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.