While the Phoenix rover is getting all of the press these days, the new study actually used records from older probes like Viking, Pathfinder and Opportunity. The study's authors looked at data collected between 1976 and 2006 at different locations on Mars. In the various samples the researchers looked at, the soil had been depleted of elements that should have been present at the formation of the particular rock types. The absence of those elements indicates that water washed out the soil while moving downward. In contrast, had the water rose through the surface as previously thought, the soil would have been enriched with minerals from lower levels of rock.