USING GRIDVIEW TO COMPARE TOPOGRAPHIC AND CRUSTAL THICKNESS SIGNATURES OF LARGE LUNAR IMPACT BASINS

The GRIDVIEW computer program has proven to be very useful tool to study the structure of the crust of the Moon and Mars. It was used to map Quasi-Circular Depressions (QCDs) on Mars which represent a very large but buried population of impact craters, and to help identify Circular Thin Areas (CTAs) in model crustal thickness data which indicate the presence of several previously unknown very large impact basins on Mars. More recently GRIDVIEW was used to map QCDs and CTAs for the Moon, which, in combination with the more recent LOLA and LOLA-derived data, indicates a population of impact basins significantly larger than previously thought. Detailed study of these QCDs and CTAs for basins > 300 km in diameter revealed that, for well-known basins, the ring of thickened crust in the CTA signature sometimes was situated well inside the most prominent topographic ring normally taken as the basin diameter. This appears to be especially true of some multi-ring basins but not the case for larger craters 200-300 km in diameter, just below the size cutoff for lunar basins. In order to better understand the relationship between the subsurface crustal structure represented by the model CTA signatures and the surface topographic structure, we began a systematic comparison of the QCD and CTA signatures for basins as a function of diameter, location on the Moon, and multi-ring status. We also noted whether or not mare fill was related to the location of the CTA peak ring compared to the topographic peak ring. These comparisons were done both using the fit-a-ring feature in GRIDVIEW and its profiling tool. We find for basins < 300 km diameter that the CTA diameter and QCDS diameter are most always the same. For lunar basins in the 300-700 [???] diameter range, the CTA diameter is often but not always about 80% the QCD diameter. A subset of these can have the CTA/QCD diameter as small as 0.6; these are mostly mare-filled basins and it may be the mare fill has affected the apparent model crustal thickness, in addition there a few cases where mare-filled basins have CTA/QCD ratios >1.3. For much larger basins the CTA/QCD diameter approaches 1 again as for the small features. Additional variations appear in our limited preliminary study to depend on the multi-ring nature of basins, but further work is required to understand these trends.