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Modeling the Origin of the Orientale Basin Mascon Sciences David M Earth, Atmospheric, & Planetary Modeling the Origin of the Orientale Basin Mascon Sciences David M. Blair*, Brandon C. Johnson, Andrew M. Freed, H. Jay Melosh, Gregory A. Neumann, Sean C. Solomon, and Maria T. Zuber Abstract This Poster * Point of Contact: [email protected]. Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN; NASA Goddard Space Flight Center, Greenbelt, MD; Department of Terrestrial Magnetism, Carnegie QR Institution of Washington, Washington, DC; Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY; Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA. Introduction Step 1: Hydrocode modeling The origin of gravity anomalies in large lunar basins is a puzzle that has endured We use the hydrocode iSALE to simulate the time period from impact through collapse of the transient crater several hours later. We constrain since the late 1960s [1]. High-resolution gravity data obtained from NASA’s dual our models to match the location of the annulus of thickened crust [7, 8] and with a regional crustal thickness of 40 km inferred from GRAIL Gravity Recovery and Interior Laboratory (GRAIL) spacecraft have provided data [3]. The thermal gradient of the Moon and the impactor diameter are treated as variables. unprecedented high-resolution measurements of the gravity anomalies associated with lunar impact basins [2,3]. These gravity anomalies are the most The rst few hours: hydrocode results The annulus of thickened crust is formed by ejecta falling onto intact striking and consistent features of the Moon’s large-scale gravity eld and crust outside of the excavation cavity. This crustal “collar” then 0 100 200 300 400 500 600 generally consist of a central region of positive gravity anomaly (a “mascon”), subsides and slumps back into the transient cavity, creating a 1800 surrounded by an annulus of negative anomaly (here called a “masdef”) and a sub-isostatic geometry. The impact leaves a melt pool ~160 km 0 distal annular mascon. across and more than 270 km deep, capped by ~10–20 km of (K) unmelted crust, despite sloshing of mantle material onto the 1400 e Left: GRAIL r surface. tu Right: Ask about (or scan QR code for) a The lunar gravity eld from GRAIL 100 global free-air a r movie of our best-t iSALE calculation. Movie de near s gravity anomaly si id 1000 far 0° E e map [2]. Basins 60° N are characterized empe Left: Our best-t iSALE model for the formation of Orientale T 180° E 200 270° E by bull’s-eye Basin. This run involves a 70 km bolide impacting at 15 km s into 30° N gravity patterns. 600 a Moon with a near-surface thermal gradient of 30 K km. The basin-central (11 MB .mov) 0° positive Distance from impact origin (km) anomaly—the 30° S mascon—is stronger on the 30° S near side due to Step 2: Finite element modeling the greater An FEM built from hydrocode results amount of Free-air gravity anomaly volcanic inlling 0 100 200 300 400 500 600 Evolution over geologic time: FEM results (mGal; 1 mGal = 10 m s) -300 -150 0 150 300 in those basins. 1800 0 600 Shelf Inner Outer Rook Cordillera Rook The multi-ringed Orientale Basin presents a mixed case between the gravity (K) 1400 e 300 signature of nearside and farside basins; it displays both a prominent annular r ) tu 100 masdef like most farside basins, and a central mascon comparable in magnitude a r 0 m s to that of Mare Smythii or Mare Moscoviense. Orientale does contain its own Mare, although it is comparatively small (~250–300 km in diameter with an average 1000 empe T depth of ~200 km) [4]. At ~960 km in diameter, Orentale Basin is substantially 200 -300 larger than the Humorum and Freundlich-Sharanov Basins considered in similar 600 studies [5, 6; Tues. 3:30–4:00 PM in Ballroom 6]. 1 mGal = 10 Mare 1km in middle 200 m thick Mare Distance from impact origin (km) -600 end of FEM In this study, we attempt to explain the free-air gravity pattern over the Orientale (mGal; Basin. We do this with a combination of modeling techniques that allow us to track anomaly Free-air gravity end of hydrocode Above: Translation of the end state of the hydrocode model into a nite element -900 observed the history of the basin from the time of impact through long-term geophysical model, showing the starting temperature conguration of the FEM for evolution. comparison to the hydrocode model shown above. 0 200 400 600 800 The nal state of the hydrocode model is used as the starting condition Distance from basin center (km) Below: The observed free-air gravity anomaly over Orientale basin, from a spherical for our nite element models (FEMs), taking into account both harmonic expansion of GRAIL data to degree and order 420 [2]. The anomaly in the basin ranges from ~400 mGal positive to ~700 mGal negative (1 mGal = 10 m s). geometry and temperature and density structure. We use the FEMs Above: Our best-t nite element model of the formation of the gravity to simulate the evolution of the gravity anomaly over geologic time following anomaly pattern in Orientale Basin, showing its state before (blue line) crater collapse. This includes viscoelastic ow of the mantle driven by pressure and after (red line) relaxation of the basin, and after the emplacement Free-air gravity anomaly over Orientale Basin gradients caused by sub-isostatic geometry (i.e., isostatic adjustment), thermal of mare material into the basin (yellow lines; thickening mare to 1 km at the basin center provides a better t to observed gravity). contraction of the melt pool, and development of a lithosphere over the 300 cooling basin center. These processes evolve in association with a pressure- and ) temperature-dependent density structure and a temperature-dependent rheology. Our models are Pressure gradients 10 m s axisymmetric and are built and run in the Abaqus modeling suite. After cooling and viscoelastic 0 200 400 600 800 relaxation have reached steady state, we calculate the eects of mare emplacement into the basin. 0 0 -10 10°S Comparing the gravitational acceleration over our models to the acceleration over a corresponding -20 150 geoid model (a model with at-layered geometry and no basin) allows us to generate the synthetic -30 1 mGal = 10 free-air gravity anomaly for comparison to the GRAIL data shown in the gures here. 150 -40 -50 At the start of the FEM (the end of the hydrocode), the distal mascon annulus and annular masdef (mGal; 300 Pressure 20°S are already recognizable in the gravity signature, due to the ejecta and crustal collar discussed above. 0 Distance from impact origin (km) (MPa) The initial subisostatic geometry causes a pressure gradient that drives mantle toward the basin from the exterior, generating uplift of both the basin center and the crustal collar. Mechanical continuity Above: Pressure gradients form under the between the basin center and the crustal collar leads to a basin that is generally in isostatic basin due to subisostatic crustal geometry and equilibrium (some support from lithospheric stresses surface topography generated by the impact. These pressure gradients drive upwards motion 30°S notwithstanding), though locally the crustal collar -150 of much of the interior of the basin, and are Exploring model space remains subisostatic and the basin center becomes largely responsible for the formation of the superisostatic. The emplacement of 200 m of mare mascon in Orientale. The rough outline of the material into the basin adds only a small amount to crust is shown by the black line. 600 Free-air gravity anomaly Free-air gravity ) the central mascon; a 1km mare unit, however, brings 0 km 500 300 the mascon up to observed levels. -300 m s 0 Density structure 250°E 260°E 270°E 280°E Left: Variations on our nite element model that did not 3300 Right: 0 100 200 300 400 500 3190 produce as prominent a mascon as the best-t model. 3100 Density 0 -300 (Green line) Insucient lithosphere formation over the structure at 1 mGal = 10 basin center during cooling leads to reduced -600 observed the start of mechanical coupling between the basin center and the 100 References lithosphere too thin the best-t no thermal contraction crustal collar. (Brown line) Neglecting thermal 2600 -900 (mGal; [1] Sjogren, W.L., and P. M. Muller (1968), Science 161, 680; [2] after Zuber, M. T. et al. (2013) Science 339, 668; [3] Wieczorek, M. A. et al. anomaly Free-air gravity model. 2500 contraction means that the mascon cannot benet from 200 (2013) Science 339, 671; [4] Whitten, J. et al. (2011) JGR 116, E00G09; [5] Johnson, B. C. et al. (2013), LPSC 44, #2043; [6] Freed, A. M. et al. 0 200 400 600 800 (2013), LPSC 44, #2037; [7] Neumann, G. A. et al. (1996) JGR 101, E7; [8] Wieczorek, M. A., and Phillips, R. J. (1999) Icarus 139, 246; its anomalously high density late in the simulation. Distance from Density [Background image] Lunar Orbiter frame 4187. Distance from basin center (km) impact origin (km) (kg m).
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