A Spectral Study of the Caloris Basin on Mercury and the Origin of Associated Volcanic Smooth Plains.

Results from the NASA/MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission showed that 27% of Mercury's surface is covered by smooth plains mostly produced by extensive effusive volcanism. The Caloris impact basin is associated with two, mostly volcanic, smooth plains, one in its interior and one surrounding the basin. Previous studies have shown that it is difficult to estimate the relative ages of the interior and exterior plains and to explain their spectral and compositional differences. We perform an exhaustive spectral analysis of the basin based on data obtained by the Mercury Atmospheric and Surface Composition Spectrometer spectrometer onboard MESSENGER between 300 and 1,450 nm with a 5 nm resolution. Unlike previous results based on crater counts, we found that the exterior plains were emplaced before the interior plains although both postdate the formation of the basin. We propose a decrease in the partial melting degree and/or partial melting depth over time to explain the spectral and compositional differences. The study highlights spectrally heterogeneous exterior plains associated with a compositional heterogeneity and the presence of low reflectance material (LRM) deposits. This heterogeneity in the exterior plains could be related to the formation of the basin and asymmetry of the ejecta deposits or to a heterogeneous distribution of LRM present at depths before the formation of the basin. Plain Language Summary: Results from the NASA/MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission highlighted several geological units at the surface of Mercury. The volcanic plains are defined as a sparsely cratered unit with several embayments of other landforms. Most of these plains formed by effusive volcanism before 3.5 Ga. Caloris, the largest well‐preserved impact basin, is associated with two plains, mostly volcanic, one filling the basin and one surrounding the basin, which have different spectral and compositional properties. Previous studies have shown that it is difficult to estimate the relative ages of the interior and exterior plains. In this work, we perform an exhaustive spectral analysis of Caloris based on MESSENGER data. Unlike previous results, we found that the exterior plains are older and were emplaced before the interior plains although both postdate Caloris formation. We propose an evolution of the volcanic source conditions over time to explain the spectral and compositional differences between the exterior and interior plains. A decrease in the partial melting degree and/or partial melting depth over time leads to modifications of magma properties and different compositional and spectral properties of lava. This study highlights heterogeneous exterior plains with east‐west asymmetry. Key Points: The exterior volcanic plains of Caloris were emplaced in and around the basin followed by the interior plains within the basin onlyThe melting degree and/or melting depth decrease over time between the formation of the exterior and interior plainsSpectral and compositional data show heterogeneous exterior plains with east‐west asymmetry [ABSTRACT FROM AUTHOR]