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GRAIL-Identified Gravity Anomalies In Solar System Exploration Division, GSFC Code 690 GRAIL-identified gravity anomalies in Oceanus Procellarum: Insight into subsurface impact and volcanic/magmatic structures on the Moon Ariel N. Deutsch1, Gregory A. Neumann2, James W. Head1 1Department of Earth, Environmental and Planetary Sciences, Brown University, 2NASA Goddard Space Flight Center Introduction: Lunar gravity anomalies. Positive Bouguer gravity anomalies. • Four distinctive positive Bouguer gravity anomalies are • Previous work has suggested that these four positive gravity anomalies may be due to: -Subsurface volcanic sills [2]. • New, higher-resolution GRAIL data [3] allow for the re- analysis of these anomalies. • Understanding the subsurface density structures that contribute to these anomalies is important in order to discuss regional impact and volcanic histories, and the evolution of the lunar crust in Oceanus Procellarum. Objectives. 1. Constrain subsurface structures that contribute to the . four positive Bouguer gravity anomalies. 2. Discuss the hidden impact and volcanic histories of . the Moon. Methods. Results: Filled and buried impact craters. RESULTS: MANTLE UPWELLING • Six geologic end-member scenarios are explored to 20 200 1. Filled and Buried Impact 2. Southern Aristarchus Plateau analyze the four observed gravitational anomalies. 15 Model 3 10 GRAIL 100 • Impact crater parameters [e.g., 4] are estimated to C Gravity 5 B from uplift Gravity from A 0 0 km -3 km km ρ = 3150 kg m -5 mGal mGal • Analyses of the generation, ascent, and eruption of -3 mGal ρ = 2800 kg m crater -10 Surface topography -100 -3 of subsurface magmatic structures and also the . -15 ∆ρ = 600 kg m Highland crust -20 -200 Mantle interpretation of surface volcanic features. -25 uplift Highland crust Longitude (°) 310 Longitude (°) 315 MAGMATIC INTRUSION SCENARIOS GRAILDensity ContrastResidual • Does not provide enough density contrast. • Provides enough density contrast. • • Consistent with postulated buried craters [1]. • Anomalies are high magnitude and concentrated, in contrast to the • 100 km 2 km thick 105 km gravitational signatures that are associated with FFCs [9]. • Requires crustal thickness of 18-28 km. 110-km diameter Results: Volcanic and magmatic intrusions. • We obtain Bouguer anomalies from GRAIL gravity data 20 200 20 200 Northern Marius Hills Northern Marius Hills [3] by removing the surface topography attraction, 15 15 10 100 3 GRAIL assuming a crustal density of 2800 kg/m 5 GRAIL Model 6 10 100 0 Topography 0 3 -3 • We use a bulk density of 3150 kg/m for the maria, as Model 4 Gravity from -5 ρ = 3150 kg m 5 Gravity -10 crater -100 km -3 km km km ∆ρ = 150 kg m mGal mGal km mGal was suggested for basalt of intermediate Ti-content [2]. from sill mGal -15 ρ = 2800 kg m-3 0 0 -20 -200 • We remove the longest wavelength variations in Topography -25 -3 -5 ρ = 3150 kg m Dike crustal structure, windowing the anomalies to spherical crater -30 swarm -300 ρ = 2800 kg m-3 Sill -35 -10 -3 -100 ∆ρ = 600 kg m -40 -400 305 Longitude (°) 310 305 Longitude (°) 310 • The density contrasts for each geologic scenario are Longitude b m 3 to model the anomalies. • Does not provide enough density contrast. • Provides enough density contrast. • The total modeled gravity anomaly is the sum of the • Horizontal, subsurface sills beneath the Marius Hills anomalies require a • thickness >10 km in order to match the observed signal. • • • Crust occupied by 25% dikes [11], fed by mantle. References. Conclusions. GRL, 43, JGR Planets, 118, Science, 339, GRL, 40, 38–42. [5] Wilson L. and Icarus, 283, Icarus, 283, GRL, 41, JGR, 2012, Icarus, 283, 224–231. [10] Wilson L. and Head :S. AP and N. Flamsteed JGR, 107, JGR, 86, :N. and S. Marius Hills). Acknowledgment: This work is supported by NASA under Grant Number ISFM End-of-Year Review – September 21, 2018 – NASA GSFC.
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