Mars was once a wet planet, but despite losing its oceans, a water cycle still shapes the planet's climate. Significant knowledge gaps remain concerning the low-altitude vertical distribution of water vapor and atmosphere-surface water exchange. We propose to use a multi-instrument and modeling approach, combining surface and orbital data to investigate the vertical distribution of water vapor from top of the atmosphere to the subsurface for the first time. We will isolate seasonal/latitudinal variations of near-surface water vapor, assess soil humidity and the extent of surface-atmosphere exchange. This study will enhance our understanding of Mars’ climate, geochemistry, glaciology, mineralogy, and potential habitability.