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| Excavation on the floor of Abel C. A bench crater in the pyroclastic deposit mantling the floor of Abel C. The impactor appears to have punched through multiple layers, giving the 100 meter crater an irregular, stepped appearance. LROC NAC observation M1153673248R, LRO orbit 21990, May 2, 2014; 68.8° incidence angle, resolution 75 cm from 72.53 km [NASA/GSFC/Arizona State University]. |
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Lunar craters less than between approximately 15-20 km in size are usually bowl-shaped, so the crater above, excavating the floor of Abel C (41.5 km;
36.72°S, 82.5°E) has a somewhat irregular morphology relative to most craters of its size (about 100 meters).
This crater is characterized by its block-strewn, hummocky floor and low-relief rim. The step-like or benched appearance most evident in the northeastern portion of the wall is due to a strength discontinuity. Such discontinuity indicates that the impact penetrated through two materials of different strengths. Lab experiments have shown that the bench crater morphology forms when the surface layer is composed of thin and poorly consolidated regolith and the subsurface is composed of harder, more coherent material.
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| Small relatively fresh excavation, the 100 meter crater right of center shown in context of the full, 3.6 km-wide field of view swept up in LROC NAC observation M1153673248R, May 2, 2014 [NASA/GSFC/Arizona State University]. |
In some cases, the underlying material is bedrock, evidenced by an abundance of boulders. So, what did the impactor punch through at the surface? Usually regolith is the culprit, but further investigation of the floor of Abel C suggests that it may not be regolith alone.
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| Ancient Abel C (41.5 km; 36.72°S, 82.5°E), on the northwest frontier of Mare Australe, has a smooth floor of relatively low reflectance, compared with it's equally eroded neighbors. Arrow marks the location of the 100 meter crater presented at high resolution above. LROC 100 meter Wide Angle Camera (WAC) global mosaic [NASA/GSFC/Arizona State University]. |
Abel C is a 36 km diameter crater on the southeastern limb of the Moon, just off the northwest edge of
Mare Australe, a large region dominated by volcanic activity.
Clementine (1994) color ratio images are extremely useful in identifying nonmare deposits since non basaltic compositions tend to stick out when Clementine images are placed in the proper color space. Three bands (415 nm, 750 nm, and 1000 nm) from the Clementine
UVVIS camera were used to create the false-color image below.
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| Clementine (1994) false color image of the Mare Australe region with Abel C off the western edge. The floor of Abel C appears more red than the surrounding highlands, along with a clear indication of similar pyroclastic exposure like the multiple indications within Mare Australe [NASA/GSFC/Arizona State University]. |
The three bands were ratioed to control the colors of the false-color image. The 750/415 ratio controls the red component, which is an indication of low titanium or high glass content as found in mature
lunar regolith and to a greater degree
pyroclastic deposits. The 750/1000 ratio controls the green component and is an indicator of the amount of iron on the surface. The 415/750 ratio controls the blue component and indicates high titanium or bright slopes and albedos. In the Clementine false-color image, Abel C stands out from the highlands and more closely resembles the nearby mare. The strong red component, combined with the low-reflectance, mantled floor seen in Abel C is indicative of a pyroclastic deposit.
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| Abel C is shown in relation to the terrain of the Moon's southeastern limb, between Humboldt crater and Mare Australe. LROC WAC-derived digital elevation model and 100 meter global mosaic data [NASA/GSFC/Arizona State University]. |
This pyroclastic deposit encompasses approximately 190 km2 and has been identified in studies of pyroclastic deposits across the Moon (
Compositional analyses of lunar pyroclastic deposits by Gaddis et al. 2003). Because pyroclastic deposits have a mantled appearance, they must be relatively thin. It's possible that the thin upper layer the impactor penetrated was partially composed of pyroclastics.
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