THE GEOLOGY OF MEDIUM-SIZED IMPACT BASINS ON MERCURY

Peak-ring basins, characterized by an outermost basin rim and an interior ring of contiguous peaks, are a common landform on Mercury and provide a window into how surface and interior processes have changed over time on the innermost planet. In this paper we use orbital observations by NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft to examine the geological evolution of three young peak-ring basins on Mercury. Rachmaninoff (290 km in diameter), Raditladi (265 km), and Mozart (236 km) all contain smooth plains deposits within their interiors and pockets of what appear to be solidified impact melt atop their continuous ejecta deposits. Smooth plains material within Rachmaninoff’s peak-ring appears, on the basis of crater size-frequency distributions and color contrasts, to be the result of volcanism that occurred well after the impact basin formed. Inside Rachmaninoff’s rim is a bright diffuse deposit, similar to deposits elsewhere that have been interpreted to be of pyroclastic origin. Raditladi is one of the youngest basins on Mercury, with an age of perhaps only ~1 Gy. It is partially filled with smooth plains that are spectrally indistinct and not clearly different in age from the basin; whether these plains consist of impact melt or volcanic material emplaced shortly after basin formation is unclear. Mozart contains sufficient smooth plains to obscure more than half its peak ring. Despite apparent differences in age, relative volume, and perhaps origin of the materials filling the basins, all three contain sets of approximately concentric troughs in smooth plains within their central peak rings. Somewhat similar patterns of troughs are found within some ghost craters in Mercury’s northern volcanic plains and in plains exterior to the Caloris basin, although these do not display the dominantly concentric patterns of the troughs within the three basins. Among models proposed for the formation of the troughs within the three peak-ring basins, the most promising is a combination of flexural stresses from the interior response to the formation of the basin and thermal stresses accumulated during cooling of plains material.