Rembrandt Impact Basin on Mercury: Determining the Origin of Low- and High- Albedo Smooth Plains

Introduction: Rembrandt impact basin (~715 km in diameter) is one the largest and most well-preserved impact basins on Mercury [1]. Crater density measurements indicate that Rembrandt is comparable in age to the Caloris impact basin, the largest and also one of the youngest basins on Mercury [e.g., 2, 3]. The basin rim-crest is visible and several massifs define an inner ring approximately 450 km in diameter. A second inner ring is suggested by a circle of wrinkle ridges in the innermost part of the impact basin [2]. Rembrandt basin is both surrounded by and contains smooth plains deposits of varying albedo. This observation has been made using the MErcury Surface, Space ENvironment, GEochemistry and Ranging (MESSENGER) Mercury Atmospheric and Surface Composition Spectrometer (MASCS) and Mercury Dual Imaging System (MDIS) datasets [4-6]. Like most of the impact basins on Mercury, Rembrandt is filled with and surrounded by HRP [7] smooth plains which are interpreted to be formed volcanically [8]. However, low-albedo plains can also be found within and around the exterior of Rembrandt. This low-albedo material is classified as low-reflectance material (LRM [7]) and has been found around several other large impact basins including Tolstoj and Caloris; this LRM material is interpreted to be excavated lower crustal or upper mantle material [9]. Most of the plains deposits on Mercury have been interpreted as volcanic, including the smooth plains (i.e. HRP) [7, 8] and the intercrater plains [e.g., 10]. However, as exemplified by deposits on the Moon, smooth plains can also form by impact related processes , either as ponded impact melt or fluidized impact ejecta [e.g., 11]. Impact melt scaling laws indicate that impacts on Mercury should produce proportionally more impact melt [12], but there is little conclusive evidence for extensive impact melt deposits around large impact basins. The variable albedo of smooth plains around Rembrandt basin inspired us to assess the importance of basin impact melt on the surface modification of Mercury. Specific questions to address include: Are there different types of smooth plains deposits around Rembrandt? Are all of the smooth plains formed by the same process? If so, are the plains formed volcanically or by impact-related processes? What was the sequence of geologic events at Rem-brandt impact basin? Methods: The MESSENGER MDIS dataset was used in this study to map and characterize the smooth