Impact-induced compositional variations on Mercury

- Mercury's early bombardment originated from an extended primordial asteroid belt.
- Repeated and overlapping impacts produce a heterogeneous surface.
- The maximum volume fraction of mantle on the surface is ∼30%.
- The darkening agent producing the LRM may originate from ∼30 km deep.

Remote sensing data suggest Mercury's surface has compositional variations spatially associated with crater and basin ejecta, the so-called “Low-Reflectance Material” (LRM), which has been suggested to be enriched in a subsurface native darkening agent that is excavated and redeposited onto the surface. This unit may record the evidence of impact-induced mixing of Mercury's outer layers during its early history. Here, we develop a fully three-dimensional Monte Carlo model of impact cratering, excavation, and ejecta blanket deposition on a global scale for Mercury.New dynamical simulations of the early evolution of the asteroid belt hint at the presence of additional asteroids in a region interior to the present-day belt, known as the “E-belt”. We use Monte Carlo methods to show that the predicted bombardment from this population matches the observed spatial crater densities on Mercury. Impacts large enough to pierce through the crust create surface ejecta deposits rich in mantle material. Later impacts onto enriched ejecta deposits redistribute mantle material away from the basins. For the suggested average mercurian crustal thickness of 50 km, the surface has, on average, ∼0.4% mantle material by volume; the most enriched areas have ∼30% mantle by volume.The regional coverage of impact-induced compositional changes is strongly dependent on the thickness to the subsurface source. Because observations indicate LRM covers ∼15% of Mercury's surface, our model suggests the darkening agent is ∼30 km deep. Considering the current estimated average mercurian crustal thickness of 50 km, this implies the darkening agent is likely located within a chemically distinct lower crust.