Observation of Water Ice Halos and Bright Cores Within Dark Dune Spots on Mars: Implication for a Solid Greenhouse Effect

Dark dune spots are seasonal low albedo features observed in the Martian polar regions. A formation mechanism of gas jets has been proposed for the formation of dark dune spots. However, the composition and state of the volatile species involved remain debated. Here we report the observations of dark central deposits, bright halos and cores within dark dune spots using the High-Resolution Imaging Science Experiment images. Time series observations show that after the disappearance of bright halos, the bright cores are later exposed on the surface as a result of the removal of the dark deposits above. The Compact Reconnaissance Imaging Spectrometer data analysis suggests that the bright halos consist of a combination of H₂O and CO₂ ice. During the season when bright cores are observed, water-ice halos have disappeared and the diurnal maximum temperature is up to 278K, close to the sublimation point of H₂O ice. Both the spectral data and surface temperature suggest a significant role of H₂O ice during the formation of dark dune spots. We propose that the subsurface heating of a chamber filled with H₂O ice under the seasonal CO₂ ice slab resulted in the phase change of H₂O ice during the evolution of dark dune spots, which has been proposed as the solid-state greenhouse effect (Matson & Brown, 1989, https://doi.org/10.1016/0019-1035(89)90007-9). The results suggest a relatively mild environment with raised temperature and pressure compared to the Martian surface under the seasonal ice caps in the south polar regions of Mars.