AS15-91-12366

Figure 207

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The large (approximately 100-km) crater Posidonius is filled with mare lava to a level higher than the surrounding surface of Mare Serenitatis. The most interesting and perplexing feature of this crater is Rima Posidonius II - the highly sinuous rille that follows a devious course from the north rim of the crater at upper right (outside the photograph) through the breach in the crater rim at center. The rille is topographically controlled in part, hugging the juncture between lava and crater material. Erosion by some sort of fluid may have formed the rille; material appears to have been removed from it. An altemative explanation might be that the feature represents a drained and collapsed lava tube. The fluid involved probably was emitted from the craterlike depression at the head of the rille in the crater's north wall. If the rille is assumed to be contemporaneous with the lava filling, a lava of low viscosity would seem to be required to explain the channel's high sinuosity.    - Carroll Ann Hodges