During the past several decades, a shift of a substantial quantity of scientific real estate has occurred, as spacecraft data have transformed the planets from astronomical objects into geologic worlds. Mars is the current focus of planetary exploration, and NASA’s objectives for this effort are based on the theme, “follow the water.” This issue will address new discoveries from spacecraft and from Martian meteorites about where water or ice was (or is) located, and new insights into the role of water in determining the mineralogy, petrology, and geochemistry of the Martian surface.
