A Structural Solution for the Formation of Dunes in the Martian Polar Region

Introduction: Exotic sand dunes on Mars have been known since 1972 when NASA's Mariner 9 spacecraft transmitted images of their interesting shapes. This is a common feature across the surface of Mars. Despite three decades of studying these features [e.g., 1-16], many questions remain regarding their composition, sources of sediment, morphology, age , origins, and dynamics under present and past climatic conditions. One interesting observation is that they have not moved in the past thirty years [12,15], nor, perhaps, in millions of years according to the interpretation of impact craters preserved on some dunes [16]. The most extensive dune deposits on Mars completely encircle the residual water ice deposit [10]. Research shows that dune apparent thermal inertia ranges from modest to low [4], indicating that the dunes in the polar area are made up of irregularly-shaped cemented dust and sand fragments [5] or mixtures of ice and silicate dust [6-7] that are not cemented in bulk to form a cohesive mass. Acquired visible images from MOC and HIRISE indicate that induration took place in some dunes and that niveo-aeolian deposits may be contained in some dunes [9]. [10] studied the water content of the north-polar sand dunes within Olympia Undae using Mars Odyssey Neutron Spectrometer (MONS) epithermal neutron data. Their results are consistent with a fully ice-filled pore volume at depth covered by a relatively desiccated, loose sand cover. Such a structure would then have a relatively low thermal inertia as determined using Viking Orbiter data [11], yet be relatively immobile because it would be fully cemented at depth. This suggests that frozen water (ice or snow) is present in the polar sand dunes on Mars and supports the assertion that the dunes in the north-polar region are composed of niveo-aeolian deposits [12, 13]. [14] observed the Dark Dune Spots (DDS) and their clusters on the MOC narrow angle images from the year 1998 to 2002. The shape, location, development and other features of the DDS prompted them to suggest that some fluid phase must be invoked in the explanation for the formation of the dunes, which under the given circumstances cannot be anything else but liquid water. In this case we can infer that frozen water may be the cement for the induration of the dunes. This induration may be partially responsible for the lack of movement observed in many of the larger dunes on Mars. A similar result is gained from research on dunes in cold climate regions on the Earth [17]. One question needing an answer is “what controls the spatial geometry and location of the dunes?” From the observation of more than 200 MOC images, we found that fractures in the polar region provide a possible solution. In this paper, we first show evidence of conjugate fractures and en echelon fractures developed in the polar regions of Mars. Then we suggest that the distribution of some of the dunes is controlled by these two types of fractures in these areas.