Thursday, November 10, 2011

Lobate Scarp or Fluidized Ejecta?

A double merges into a single escarpment, part of a dramatic scene near 32.8°N, 215.08°E on the floor of an unnamed crater superimposed on the larger farside highland crater Blazhko D. LROC Narrow Angle Camera (NAC) observation M105463860R, LRO orbit 694, August 21, 2009; field of view around 1500 meters at an incidence angle of 51.02° from an early Commissioning Mission altitude of 166.2 km. See the full size original LROC Featured Image HERE [NASA/GSFC/Arizona State University].
James Ashley
LROC News System

Is this meandering escarpment an example of 1) a lobate scarp, or 2) imbricated deceleration lobes? Lobate scarps represent portions of the lunar crust that thrust-faulted, with one portion riding up and over the other in a process that is suggestive of crustal shrinking. Imbricated deceleration lobes result from fluidized impact ejecta deposits, with some deposits riding up and over others in a similar-looking way. Fluidization in this sense should not be confused with liquid or water - these materials are totally dry!

The cause of this type of fluidized flow remains somewhat unknown, but may involve acoustic energy within the moving mass of debris. Some of this energy may be supplied by the surface beneath the debris flow as secondary impacts and tectonic readjustments keep it vibrating following the large impact that initiated the flow. Try pouring some sand on a slanted board while you drum on the board with your fingers. You will see that the drumming removes most of the friction and allows the sand to flow freely. These type of flows often travel great distances before losing energy as they encounter slopes too steep to climb. On Earth we call them sturzstroms.

A complication in the case of today's Featured Image is the presence of scarps that are clearly related, but which oppose each other in their orientation (left side of frame).

Extensions of the scarp are visible far beyond the field of view of the Featured Image (white square) in much larger 5500 meter-wide segment from LROC NAC frame M105463860R [NASA/GSFC/Arizona State University].
The presence of this feature within the deposits of a highland crater floor might argue that fluidized ejecta are responsible because these debris flows often collect (some even appear to 'pool') within the low-lying crater floors. The material could be seen as so fluid that even after portions of the material have climbed the crater wall and stalled, other portions are able to slide backward to create the secondary 'reverse' escarpment in the Featured Image. Similar features within fault escarpments are not unheard of, however, and are referred to as splay faults. Thus the discussion remains an open one...

From the LROC QuickMap a contextual image showing both the left and right frames of LROC NAC observation M105463860 juxtaposed surrounding the field of view (white square) in the 5500 meter-wide image immediately above  [NASA/GSFC/Arizona State University].
LROC Wide Angle Camera (WAC) 100 meter resolution context image superimposed upon the LOLA Digital Elevation Model from the NASA ILIADS lunar mapper program. The elevation in meters above the global mean is shown at selected points around the spotlighted area [NASA/GSFC/Arizona State University],
Additional indications of fluidized ejecta are located throughout the surrounding region in the above context image. However, although a great many highland craters are present within this region of the Moon, none stand out as being the unambiguous source of these deposits. Knowing the true mode of origin for features like these is basic to the goals of planetary science. A properly equipped team of astronauts conducting seismic measurements is probably required to make a definitive determination. Are there any additional clues visible in the full NAC image that might help solve the mystery? Examples of lobate scarps can be found in Schrödinger, and Xenophanes craters. Examples of fluidized ejecta include the Lavish Lobes of Necho R and King Crater Ejecta Deposits.

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