Intercrater plains on Mercury: Insights into unit definition, characterization, and origin from MESSENGER datasets

Orbital observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft are used to re-evaluate the nature and origin of the oldest mapped plains deposits on Mercury, the intercrater and intermediate plains units defined by Mariner 10 investigators. Despite the large areal extent of these plains, which comprise approximately one-third of the planetary surface area viewed by Mariner 10, their formation mechanism was not well constrained by Mariner 10 imaging. One hypothesis attributed plains formation to ponding of fluidized impact ejecta to create relatively smooth surfaces. Another hypothesis was that these plains are of volcanic origin. To assess the origin of these older plains and the contribution of early volcanism to resurfacing on Mercury, we have used MESSENGER data to analyze the morphology, spectral properties, impact crater statistics, and topography of Mar-iner 10 type-areas of intercrater and intermediate plains. On the basis of new criteria for the identification of intercrater and intermediate plains derived from these observations, we have remapped 18% of the surface of Mercury. We find that the intercrater plains are a highly textured unit with an abundance of secondary craters, whereas the intermediate plains are composed of both intercrater and smooth plains. We suggest that the term ''intermediate plains'' not be used to map the surface of Mercury henceforth, but rather this unit should be subdivided into its constituent intercrater and smooth plains units. We argue that a substantial percentage of the intercrater plains are composed of volcanic materials on the basis of (1) examples of areas where ejecta from a small number of superposed craters have transformed smooth plains deposits of volcanic origin into a unit indistinguishable from intercrater plains; (2) the range in ages of intercrater plains deposits as interpreted from crater size–frequency distributions; and (3) the near-global distribution of intercrater plains compared with the uneven distribution of impact basins and their associated ejecta deposits. The first spacecraft images of Mercury were obtained by Mari-ner 10 (M10) during three flybys in 1974–1975. M10 imaged approximately 40% of the planet's surface (Fig. 1), $55% of which was covered with several different plains deposits. The earliest geological interpretation of images from the first M10 flyby (Murray et al., 1974) included a regional map showing three distinct geologic units: plains material, hilly and lineated terrain, and heavily cratered terrain. The intercrater plains unit shortly thereafter was identified as a subdivision of this ''heavily cratered terrain'' marked …