Five spacecraft are circling Mars and creeping across its ruddy surface, looking for traces of long-gone waters and signs that the cold, arid planet may once have been hospitable to life. The robotic martian invasion - three orbiters and two six-wheeled rovers - has already uncovered strong evidence that water once flowed on Mars and is now locked in subsurface ice. But big questions about water on Mars remain. When did it flow? How long did it last? How much was there? Where did it come from? Where did it go? Perhaps the most tantalizing question: Were there long-lived watery environments where microbial life could have gained a foothold?