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Sinuous Chain of Depressions
A single depression from a larger sinuous chain of pits located at 34.6°N, 43.5°W in NAC frame M102443238R. Image is 1.5 km wide and light is incident from the left at an angle of 78° [NASA/GSFC/Arizona State University].
This unnamed sinuous chain of pits was suggested to be a collapsed lava tube (see Wilhelms' Geologic History of the Moon). New high resolution NAC images (e.g. M102443238R) provide a new look at the area. This particular feature transitions from a discontinuous sinuous rille into a wrinkle ridge. Some scientists have suggested that wrinkle ridge faults interact with lava tubes, with the wrinkle ridge exploiting a zone of mechanical weakness (the lava tube) in the preexisting basalt deposit. In this NAC image, the topographic depressions are non-circular, with collapse rims. Many of the pits have boulders on the interior walls. If these depressions were created by impacts, each pit would have a raised rim and an ejecta blanket.
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This section of WAC frame M117773324 shows the area where the feature transitions from a chain of collapse pits to a continuous uncollapsed segment. A large depression at the northern tip of the chain maybe a possible source region for the flow of lava across this region. The chain is approximately 50 km long. Image resolution 58.9 m/pixel [NASA/GSFC/Arizona State University].
Geologists suspect that lunar lava tubes form similarly to terrestrial lava tubes. However, lunar lava tubes are likely much larger, due to the lower gravity and the lack of an atmosphere. Studies of lava tubes on Earth show that most are hollow. If lunar lava tubes form in a similar way, then they too are most likely hollow. A lava tube may form when an active basaltic lava flow develops a continuous crust. For instance, an open lava channel may form a crust of hardened rock that extends from the sides and, over time, meets in the middle, forming a roof. Even if a lava tube develops a roof, it still has lava running through it. There is a possibility that the cooling lava would solidify inside the tube and block it. However, on Earth most lava tubes do not "plug up." As the rate of lava flowing from the source diminishes over time, the level of liquid in the tube drops, leaving an empty space between the top of the flow and the roof of the tube.
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