Indications of brine related local seepage phenomena on the northern hemisphere of Mars
Abstract Springtime low albedo features, called Dark Dune Spots, on the seasonal frost covered dunes on Mars between 77°N and 84°N latitude have been analyzed. Two groups of these spots have been identified: “small” and “large” ones, where large spots have diameters above 4 m, and complex internal structure. From these “large” spots branching seepage-like features emanate and grow on the steep slopes. They show a characteristic sequence of changes: first only wind-blown features emanate from them, while later a bright circular and elevated ring forms, and dark seepage-features start from the spots. These streaks grow with a speed between 0.3 m/day and 7 m/day respectively, first only from the spots, later from all along the dune crest. During this “seepage period” the temperature is between 150 K and 180 K at a 3–9 km spatial resolution scale, indicating that CO2 ice-free parts must be present at the observed dark spots. Around the receding northern seasonal CO2 cap, an annulus of water ice lags behind, which is probably present in the spots too where the CO2 frost has sublimated. Our model estimates show in the present work and in Kereszturi et al. (Kereszturi, A., Mohlmann, D., Berczi, Sz., Ganti, T., Kuti, A., Sik, A., Horvath, A. [2009b]. Icarus 201, 492–503) that the warming driven by solar insolation may produce not only interfacial water, but also bulk brines around the dune grains. The brine can support the movement of liquids and dune grains, enhances the sublimation of CO2 frost, and produce the dark features, as well as liquid modifies the optical properties of the surface. Signs of movement of dune material after the total defrosting of the terrain is also visible but it is uncertain because of the limit of resolution. In our previous work (Kereszturi et al., 2009b) we showed that resembling seepage-like streaks at the southern hemisphere might have been formed by ephemeral interfacial water, as well as these northern features. Such wet environments may have astrobiological importance too.