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Spanning over 130 km in length, Rimae Posidonius is a sinuous rille winding across the floor of Posidonius crater. LROC WAC mosaic at 100 m/pixel, arrow points to the rille and location of an LROC NAC close-up [NASA/GSFC/Arizona State University].
Sinuous rilles are remarkable features resulting from turbulent flow of low viscosity (very fluid), high temperature lavas that erodes the pre-existing surface. In turbulent fluid flows, eddies and vortices form that can be highly erosive and result in the twists and turns seen in many rilles. This rille, located on the western edge of Posidonius crater (~100 km diameter, floor-fractured and partially mare-filled), tightly winds against the northern crater wall and then veers away in a southerly course.
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Close-up view of boulders, derived from the mare lavas that flooded the crater, outcropping from the eastern rille wall. The rille is on the left of the exposed rocks, image width is 500 m [NASA/GSFC/Arizona State University].
Why are scientists captivated by sinuous rilles? Part of the reason is purely aesthetic - each sinuous rille is different. Some sinuous rilles are less curvy, some - like Rimae Posidonius - look like squiggles, while yet other rilles are so windy that they have horseshoe-shaped curves. Scientifically, however, sinuous rilles are exciting because it's possible to see layers of mare lavas that were cut through during the rille formation - as long as the regolith slumping down the walls isn't thick enough to obscure them! Besides exposing layers in the lava flows, rilles also give you an almost dynamic look at where lava flowed, suggesting very high effusion rates over long periods of time (often much higher than typical of those on Earth). Some rilles are believed to contain pyroclastics, which can tell scientists something about the volcanic history of the rille. Sinuous rilles in the Rimae Prinz region of the Moon may even host a lava tube network.