WIDESPREAD AND VOLUMINOUS FLOOD VOLCANISM IN THE NORTHERN LOWLANDS OF MERCURY REVEALED BY MESSENGER

The presence and importance of volcanism on Mercury was extensively debated following the Mariner 10 flybys in 1974–75. Some supported a volcanic origin for Mercury’s plains, but others noted their similarity to lunar plains formed by fluidized impact ejecta. Uncertainty persisted until the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) flybys in 2008–2009 confirmed that volcanism had been widespread on Mercury. After those flybys, questions that remain include: the global extent of volcanic plains, their style and mode of emplacement, their temporal distribution, and their association with other geological features. MESSENGER high-resolution images of Mercury obtained from orbit provide insight into these issues and reveal that a large contiguous area of smooth plains that exceed ~6% of the surface of the planet occupies much of Mercury’s high northern latitudes. Smooth surface morphology, embayment relationships, color data, candidate vents and flow fronts, and a population of partly to wholly buried (ghost) craters provide evidence for a volcanic origin for these plains and emplacement in a flood lava mode to depths in excess of a kilometer. Ghost craters can provide clues as to the thickness of the flooding deposits. Assuming that the ghost craters were fresh at the time of flooding, morphometric relations (depth versus diameter; height of rim crest above adjacent terrain) can be used to assess the amount of flooding. Mapping of all partly buried and ghost craters visible within the smooth plains reveal buried craters >60-70 km in diameter; thus, if the smooth plains are filling a large degraded impact basin, that basin must be extremely old, comparable to some of the oldest terrain on Mercury. Ghost craters >100 km in diameter in the interior of the smooth plains suggest plains thicknesses locally in excess of 1-2 km, with the plains appearing to thin toward their margin, where smaller-diameter ghost craters are more common. The age of these plains is similar to those associated with and postdating the Caloris impact basin, confirming that volcanism was an important process in the post-heavy-bombardment era on Mercury.