Structure of the Moon’s Orientale Basin from Gravity Recovery and Interior Laboratory (GRAIL) Observations

Tuesday, 16 December 2014: 11:20 AM
Maria T Zuber1, David E Smith1, Sander J Goossens2, Sami W Asmar3, Alex S Konopliv3, Frank G Lemoine4, Jay Melosh IV5, Gregory A Neumann4, Roger J Phillips6, Sean C Solomon7,8, Michael M Watkins3, Mark A Wieczorek9, Jeffrey C Andrews-Hanna10, James W Head III11, Walter S. Kiefer12, Patrick Joseph McGovern Jr12, Francis Nimmo13, Jason M Soderblom1, Jeffrey Taylor14, Brandon C Johnson1, Erwan Mazarico1,4, Katarina Miljkovic1, Ryan S Park3 and Dah-Ning Yuan3, (1)Massachusetts Institute of Technology, Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States, (2)UMBC CRESST/ NASA GSFC, Greenbelt, MD, United States, (3)Jet Propulsion Laboratory, Pasadena, CA, United States, (4)NASA Goddard SFC, Greenbelt, MD, United States, (5)Purdue University, West Lafayette, IN, United States, (6)Southwest Research Institute, Boulder, CO, United States, (7)Carnegie Institution of Washington, Department of Terrestrial Magnetism, Washington, DC, United States, (8)Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY, United States, (9)Institut de Physique du Globe de Paris, Paris, France, (10)Colorado School of Mines, Golden, CO, United States, (11)Brown University, Providence, RI, United States, (12)Lunar and Planetary Institute, Houston, TX, United States, (13)University of California-Santa Cruz, Department of Earth and Planetary Sciences, Santa Cruz, CA, United States, (14)Hawai'i Institute of Geophysics and Planetology, Honolulu, HI, United States
The Gravity Recovery and Interior Laboratory (GRAIL), a dual-spacecraft, gravity-mapping mission that is a component of NASA’s Discovery Program, successfully concluded its Primary and Extended Missions, mapping the lunar gravity field from average altitudes of 55 km and 22.5 km, respectively. The mission is currently in its science analysis phase. During the final weeks (the “endgame”) of the mapping mission, the orbital altitudes of the two spacecraft were lowered to an average of 11 km above the lunar surface. The endgame mapping strategy was designed to provide the highest-resolution coverage over the Orientale basin, to be used to develop a gravity map of a multi-ring impact basin at unprecedented resolution. In order to achieve the highest-resolution gravitational model of Orientale, we performed a short-arc analysis of GRAIL’s Ka-band range rate (KBRR) observations by adjusting a priori field GRGM900A while embedding neighbor smoothing. The combination of a spherical harmonic solution and the local analysis resolves the gravitational structure of Orientale and its environs to 3–5 km, suitable for detailed investigations of basin structure and evolution. The map reveals a correspondence of the inner depression with an excavation cavity that reflects removal of approximately 30 km of crust. The crustal thickness beneath the basin cavity may be as little as 6 km, depending on assumptions regarding the global mean thickness of the crust, the densities of the crust and mantle, and the thickness and density of the impact melt and mare fill. An annulus of negative free-air anomalies is strongest between the Inner and Outer Rook Mountains and is likely due in part to an annulus of thickened crust surrounding the excavation cavity, most likely caused by the crustal overturn during excavation (as suggested by hydrocode models). Gravitational signatures of basin rings are well resolved and distinctive, reflecting substantial fracturing or porosity as well as evidence for localized magmatic intrusion. There is no preserved evidence, however, for a transient cavity that correlates with any of the basin rings. A record of the energetics of this and other impacts is preserved in the excavated volume, the fractured crust and ring structures.