Volcanism on Mercury: Characteristics and Distribution from the First MESSENGER Flyby

The first MESSENGER flyby of Mercury obtained Mercury Dual Imaging System (MDIS) image data for 21% of the surface unseen by Mariner 10. These data have helped to address and resolve a series of questions related to the existence, nature, and distribution of volcanism outstanding since 1975. Numerous volcanic vents, in the form of irregularly-shaped rimless depressions, are seen around the interior margin of the Caloris basin; some have surrounding shield-like or flow-like structures, in one case a shield in excess of 100 km in diameter. In several cases, the vents appear to be sources of explosive volcanism that formed bright haloes around the vents. The interior of the Caloris basin is filled with plains units spectrally distinctive from the basin rim deposits, and impact crater stratigraphy shows evidence that these units are volcanic in origin. Some of the proximal smooth plains surrounding the exterior of the Caloris basin show distinct differences in color and morphological properties, supporting a volcanic origin; many of the circum-Caloris smooth plains, however, show no spectral distinctiveness compared with surrounding crustal material, and thus an impact- ejecta-related origin for some of these plains remains a viable hypothesis. Some smooth and intercrater plains units distal to the Caloris basin show evidence of flooding and embayment relations unrelated to Caloris ejecta emplacement; local and regional geological and color relationships support a volcanic origin for these plains. Large impact craters show a sequence of embayment of interior floor and exterior ejecta deposits that supports a volcanic origin for the embayment and filling processes. 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; these thicknesses are less than the amount thought to be required to cause exterior loading of the Caloris basin that would result in uplift of the basin interior. 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 interpreted as a radial dike swarm linked to the late-stage evolution of the Caloris basin interior, although hypotheses for this feature unrelated to magmatism have also been advanced. These new data provide evidence that volcanism was important in shaping the surface of Mercury.