Mercury's Caloris basin: Continuity between the interior and exterior plains

The smooth plains on the floor of Mercury's Caloris basin and those almost entirely surrounding it beyond its rim are usually accepted to be younger than the rim materials and to be lava flows rather than impact melt. High-resolution imaging shows that the emplacement of interior and exterior plains was concurrent, with evidence of both inward and outward flow while they were being emplaced. The Caloris rim is breached in two places by continuous smooth plains that seamlessly connect interior and exterior plains. The gross-scale spectral and compositional distinctiveness of interior and exterior plains is blurred on a scale of several tens of kilometers, which could reflect interfingering of flow units less than a few hundred kilometers long that tapped melt sources of different composition and/or depth inside and outside the basin followed by local mixing of regolith. Flows occurring both inside and outside the basin should be included in estimates of the total erupted volume. Plain Language Summary The smooth plains interpreted to be lava flows inside and outside Mercury's Caloris basin have different color and different composition when viewed on broad scale. However, study of the highest-resolution images from NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission reveals places where there is a continuous lava surface between the interior and exterior plains and a gradual, not sharp, color change. There are also places where exterior plains lava has cascaded over the basin rim toward the basin floor and others where the interior plains seem to flood outward against or over the rim. It seems that interior and exterior plains were made by flood volcanism occurring during an overlapping time period. The lack of a sharp color boundary between the interior and exterior plains is evidence of multiple individual flow units that were less than a few hundred kilometers in length.