Recent Dark Pyroclastic Deposits on Mercury

• Published in: Geophysical research letters Vol. 48; no. 9
• Main Authors: Xiao, Zhiyong, Xu, Rui, Wang, Yichen, Chang, Yiren, Xu, Ru, Cui, Jun
• Format: Journal Article
• Language: English
• Published: 16.05.2021
• Subjects: country rock; Mercury; MESSENGER; pyroclastic deposits; volcanic pit
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2021GL092532

Understanding 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. Plain Language Summary The prevailing explanation for the low reflectance of Mercury is graphite, which was possibly differentiated from a hypothesized magma ocean. Graphite is predicted to be stable in Mercury's mantle, and oxidization of graphite may be a major source of volatiles that drove explosive volcanism on Mercury. Typical pyroclastic deposits on Mercury have higher and steeper reflectances than the global average, which were ascribed to the loss of graphite. For the first time, dark pyroclastic deposits are discovered on Mercury, appearing as low‐reflectance and diffuse‐edged thin cover around rimless depressions. We found two candidate dark pyroclastic deposits that were formed in Mercury's most recent history, and the eruption occurred along pre‐existing crustal weaknesses. For one such case, an absorption feature attributable to graphite is visible in both the country rocks and diffuse pyroclastics, indicating that graphite in the magma was incompletely oxidized, or blasted country rocks that contain graphite exist in the pyroclastics. For another candidate, the graphite absorption feature is visible in the country rocks but absent in the pyroclastics. The pyroclastics have redder reflectance spectra, indicating metallic iron is the possible darkening phase. Approximately 12.5% of recognized low‐reflectance materials on Mercury does not exhibit a graphite absorption either. Key Points We describe candidate dark pyroclastic deposits that were formed by vulcanian eruptions in Mercury's Kuiperian age One deposit has an absorption attributable to graphite, suggesting a dominance of country rock or incomplete exsolution of magmatic carbon A second deposit exhibits no graphite or other absorption features, but metallic iron is a possible darkening phase