Thursday, August 7, 2014

Dark Halo "Patch" west of Neper D

A roughly 470 meter wide, 560 meter long, unusually large and cohesive fan of dark halo ejecta from an unnamed but freshly prominent crater in the Neper group, southwest of Mare Marginis. 1020 meter-wide field of view from LROC NAC observation M1136029635L, LRO orbit 19510, October 10, 2013; 32.5° incidence, resolution 1.17 meters from 116.96 km over 8.73°N, 79.36°E [NASA/GSFC/Arizona State University].
Hiroyuki Sato
LROC News System

Today's Featured Image highlights an odd shape, or texture in impact ejecta. An unnamed, approximately 600 meter-wide fresh crater on a relatively high, far older crater rim, between Mare Marginis and Mare Undarum (9.5122°N, 79.40242°E) hosts teardrop-shaped low reflectance patches in its ejecta, northwest of the crater,

The larger teardrop patch in the opening picture has a sharp boundary along its western edge, perhaps implying a partially elevated surface relative to the surrounding high-reflectance ejecta.

4 km-wide field of view from LROC NAC observation M1136029635L, October 10, 2013 [NASA/GSFC/Arizona State University].
Note the faint high-reflectance ray overlying this patch, indicating that the low reflectance patch was emplaced before the completion of the impact event. Similar dark patches that are smaller and less pronounced are found at the top of the opening image.

How were these peculiar dark patches formed? If these patches are elevated, they could represent preexisting flat dark mounds that were swept by the saltating ejecta materials. Or the excavation of low-reflectance materials by the impact could have been thrown out in one direction and resulting in this unusual patch of ground. If, on the other hand, the patches are topographic lows rather than elevated, these depressions could have simply been shielded as the ejecta passed overhead. It is also possible that the topography of the rim controlled the direction of the outthrown ejecta such that there were "no ejecta" zones that resulted in the dark patches.

Fifty kilometer-wide field of view of the complex terrain southwest of Mare Marginis, west of Neper and Neper D. The unusually cohesive "fan" of apparent dark halo material from the unnamed relatively fresh 600 meter crater is marked at 9.5122°N, 79.40242°E (arrow) and may indicate the presence of now-buried optically mature material widely distributed in the area. The circle southwest of Neper D is a patch of isolated mare material bright in Clementine iron oxide surveys, often indicative of cryptomare.  Crater counts might eventually determine whether its age and whether or not this area is consistent with Mare Marginis or ejecta from the Crisium impact, or something much older. LROC WAC global 100 meter mosaic over LOLA laser altimetry [NASA/GSFC/ASU/USGS].
High resolution topography would really help unravel this mystery! NAC to the rescue! The LROC targeted this area for upcoming NAC stereo pairs from which meter-scale topography will be extracted. In a few months, we will post the topography and will see if we solved the mystery!

Explore this peculiar dark patch and surrounding terrain in the following full NAC frame, HERE.

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