Impact melts of the Orientale and Imbrium basins

Abstract:

The largest impacts on the Moon – those that form the multi-ring basins – can produce thousands of cubic kilometers of melt. This melt is largely concentrated inside the basin, although some is ejected along with the clastic materials that make up the continuous ejecta blanket that surrounds basins. Impact melt is important because it contains information on the crustal target for basins as well as being the most suitable material to date basin-forming events. New geological mapping of the lunar Orientale and Imbrium impact basins has identified likely deposits of both types of impact melt. The Orientale basin (930 km diameter) is well preserved and only partly flooded by later mare basalts. The basin interior melt sheet is represented by the Maunder Formation, a smooth-to-cracked surface unit that covers the innermost basin ring. Study of the composition of the Maunder Fm. as determined by remote sensing shows that it is remarkably uniform both laterally and vertically, with no evidence of differentiation. Surrounding the basin are vast ejecta deposits, most of which are probably made up of clastic material. However, a few isolated deposits contained within basin secondary craters appear melt-like, with low albedo and a cracked surface texture (e.g., Struve L, 20.7° N, 76° W). The larger (1160 km diameter) and slightly older Imbrium basin is mostly filled with mare basalt lava, concealing most of the basin floor. The Imbrium basin exterior shows isolated deposits of melt-like material in several locales, including on the floors of the craters Parrot C (18.5° S, 1.2° E) and Murchison (5.1° N, 0.1° W). These deposits have low albedo and show cracked surfaces, with evidence of ground flow after deposition. Their composition is remarkably similar to highland basaltic impact melts found in the Apollo collections, such as the Apollo 17 impact melts. These features offer the possibility of examining basin impact melt at distances far removed from basin interiors or from basins that are deeply flooded by mare lava, covering the interior melt sheet. As such, these melt deposits are important sites for future robotic sample return missions.