LOLA TEST OF CANDIDATE LARGE LUNAR IMPACT BASINS PREVIOUSLY FOUND USING OLDER TOPOGRAPHY AND CRUSTAL THICKNESS DATA

Frey (2010) previously suggested the cumulative population of large impact basins on the Moon was at least a factor 2 larger than traditional inventories based on photogeologic mapping. ULCN2005 topography and model crustal thickness data based on the same Clementine topography revealed 38 Quasi-Circular Depressions (QCDs) over and above topographically expressed known named basins, and 27 additional Circular Thin Areas (CTAs), larger than 300 km diameter. Lunar Orbiter Laser Altimeter topography recently provided to the Planetary Data System has far better resolution than ULCN2005 and provides an opportunity to assess how robust were the recognition of QCDs in the earlier ULCN data and also to search for even more subtle topographic signatures of possible additional large impact basins. As before we used a 0 to 5 “topographic expression” scoring system in which very obvious, easily recognized large basins (e.g., named basins like Serenitatis or Crisium) are 5’s and basins (including some named features) with no circular topographic low signature are rated 0. Preliminary assessment of the LOLA data suggests that of the 38 new QCDs identified in the ULCN data, 23 received the same topographic expression score, 6 were more easily recognized in the LOLA data (higher expression scores), and 11 had weaker expression scores. 6 previously identified weak candidates received LOLA scores of 0 and 3 others dropped from 2 (possible) to 1 (unlikely). These rare cases were mostly due to resolution differences: what appears in ULCN as a single large roughly circular depression can in the higher resolution LOLA data be a cluster of much smaller QCDs (impact basins). We also find evidence for more than a dozen possible new candidate basins, 9 of which have a preliminary score of 3. Though some basins from the original ULCN study have been eliminated, most have been retained and some new candidates added. The original conclusion that the likely population of very large impact basins on the Moon is at least a factor 2 greater than the traditional number of ~45 is supported by the LOLA data.