Acknowledgments

This work was supported by a grant from the NASA Exobiology Program. C.C. was supported by a National Research Council Associateship. Computer resources for this work were provided in part by the Numerical Aerodynamics Simulator (NAS) Program and by National Cancer Institute Supercomputer Center. We are grateful to the UCSF Computer Graphics laboratory (NIH RR-1081, Thomas Ferrin, P.I.) for graphics support. and the Computer Graphics Facility at the College of Chemistry, UC Berkeley (NIH S10 RR-5651A) for graphics support.