Dark streaks that hint at seasonally flowing water have been spotted near the equator of Mars1. The potentially habitable oases are enticing targets for research. But spacecraft will probably have to steer clear of them unless the craft are carefully sterilized — a costly safeguard against interplanetary contamination that may rule out the sites for exploration.
River-like valleys attest to the flow of water on ancient Mars, but today the planet is dry and has an atmosphere that is too thin to support liquid water on the surface for long. However, intriguing clues suggest that water may still run across the surface from time to time.