Crustal Thickness Evidence for More Previously Unrecognized Large Lunar Basins

Lunar topographic data revealed a large population of large impact basins not previously recognized by standard photogeologic mapping. Crustal thickness model data reveal the presence of even more. The total number of lunar basins > 300 km diameter may exceed 150, more than 3 times that determined by photogeologic mapping alone. Introduction: Photogeologic mapping suggested the cumulative number of impact basins on the Moon larger than 300 km diameter was 45 [1]. Early analysis of Clementine topography revealed several new large impact basins not previously known [2,3]. A recent systematic search [4] for Quasi-Circular Depressions (QCDs) using ULCN 2005 [5] gridded topography suggested 92 > 300 km diameter. While many of the Wilhelms basins have significant topographic structure and appear as QCDs in the ULCN data, 10 do not have obvious topographic structure and 10 others have a topographic structure that suggests a different basin center or ring diameter than was derived from photogeology [4]. A comparison of the cumulative frequency curves for basins identified by photogeologic mapping and those derived from topography shows general agreement down to a diameter of about 500 km; both follow a roughly -2 powerlaw curve. Below that the Wilhelm’s listed basins [1] fall off the curve but the topographically-derived basins continue along it down to the cutoff diameter of 300 km. Most of the basins missed by photogeologists are at the smaller diameters. Most previously unknown basins on Mars were found as QCDs in Mars Orbiting Laser Altimeter (MOLA) data [6,7,8] but additional presumably more deeply buried basins were found as Circular Thin Areas (CTAs) [9] in crustal thickness model data [10]. We examined lunar crustal thickness data [11] for CTA signatures that might be additional large impact basins not previously recognized in photogeologic or topographic mapping alone. Evidence for basins in lunar crustal thickness data. Gridded data from the Wieczorek crustal thickness model [11] were colored, stretched, contoured and searched for Circular Thin Areas (CTAs) surrounded by thicker crust, using the same GRIDVIEW software used for the Mars studies [12]. Figure 1 shows results for one part of the Moon. Although there are some exceptions, nearly all the basins identified using ULCN topographic data [4] have an obvious crustal thickness signature, sometimes offset from the topographic basin. This suggests crustal thickness data is indeed useful for identifying large impact basins. Additional CTAs that may be new basins not previously identified in the topographic study are indicated by the red arrows in Figure 1. A preliminary search yielded some 56 possible new impact basins larger than 300 km diameter, over and above those found in the earlier topographic study[4]. Figure 2 shows cumulative frequency curves for basins > 300 km diameter derived from different sources. The CTA population in Figure 2 is less than the topographic population, but still contributes significantly to a “total” population (CTAs plus QCDs, which include the visible basins having a topographic signature). This total population is a factor 3 greater than that based on the photogeologically determined basins only. The N(300) crater retention age for the lunar surface is probably at least 3.9, a significant increase over that suggested by the Wilhelms [1] list of basins (1.2) or even that suggested by our earlier topographic study (2.5).