Impact Melt Sheet Composition, Age And Igneous Differentiation?: Commercial Mission Goals
Crystallization of the lunar magma ocean and formation of the primary crust has been followed by over 4 Gyr [1-6] of surface modification processes, predominantly in the form of impacts, a powerful erosive agent, modifying the primary crust through fracturing, brecciation, physical mixing, and through shock-induced melting. The shock-induced melting caused by large impact events (e.g. those producing a crater ~300 km in dimater or greater; ) is predicted to generate significant volumes of melt . Given the concentration of large impacts up to ~5% of the lunar crust may now be comprised of impact melt products . The processes involved in impact melt sheet cooling and crystallization are not well understood. After an impact event, melt is collected in the excavated crater and undergoes cooling and solidification [8,9]. Two end-member cases for the crystallization of the impact melt sheet are: (1) It undergoes igenous differentiation during solidification [8-10], resulting in a newly developed crustal stratigraphy, or (2) It may undergo homogeneous solidification and crystallize in equilibrium , thus homogenizing crustal stratigraphy. We performed a case study on the Orientale basin and explored the possibility for igneous differentiation of the Orientale impact melt sheet  by assessing the thermal and physical processes driving cooling and crystallization . Morphologic measurements of the Orientale impact structure suggest the thickness of the initial molten impact melt sheet was ~15 km . Given the observed radius of the melt sheet, this yields an initial melt volume of ~1.5x106 km3 , in agreement with scaling law predictions . Estimates of convective cooling rates of the Orientale impact melt sheet suggest cooling of the system occurred rapidly, with complete solidification occuring in ~5.5 kyr. Rapid convective heat loss results in cooling rates which are substantially more rapid than the residence time of suspended crystals during the initial stages of melt crystallization. As a result, crystal settling is effectively prevented until the residence time of suspended crystals decreases below the cooling time scales at a solid fraction of ~0.6 (primarily due to dramatically increased viscosity and significantly slower convection). At this point crystal settling at the non-convecting boundaries becomes efficient and the settled crystal fraction rapidly increases. However, at a solid fraction of ~0.6 the melt/crystal mixture is predicted to undergo a sharp rheological transition beyond which the material takes on a solid-like behavior . This would subdue convection as well as crystal settling , and any remaining crystallization would likely take place in situ. The results of this analysis suggest that crystal-melt separation through settling is not an efficient process in the convectively cooled Orientale impact melt sheet. Therefore, equilibrium crystallization is predicted to dominate during solidification of the Orientale impact melt sheet. This is in agreement with petrologic predictions suggesting the Orientale melt sheet underwent equilibrium crystallization (giving a final stratigraphic sequence of dunite, overlain by pyroxenite, superposed by a thick layer of norite; ). These results suggest up to ~5% of the lunar crust may have been homogenized by impact melting associated with large impacts. We outline a series of landing sites and commercial lander, rover and sample return missions that can address the questoins of the origin, age and evolution of the Orientale impact melts sheet. References:  Wood (1975) Proc. Lunar Sci. Conf., 6, 1087.  Walker et al. (1975) Proc. Lunar Sci. Conf., 6, 1103.  Warren (1985) Ann. Rev. Earth Planet. Sci., 13, 201.  Elkins-Tanton et al. (2011) EPSL, 304, 326.  Elardo et al. (2011) Geo. et Cosmo. Act., 75, 3024.  Nemchim et al. (2009) Nat. Geosci., 2, 133.  Vaughn et al. (2013) Icarus, 223, 749.  Cintala and Grieve (1998) Meteorit. Planet. Sci., 33, 889.  Grieve et al. (1991) JGR, 96, 22753.  Morrison (1998) LPSC, 29, 1657.  Head (1974) Moon 11, 327.  Davaille and Jaupart (1993) GRL, 20, 1827.  Martin and Nokes (1988) Nature, 332, 534.  Suckale et al. (2012) JGR, 117, E08004.  Suckale et al. (2012) JGR, 117, E08005.  Solomatov (2000) Origin of the Earth and Moon, 1, 323.  Spudis et al. (2014) JGR, 119, 19.  Cassanelli and Head (2016) GRL, 43, 156. Fig. 1. Maunder Formation impact melt sheet.