Analysis of Surface Volcanism on Mercury

Since the confirmation of widespread volcanism on Mercury by MESSENGER spacecraft observations, a key objective has been to understand how volcanic deposits were emplaced, what volume of lava was erupted, and over what timescale volcanism occurred in a given area. Many of Mercury's smooth plains, including a broad expanse of such plains at high northern latitudes well imaged for the first time, show evidence for a volcanic origin. Using orbital images from MESSENGER's Mercury Dual Imaging System (MDIS), we have identified a set of flow features proximal to these northern smooth plains deposits that provide a framework for understanding how smooth plains are emplaced across Mercury. Here, several channels cut through intercrater plains material, and strongly resemble channelized surface flow landforms on Earth and Mars. The channels exhibit the braided textures, deltas, and elongate, teardrop-shaped "islands" characteristic of erosion by the above-ground movement of a low-viscosity liquid - consistent with high-temperature lavas with komatiite-like compositions reported by MESSENGER's X-Ray Spectrometer (XRS) team. One channel links a set of candidate volcanic vents with a peak-ring basin that hosts smooth plains deposits, possibly representing a complete extrusive system from eruption to emplacement. Additionally, the surrounding intercrater plains display a range of topographic expressions, from rough to almost entirely smooth, implying flooding to various depths by lavas. These observations suggest that Mercury's smooth plains were emplaced, at least in part, by many discrete, low-viscosity, komatiite-like lavas that reshaped the surface over which they flowed. We use the linear textures in the channels to determine flow directions and to place constraints on the local paleotopography, and by mapping the area of flooded terrain in a geographical information system (GIS), we give a minimum value for the amount of material erupted in this region. Together with topographic profiles obtained from the Mercury Laser Altimeter (MLA), we provide possible effusion and flow rates within these channels for a range of lava viscosities, yielding an estimate for the time taken to form these northern deposits. The flooded textures seen adjacent to the northern smooth plains are replicated elsewhere on Mercury, and a search for other examples of flow channels now underway will act as a basis to extend our analysis of flow characteristics near these northern smooth plains to other areas, and eras, on Mercury.