Lunar Crustal Properties: Insights from the GRAIL Gravity Signatures of Lunar Impact Craters

Tuesday, 16 December 2014: 10:50 AM
Jason M Soderblom1, Jeffrey C Andrews-Hanna2, Alexander J Evans3, Brandon C Johnson1, Jay Melosh IV4, Colleen Milbury4, Katarina Miljkovic1, Francis Nimmo5, Roger J Phillips6, David E Smith1, Sean C Solomon3,7, Mark A Wieczorek8 and Maria T Zuber1, (1)Massachusetts Institute of Technology, Cambridge, MA, United States, (2)Colorado School of Mines, Golden, CO, United States, (3)Columbia University of New York, Palisades, NY, United States, (4)Purdue University, West Lafayette, IN, United States, (5)University of California-Santa Cruz, Department of Earth and Planetary Sciences, Santa Cruz, CA, United States, (6)Southwest Research Institute, Boulder, CO, United States, (7)Carnegie Institution of Washington, Department of Terrestrial Magnetism, Washington, DC, United States, (8)Institut de Physique du Globe de Paris, Paris, France
Impact cratering is a violent process, shattering and melting rock and excavating deep-seated material. The resulting scars are apparent on every planetary surface across our Solar System. Subsurface density variations associated with the resulting impact structures contain clues to aid in unlocking the details of this process. High-resolution gravity fields, such as those derived from the Gravity Recovery and Interior Laboratory (GRAIL) mission, are ideal for investigating these density variations. With gravity measurements from GRAIL and topography from the Lunar Orbiter Laser Altimeter (LOLA), we derived high-resolution Bouguer gravity fields (i.e., the gravity field after the contribution from topography is removed) that we correlated with craters mapped from LOLA data. We found that the mass deficit beneath lunar impact craters relates directly to crater size, up to diameter ~130 km, whereas craters larger than this diameter display no further systematic change. This observation, coupled with the greater depth of impact damage expected beneath larger craters, indicates that some process is affecting the production and/or preservation of porosity at depth or otherwise altering the mean density beneath the larger craters (note, measurable mantle uplift is observed for craters larger than ~184-km diameter). The observed crater gravity anomalies, however, exhibit considerable variation about these mean trends, suggesting that other factors are also important in determining the bulk density of impact crater structures. Milbury et al. (this conference) have demonstrated that pre-impact crustal porosity strongly influences the resulting density contrast between the impact damage zone beneath a crater and its surroundings. Herein, we extend these studies using the same GRAIL- and LOLA-derived maps to further investigate the effects that crustal properties have on the bulk density of the rock beneath lunar impact features. We focus, in particular, on the processes that influence the generation and preservation of porosity beneath complex craters 100–150-km in diameter.