A NEW INVENTORY OF LUNAR IMPACT BASINS FROM LOLA AND GRAIL. G.A. Neumann1, D. E. Smith2, M. T. Zuber2, S. Goossens1,3, J. W. Head4, F. G. Lemoine1, E. Mazarico2, J. Andrews-Hannah5, M. H. Tor- rence6, M. A. Wieczorek7, M. T. Zuber2. 1NASA GSFC, Code 698, 8800 Greenbelt Road, Greenbelt MD 20771, 2Massachusetts Institute of Technology, Cambridge MA 02139, 3CRESST, University of Maryland Baltimore County, Baltimore MD 21250, 4Brown University, Providence, RI 02912, 5Colorado School of Mines, Golden, CO 80401, 6Stinger Ghaffarian Technologies, Greenbelt, MD 20770, 7Institute de Physique du Globe, 75205 Paris, FR.

Introduction: Impact basins are circular depres- Bouguer residual (black) can be modeled as variations sions typically defined by the presence of at least two in the depth to the lunar mantle as well as density topographic rings, a rim crest and an interior ring of variations arising from the impact process. peaks. The onset of basins on the Moon occurs at about 200 km in diameter with the formation of peak-ring 0.0001 basins, which exhibit two topographic rings; multi-ring basins occur at larger diameters and exhibit three or 1e-05 1.0 more topographic rings. On the Moon, the majority of 1e-06 basins are >300 km in rim-crest diameter, but a few a occur at smaller diameters. Many basins >300 km in 1e-07 diameter only preserve a single topographic ring due to 0.5 post-impact resurfacing and proximal weathering by 1e-08 coherence later impacts. Lunar basins record the history and size distribution of impactors in the early Solar System and 1e-09 the primordial thermal state of the shallow lunar inte- rior. Previous analyses of the Moon’s gravity field re- 1e-10 0.0 vealed that virtually all lunar basins exhibit positive 0 60 120 180 240 300 360 420 internal mass anomalies arising from some combina- tion of impact, volcanic or thermal perturbations, sup- Fig. 1: Gravity spectrum of GRGM042A. Coeffi- ported by an elastic lithosphere. Bouguer gravity re- cient amplitude (blue) and amplitude predicted by sults from the Lunar Orbiter Laser Altimeter (LOLA) topography (red). Bouguer anomaly (black) as- [1] and the Gravity Recovery And Interior Laboratory sumes a density of 2560 kg m-3. (GRAIL) [2,3] missions have revealed or confirmed several new mass anomalies on the Moon that resem- Results: A Bouguer map (Figure 2) bandpassed ble those of traditional lunar basins. Conversely, the from degree l=6 to l=384 reveals many circular posi- lack of circular mass anomalies in either the free-air or tive anomalies of impact origin. All peak ring basins Bouguer gravity casts doubt on the existence of several exhibit positive Bouguer anomalies lying within the pre-Nectarian basins described as uncertain. The innermost ring The largest anomalies are expressed GRAIL data in particular extend the recognition of from degree 2-5 and represent a nearside-farside di- mascon basins to structures whose diameter is smaller chotomy which has been ascribed to an impact origin, than 300 km, some whose topographic expression has the South Pole-Aitken basin, and a possible third im- almost completely vanished. Diameters of hidden ba- pact in the farside northern hemisphere. sins can be assigned based on circular gravity signa- The maximum gradient of the Bouguer anomaly tures with positive central Bouguer anomalies. We find reveals even further the buried impact history of the a total of 41 definite, likely or possible basins with Moon. Several conjectured basins on the farside are diameters > 300 km. revealed by circular rings of steep gravity slope. These new basin-like craters are generally smaller than the Methods: The gravity spectrum of GRGM042A is nearside basins, with diameters in the range of 180-320 shown in Figure 1. The gravity predicted [4] by an km. A nearly-buried Sinus Asperitatis impact structure, LRO-LOLA topographic model, assuming a uniform- whose 600+ km diameter and inner rings are likely density crust (red), exceeds the measured gravity at buried beneath Mare Nectaris, is also clearly resolved. low degrees, indicating partial compensation. The Many of the newer impact structures are topographi- GRAIL primary mission tracking data exhibits coher- cally obscure, but are clearly revealed by gravity [5]. ence >0.9 with topography from degree l=50 to l=~390 The resulting inventory does not, however indicate a (green, scale on right) which indicates that a great deal more extensive history of large impacts as has been of the gravity signal arises from topography and can be previously suggested [6]. removed to reveal internal density variations. The References: [1] Smith D. E. et al. (2010) Space Sci. Rev., 150, 209-241. [2] Zuber M.T. et al. (2012), Space Sci. Rev. 10.1007/s11214-012-9952-7. [3] Zuber et al. (2012), Science, 10.1126/science.1231507. [4] Wieczorek, M. and R. J. Phillips (1998), J. Geophys. Res. 103, 1715. [5] Neumann, G.A. et al. (2010), LPSC. [6] Frey H. V. (2011) GSA Special Paper 477.

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Fig. 3: Bouguer gravity horizontal gradient. Scale is in milligals per km.