Neutron currents measured using the Mars Odyssey Neutron Spectrometer, seasonally varying temperatures measured using the Thermal Emis-sion Spectrometer, and visible images measured using the High Resolution Imaging Science Experiment (HiRISE) are studied to determine the water content and stratigraphy of Olympia Undae. Both the neutron and thermal infrared data are best represented by a two-layered model having a water–ice equivalent hydrogen content of 30 ± 5% in a lower semi-infinite layer, buried beneath a relatively desiccated upper layer that is 9 ± 6 g/cm 2 thick (about 6 cm depth at a density of 1.5 g/cm 3). A model that is consistent with all three data sets is that the dunes contain a top layer that is relatively mobile, which overlays a niveo-aeolian lower layer. The geomorphology shown by the HiRISE images suggests that the bottom layer may be cemented in place and therefore relatively immobile.