How can we search for evidence that other planets were once habitable?

Evidence that water was once present on a planet is evidence that the planet may once have had living organisms. In their role as student planetary geologists working to investigate the planet Mars, students investigate whether a particular channel on Mars was caused by flowing water or flowing lava. Along the way, students engage in the practices and ways of thinking particular to planetary geologists, and learn to consider a planet as a system of interacting sub-systems.