A gravitational map of Mars reveals that water-filled rivers once ran across the Red Planet’s northern hemisphere, researchers say. “Evidence is building of more water on the surface of Mars at one time,” said planetary scientist Maria Zuber of the Massachusetts Institute of Technology, who led the study. The researchers combined laser measurements of Martian topography with a gravitational analysis provided by the Mars Global Surveyor spacecraft to depict the shape and thickness of the planet’s crust.