Evidence for a carbon-rich Mercury from the distribution of low-reflectance material (LRM) associated with large impact basins

Low-reflectance material (LRM) on the surface of Mercury is thought to be darkened by 2-7 wt.% carbon, making Mercury's surface the most carbon-rich among the terrestrial planets, but the origin of this carbon is debated. We observe exposures of LRM within large impact basins, which naturally sample Mercury's outer layers, to produce the first observationally constrained estimate of the absolute quantity of carbon present in Mercury's shallow interior. We observe LRM and other spectrally distinct material associated with craters within large basins and use scaling laws to relate these observations to the stratigraphy and composition of the subsurface. We find that many large basins across Mercury's surface have thick layers of LRM in their subsurface. Based on inferences regarding the thickness of these layers, we estimate the absolute quantity of carbon present in Mercury's crust and upper mantle to be at least 1.8 x 1019 - 2.3 x 1020 kg, which permits evaluation of hypotheses as to its origin. This quantity rules out the hypothesis that carbon near Mercury's surface was delivered during late accretion of carbon-rich material, with implications for the delivery of carbon and volatiles to the terrestrial planets. It is also only marginally compatible with a magma ocean origin. Therefore, if Mercury's core and mantle equilibrated with respect to carbon, we infer that Mercury was probably carbon-saturated early in its evolution and that carbon is an abundant light element in its core, with important implications for Mercury's thermal and geological evolution. (c) 2023 Elsevier B.V. All rights reserved.