Sunday, November 23, 2014

Dark splotches over high albedo, under a high sun

Unnamed crater (2.2784°N, 116.2125°E) southwest of King, presenting a unique albedo variation in 1.8 km-wide field of view from LROC NAC observation M123812230R, LRO orbit 3380, March 21, 2010; 8.3° incidence angle, resolution 57 cm from 55.36 km over 2.25°N, 116.16°E [NASA/GSFC/Arizona State University].
Hiroyuki Sato
LROC News System

Impact craters routinely excavate subsurface materials, exposing them in crater walls and in ejecta. The Featured Image highlights an unnamed fresh crater (480 meters in diameter) with numerous dark splotches.

Inside the crater cavity, dark splotches (low reflectance materials) occur from the middle to the trim of the crater and spread outward beyond the rim crest.

Several small craters (less than 100 meters in diameter) with similar dark splotches also occur in this region (outside the area shown above, see next image), suggesting that the dark materials were likely excavated from an extensive subsurface layer. The distribution of the dark halo craters informs us about the horizontal extent of these subsurface materials.

Small craters a few thousand meters north of the dark halo crater (DHC) of interest, above, from the same LROC NAC frame M123812230R. Note the crater right of center bottom may be superposed on the rim of a more ancient depression [NASA/GSFC/Arizona State University].
The crater in the opening image is found 116 km from the southwestern rim of King crater (76.2 km; 4.96°N, 120.49°E), located in the farside highlands. Unlike in the mare, pyroclastic deposits are unlikely to be the low-reflectance material (seen in the opening image) here in the middle of the highlands with no indication of volcanic activity near here. So, what is this low reflectance layer?

Context view of the location of today's Featured Image in WAC monochrome mosaic (100 m/pix) overlayed by WAC stereo DTM (GLD100, Scholten et al., 2012). The NAC footprint (blue box) and the exact location of the opening image (yellow arrow) are indicated [NASA/GSFC/Arizona State University]. 
The rays of Necho crater (36.87 km; 5.25°S, 123.24°E) extend out around 680 kilometers (see image below) crossing over King crater and the area in today's Featured Image. Since the area of opening image is crossed by the Necho ray deposits the excavated dark layer might be the original mature surface (now covered by Necho's high reflectance rays).  

Context view of the area of interest in an orthographic LROC WAC mosaic of low-angle observations of the surrounding hemisphere. Arrow points to the location of the crater of interest, within range of ejecta from King, Necho or perhaps, less likely, Giordano Bruno or Goddard A craters, (not unlike the magnetic anomaly east of Firsov) [NASA/GSFC/Arizona State University]. 
Due to the lack of atmosphere on the Moon, the photometric effect is very strong. Thus, it is hard to identify the relationships between the different layers using low-Sun images (images with large incidence angles, near sunrise or sunset); however, high-Sun images (those with low incidence angles, near noon) display clearly the relationships between units, which helps us reconstruct the resurfacing history of this area.

View full-size view of the LROC NAC frame, HERE.

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