Basin formation on Mercury: Caloris and the origin of its low-reflectance material

Abstract

The ∼1500 km diameter Caloris basin is the largest, most well-preserved impact structure on Mercury. The interior of Caloris contains an enigmatic geological unit at depth subsequently excavated by smaller impacts – low-reflectance material (LRM). LRM is interpreted to be the original basin floor material and be composed of melted and re-crystallized mantle material. In this work, a comprehensive numerical modeling study of Caloris basin formation is undertaken to provide insight into basin formation on Mercury (which may differ to that on the Moon), and to address the origin of the LRM. Analysis of the modeled basin attributes demonstrates that the Caloris impact would have penetrated to depths approaching the core/mantle boundary, excavating mantle material to the surface and producing melt volumes on the order of 107 km3. Such a melt volume could differentiate, forming a layer (buried beneath later volcanism) that could be the LRM, in agreement with previous spectroscopic analyses. Furthermore, LRM deposits at other smaller basins on Mercury may also represent impact melted mantle material. A comparison to lunar basin counterparts demonstrates that basin formation on Mercury is similar to that on the Moon. This observation suggests that the observed difference in the number of large basins between the two bodies (fewer on Mercury) is due to processes that modify basins (i.e., greater impact melt volume and more extensive volcanism on Mercury), rather than first-order impact process differences. The greater average impact velocity and gravity exert a slight effect on some basin properties, including an increased excavation depth-to-transient crater diameter ratio on Mercury.