SURFACE LAVA FLOW FEATURES ON MERCURY

The nature of volcanism on Mercury has important implications for terrestrial planetary evolution. Orbital data from the MESSENGER mission have affirmed earlier observations of an extensive area of smooth plains at high northern latitudes on Mercury; superposition relations and color data are consistent with a volcanic origin for these deposits. Few candidate vents and lava flow features have been identified, however, raising questions as to how these plains were emplaced, what volume of lava was erupted, and over what timescale volcanism occurred. We report on several channels proximal to the northern plains that cut through the surrounding intercrater terrain and strongly resemble surface flow feature observed on Earth and Mars. The channels exhibit braided textures and elongate, tapered “islands”, characteristic of erosion by the above-ground movement of a low-viscosity fluid. One channel adjoins two rimless depressions that may have coalesced from smaller pits, and which we regard as candidates for volcanic vents. At the distal end of this same channel lies a peak-ring impact basin that has been flooded by lava. Parts of the inner peak ring protrude above the younger volcanic deposits, which in turn have been deformed by contractional wrinkle ridges. The pits, channel, and basin infill may therefore represent a complete extrusive volcanic system, from eruption to emplacement. Also of note is the suppressed topographic expression of the intercrater plains material nearby, possibly indicative of a veneer of lava that flowed overland outside of the well-defined channels. These findings suggest that Mercury’s smooth plains were emplaced, at least in part, by discrete flows of low-viscosity lava. These flows may have exploited existing lineations formed by ejecta from nearby impact basins, such as secondary crater chains, reshaping them into the channels we observe today. We use the linear textures in the channels to determine flow directions and to place constraints on the local paleotopography, and we then calculate the volume of flooded terrain to provide a minimum value for the amount of material erupted in this area. Together with estimates of lava viscosity and the regional topographic gradient, these data constrain possible effusion rates, a parameter not yet readily measurable elsewhere on Mercury.