (Acerinox, Algeciras, Spain) for Figs. 3 B and C and 4C; to R. Bargallo, L. E. Bertani, L. Calvo-Barrio, A. Martinez, F. Pinto, and K. Riddle for support and technical help; and to A. B. Thistle for critical editing. This work was supported by National Aeronautics and Space Administration Grants NAG5/4921 and -9845 to E.I.F., Direcció General de Recerca de Generalitat de Catalunya Grant ACES98-15/1 to J.W., and Ministerio de Educación y Cultura Grant APC1998-0045 to C.A. and J.W.
SEM-BSE, scanning electron microscopy in backscattered electron mode; SEM-SE, electron microscopy in secondary electron mode; LEDS, low-electron-density substance.
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