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Dry Debris or Liquid Flow?
A granular debris flow on the wall of Stevinus A › View larger image
A granular debris flow on the wall of Stevinus A (downhill to the right). A 7 m diameter boulder impedes the progress of the flow, which bifurcates and reconnects about 10 m downhill. LROC NAC M154893929R, image is 500 m across [NASA/GSFC/Arizona State University].

Stevinus A (31.75°S, 51.55°E) is an 8 km diameter crater with very smooth, high albedo crater walls and low albedo streamers and streaks. The high albedo material composing the crater walls may have once been an impact melt veneer that is eroding over time as micrometeorite bombardment promotes regolith formation. Sometimes, craters with smooth walls are observed because the craters are old enough (at least Eratosthenian age, where their ejecta rays have been erased) to have developed a regolith layer that blankets the interior of the crater and obscures small-scale morphology variations. However, Stevinus A is pretty young and has only recently begun to accumulate a thin regolith because there are distinct edges observed (above the boulder mentioned in the opening image, for example) as well as boulder trails (also observed in the opening image).

Stevinus A crater is home to many unique features, but the low albedo flows on the crater walls are striking. Are they impact melt flows that splashed onto the crater walls and flowed downhill, toward the crater floor? Or are these flows composed of granular, dry debris that acted like a fluid when the particles were mobilized, perhaps by a seismic shock wave from a nearby impact? Moreover, why are these streamers composed of such low reflectance material when the rest of the crater is of higher albedo?

LROC WAC monochrome mosaic of Stevinus A › View larger image
LROC WAC monochrome mosaic of Stevinus A and the surrounding area. Location of opening image noted with asterisk [NASA/GSFC/Arizona State University].

In the opening image and elsewhere in the LROC NAC image pair, there appear to be two kinds of low albedo material. One type is rubbley and contains discernible rocks, scattered across sections of the crater wall. The other type looks smoother and occurs in braided stream-like flows and cuts through the more rubbley material. Although at first glance these braided streamers seem to be impact melt flows, closer observation suggests that they are probably the result of dry debris flows of fine-grained material. Granular flows are not uncommon on the Moon, especially on steep slopes like crater walls, where the slope is greater than ~20° and not quite the angle of repose, which is ~30°. Dry granular flows are also observed on Mars and even on the asteroid, Eros! Furthermore, the braided nature of these flows suggests successive depositional periods. If these streamers are dry material, successive depositional events may be triggered by disturbance from a boulder bouncing downhill, and in the opening image there is evidence of at least one bouncing boulder.

These observations are consistent with a dry, granular debris flow from the higher elevations on the Stevinus A crater wall toward the crater floor. However, there is impact melt in the crater floor, along with bouldery and blocky ejecta debris (and probably debris flows, too!) observed in another LROC NAC pair of Stevinus A. Thus, to help make a definitive interpretation of these low albedo streamers traveling down the crater walls, a stereo pair of the crater would allow LROC scientists to measure the slope of the crater walls.

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