Spectral identification of pyroclastic deposits on Mercury with MASCS/MESSENGER data

Abstract

The pyroclastic deposits on Mercury are proof of explosive volcanism that occurred on the innermost planet of the Solar System; pyroclastic deposits are usually characterized by a central pit (vent) surrounded by a spectrally bright and red deposit (facula).The pyroclastic deposits on Mercury were mainly identified by analysing images acquired from MDIS onboard the MESSENGER mission, which revealed about 200 deposits.In this work, we focus on the identification of pyroclastic deposits on Mercury by applying spectral conditions on radiometrically and photometrically corrected data acquired by the MASCS spectrometer onboard the MESSENGER spacecraft. In addition, to further optimize the quality of the used spectra, the spectral dataset was limited by applying additional filters on observing conditions and on detector temperature.We selected 8 pyroclastic deposits, out of the 25 deposits recognized by the IAU, as a training field, and we spectrally characterized the area of the deposits by using four spectral parameters, i.e. reflectance and spectral slopes. We defined the range of variability of the parameters able to distinguish the pyroclastic deposits from the Average Mercury Terrain, and we filtered the MASCS data set by applying the spectral conditions. As a result, 52 previously recognized pyroclastic deposits and 16 new potential candidates were revealed across the portion of Mercury's surface observed by MASCS. The method also exposed extended areas such as the High-Reflectance Red Plains (HRP) in Asparangi Planitia, Borealis Planitia, and the floor of Rembrandt crater, in addition to Intermediate Terrains (IT) and Intermediate Plains (IP).Nathair Facula, Agwo Facula, Abeeso Facula and an unnamed facula that we termed “Facula 5” show a redder UV slope than most of the recognized pyroclastic deposits, possibly associated with fine regolith and/or a minor amount of Fe or C-phases in the minerals composing the facula.Among the 16 new bright and red areas, 6 potential candidates strongly suggest a nature connected to explosive volcanism and could be considered as pyroclastic deposits.