THE CALORIS IMPACT BASIN AND ITS ROLE IN MODIFYING THE SURFACE OF MERCURY

The Caloris basin is the youngest and best preserved impact structure of its size on Mercury. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft has provided important new information about the basin, allowing examination of its morphometry, filling and modification history, ejecta deposits and secondary craters, and the origin of smooth plains on its interior and exterior. Caloris is ~15% larger than was estimated from Mariner 10 images, which included only the eastern third of the basin, and appears to be moderately elliptical (1525 x 1315 km). Prominent basin-related sculpture and secondary craters from Caloris extend to distances of at least 1700 km from the basin rim, which makes the Caloris event an important stratigraphic marker. The basin interior is filled by smooth plains that are comparatively red and high in reflectance; these plains appear to embay the surrounding Caloris Montes rim material and are interpreted as volcanic in origin. In the center of the basin, these plains have been deformed into a system of radial graben, forming the Pantheon Fossae structure. This structure is unique on Mercury, and extensional deformation of any form has been observed only in two other locations, both within large impact basins. Smooth plains are also widespread outside the basin; unlike the interior plains, exterior plains display limited color contrast with their surroundings. One of the major units in the exterior plains, the Odin Formation, is particularly enigmatic and consists of plains characterized by hummocky hills or knobs a few kilometers across. It was suggested after Mariner 10 that the Odin Formation and other plains materials on the Caloris exterior may be predominantly ejecta from the basin itself. However, MESSENGER observations make clear that far fewer craters are superposed on these exterior (and interior) plains than the basin rim region, which requires that these plains units are younger than the basin. This result supports the hypothesis that these materials have been significantly resurfaced by volcanism. These findings re-emphasize the importance of understanding the Caloris basin for interpreting the geological history of Mercury.