Recent Dark Pyroclastic Deposits on Mercury

Abstract

AbstractUnderstanding the origin of volatiles and identifying opaque phases on the surface of Mercury are important tasks to be completed to understand the planet's building blocks and early evolution. Pyroclastic deposits, typically featuring higher and steeper reflectance spectra than the global average, are a bridge connecting the two tasks. We report the first discovery of dark pyroclastic deposits on Mercury, which exhibit identical morphology and geometry with typical reddish pyroclastics but comparable reflectances to low‐reflectance materials (LRM). Two Kuiperian‐aged dark pyroclastic deposits and many potential older ones are discovered. Reflectance spectra for one Kuiperian‐aged pyroclastic deposit exhibits an absorption feature attributable to graphite, indicating incomplete exsolution of C from the pre‐eruption magma and/or a dominance of blasted country rocks in the pyroclastics. Our second Kuiperian‐aged case and also ∼12.5% of the global LRM exhibit no graphite or other absorption features, and metallic iron may be the alternative darkening phase.