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The ~16.85km Dawes crater located in Southern Mare Serenitatis exhibits a morphology typical of lunar craters in the simple-to-complex tran¬sition zone: strongly terraced walls and a relatively deep, irregular and hummocky floor with no central peak. Relying on available LROC NAC/WAC images and the LOLA-WAC DEM, I have constrained variations in its rim and wall dimensions. Surprisingly, this effort revealed significant variations in the rim crest height as measured around the circumference of Dawes. While the average rim crest height, measured from the pre-impact surface is 641m, it varied from 820m in the SE quadrant to a low of 313m due North. Local maxima exceeding 800m also occur in the NW and NE sectors bracketing the northern low. The northern low is complemented by a broad southern low where the rim crest dipped below 465m.
Such antithetic variations in rim crest height are hard to reconcile with the effects of oblique impact as previously proposed. Both lows are associated with outward inflections in location of the rim crest, whereas highs correlate with modest inward rim crest inflections. Lows are also associated with broad, shallow inner walls and shallow ejecta-covered flanks, suggesting that pre-existing target weaknesses may have induced enhanced slumping along a N-S trend. Pre-existing structures are known to have controlled crater shape at the 1.1km Meteor crater in Arizona. A N-S trending curvilinear trough, likely a partially buried graben, is located ~3km to the east of the easternmost rim crest of Dawes crater and provides further support for a pre-existing structural fabric controlling this crater’s morphometry.
Morphometric databases of lunar craters typically provide a single value for rim crest height, yet more recent LROC and LOLA data for Dawes crater reveal significant rim crest height variations that correlate with plan-form deviations from circularity. The rims of many larger complex craters (including some multi-ring basins) also show linear trends and other departures from circularity. Perhaps these, too, were induced by pre-existing structural fabrics.
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