Radiometric dating, geomorphology, biostratigraphy, and diffusion physics provide conflicting evidence for the age of the ground ice found in Beacon Valley, Antarctica. The rate of vapor diffusion into a dry atmosphere is incompatible with a minimum radiometric age of 8 Ma. Recent measurements of meteorological conditions in Beacon Valley show that the humidity of the atmosphere, although small, is comparable to the saturation vapor pressure at the ice. Analogous conditions explain the survival of ground ice at the high latitudes of Mars. At the study site, atmospheric vapor slows the sublimation loss by a factor of three and the retreat will cease entirely if temperatures are lower by 5°C. Detailed model simulations show that advection, including the advection from changes in surface pressure caused by winds, has a negligible effect on sublimation loss.