Spectral investigation of Mercury's pits' surroundings: Constraints on the planet's explosive activity

Abstract The understanding of volcanic explosive episodes on the surface of Mercury has important implications for the origin, history and inventory of Mercury’s volatiles and for the thermal evolution of the planet. The hermean explosive features are pits, interpreted as endogenic pyroclastic vents often surrounded by bright and diffuse deposits, named faculae, and interpreted as pyroclastic material. We present a global spectroscopic survey of the surroundings of the listed endogenous pits, derived from spectra obtained by the Mercury Atmospheric and Surface Composition Spectrometer (MASCS/MESSENGER). The spectral properties of 26 faculae around endogenous pits were characterized. The diameter of 80% of them has been reassessed upwards by 17 km on average, implying volatile contents needed to emplace these deposits consequently higher than estimated in previous studies. These observations suggest a higher bulk volatile content or subsurface volatile enhancement. Spectral similarities between faculae located in the same crater indicate either a similar magmatic source or a large fraction of country rocks. The size, volatile content, location, and spectral properties of Nathair Facula seem to indicate an anomaly with respect to the other faculae that may be explained by a phreatomagmatic-like event by interaction between hot-magma and local carbon-rich layer