Wednesday, August 8, 2012

LROC: Debris Channels

Granular materials once flowed down the crater wall of Alpetragius B and formed these striking patterns. LROC Narrow Angle Camera (NAC) observation M170606933L, LRO  orbit 10276, September 14, 2011; angle of incidence 17.54° at half-meter resolution from an altitude of 45.12 kilometers. See the full size LROC Featured Image 600 meter field of view HERE [NASA/GSFC/Arizona State University].
Lillian Ostrach
LROC News System

Crater modification is a topic of interest to lunar scientists because most of the impact craters on Earth are heavily degraded. Meteor Crater is the best-preserved and youngest impact crater studied on Earth, and fieldwork spanning as far back as the 1800s has greatly improved the understanding of crater formation and modification. By comparing geologic features on the Moon to those that are well-studied on Earth, scientists can begin to understand the complexity of geologic processes that constantly change the lunar surface.

Granular debris flows are the result of material moving downhill due to gravity. The flows often contain fine-grained material, forming a smooth, fluid-like texture, and may be mistaken for impact melt flows. Today's Featured Image highlights the complex and diverse nature of debris flows that represent erosive events that are likely occurring on the Moon today. Located on the eastern wall of Alpetragius B crater (~10 km diameter) in Mare Nubium, this debris flow (15.119°S, 353.253°E) is composed of multiple channels and different sized materials.

Elsewhere, on the north side of the first big bend past the "Cobra's Head" (25.45°N, 49.46°W), widest part of Vallis Schroteri on Aristarchus Plateau, a 1600 meter-long debris channel fans out on the valley floor after a steep 800 meter fall. (Watch your step!). LROC NAC observation M170884438R, orbit 10137, September 17, 2011; angle of incidence 27.86° at 49 centimeters per pixel resolution, from 43 kilometers [NASA/GSFC/Arizona State University].
Rubbly, higher reflectance material (opening image, to the right) appears to be surrounding and somewhat interspersed with a lower reflectance, finer-grained material composing the flow. Where the flow is composed of separate channels (opening image, to the left), the fine-grained material is bordered by the higher-reflectance material.

LROC WAC monochrome mosaic centered on Alpetragius B (15.137°S, 353.128°E) in Mare Nubium. Location of opening image noted by asterisk [NASA/GSFC/Arizona State University].
Perhaps the finer-grained debris carved through an older flow composed of the higher reflectance material. This hypothesis may be supported by observations within the image. For example, the rubbly material is visible elsewhere on the crater wall where the finer-grained material is not. The rubbly, higher reflectance material extends beyond the fine-grained, channeled flow (middle right and middle left). Additionally, the fine-grained flow channels cross-cut one another multiple times, but the higher reflectance material does not border those channels. However, additional observations should be made to make sure that the flow in the opening image is representative of the debris flow populations in Alpetragius B and not a special case.

What evidence can you find that to support the hypothesis about debris flow formation in Alpetragius B? Grab your notebooks and take a look at the full LROC NAC image, HERE.

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