On the Cover: Our experience in exploring the Moon revealed no danger to earthly life from lunar life-forms—there were none—but the case on Mars may not be so easy. We know that some terrestrial life-forms—such as the stromatolite-building algae (insets, center and right)—are not always easily identified as living things. Martian life-forms may be more difficult to recognize. Dry, ancient riverbeds (such as Kasei Valles, background) tell us that Mars once supported liquid water and, therefore, the possibility of life. Can we be sure there is no life there now, that we will not contaminate Mars ourselves, or that samples sent to Earth from Mars will not endanger our biosphere? Background: USGS. Insets (clockwise from right): Fred Bavendam, Peter Arnold, Inc.; Schafer and Hill, Peter Arnold, Inc.; Naval Research Laboratory/USGS
3 Is It Dangerous to Return Samples From Mars to Earth? Carl Sagan poses a question for which no one has the definitive answer.
4 Protecting Our Planet, Preserving Other Worlds: Donald DeVincenzihas put together this special issue on planetary protection as an introduction to our members: