VOLCANISM ON MERCURY

The MESSENGER flybys of Mercury obtained image data for the vast majority of the planet (including most of that unseen by Mariner 10); these data have helped to resolve a series of questions outstanding since 1974 related to the existence, nature and distribution of volcanism. Numerous volcanic vents, in the form of irregularly-shaped rimless depressions, are seen around the interior margin of the Caloris basin and elsewhere on the planet, mostly on impact crater floors; several appear to be sources of explosive volcanism and in one case a depression is surrounded by a shield in excess of 100 km in diameter. The interior of the Caloris basin is filled with plains units spectrally distinctive from the basin rim deposits; impact crater stratigraphy and comparisons to the filling of the lunar Imbrium basin support a volcanic origin. Some smooth plains surrounding the rim of the Caloris basin show distinct differences in color and morphological properties, supporting a volcanic origin. Some smooth and intercrater plains units distant from Caloris show evidence of flooding and embayment unrelated to Caloris ejecta emplacement; local and regional geological and color relationships support a volcanic origin. The recently discovered 715-km-diameter Rembrandt basin shows evidence for interior volcanic fill. Large impact craters show a sequence of plains embayment of interior floor and exterior ejecta deposits that supports a volcanic origin. Crater embayment and flooding relationships suggest typical thicknesses of volcanic plains of many hundreds of meters, and local thicknesses inside impact craters of up to several kilometers. Evidence is seen for intrusive magmatic activity in the form of a floor-fractured crater with two laccolith-like structures. Pantheon Fossae, a large radial graben swarm originating in the Caloris basin center, can be plausibly interpreted as a radial dike swarm linked to the late-stage evolution of the Caloris basin interior. Regional and global plains units are being defined and classified using multispectral image data, characterized using spectrometer data, and further assessed using local and regional altimetry and topography data. Together these new data provide evidence that intrusive, extrusive and explosive volcanism were important processes in shaping the surface of Mercury.