New morphologic map and analysis of orientale basin ejecta

Introduction: Orientale Basin, an impact structure located on the western limb of the nearside of the Moon, is one of the best preserved multi-ring basins in the Solar System. Orientale is thought to have formed 3.8 billion years ago [1], marking the end of the Late Heavy Bombardment of the lunar surface. The age of Orientale, and the almost complete absence of degredational processes after it formed, facilitated the excellent preservation state of the ejecta deposits. Here we have used high resolution images and topographic datasets derived from the instruments onboard the Lunar Reconnaissance Orbiter (LRO) to map in detail the locations and extent of the various distinctive ejecta facies. The type, distribution, and interpretations of these ejecta facies reveal patterns that provide insight into the formation and emplacement behavior of the Orientale ejecta, which also provide insights into other basin-scale impact structures and their influence on the origin and evolution of planetary crusts.. Methods: Initially, ~100 m/pixel Lunar Reconnaissance Orbiter Wide Angle Camera (LROC-WAC) images [2] along with elevation data derrived from the Lunar Orbiter Laser Altimeter (LOLA), were used to characterize the surface morphology/morphometry of the ejecta deposits surrounding Orientale. We catalogued unique facies based on morphology (i.e., texture, relative tonality and structures) as well as documenting their geographic setting, surface area, and stratigraphic relation to one another. Using the Java Mission-planning and Analysis for Remote Sensing (JMARS) [3], shapefiles were drawn between areas expressing different surface morphologies. As the focus of this study was to map and interpret ejecta deposits from Orientale, we chose to focus only on the region exterior to the Outer Rook ring (~310 km from the centre), which is thought to approximate the diameter of the Orientale Basin transient cavity [1,4]. Since any ejected materials would have been emplaced outside the developing transient cavity [e.g., 5] and the Outer Rook ring provided an easily recognizable topographic feature of approximately the same diameter, it was chosen as the inner boundary for our mapping effort. Furthermore, recent mapping by Spudis et al. [6] includes detailed mapping of this region, which encompasses the basin interior, and extends outward to the Cordillera ring (~600 km). Results and Interpretations: Our mapping effort resulted in the most detailed morphologic map of the Orientale Basin ejecta materials to date (Fig. 1). We define five morphologically distinct ejecta facies associated with the basin: Facies A. Facies A (blue) is an inner facies characterized by grooves extending radially outward from the the basin centre. It extends ~300 to as much as 800 km from the Cordillera ring, which marks it inner boundary. Facies A covers a total surface area of ~1,750,000 km. This facies is interpreted as the initial ballistic ejecta blanket from the Orientale impact.