This paper was published in SPIE Proceedings 2002, Vol. 4495, pg. 137-144, "Instruments, Methods and Missions for Astrobiology IV". It was made available as a preprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in the paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.