Buried Bright, Spectrally Steep Materials on Mercury: A Possible Record of Ancient Explosive Volcanism

1. Introduction
Mercury’s surface records the cumulative effects of impact cratering, volcanic resurfacing, space weathering, and contractional deformation. These processes commonly overprint one another, making it difficult to distinguish ordinary regolith maturation from primary compositional or textural heterogeneity. Space weathering modifies reflectance and spectral slopes, while impact reworking may obscure pre-existing units or excavate shallow subsurface materials [1]. In this context, spectrally anomalous crater-related deposits can provide windows into buried materials no longer clearly expressed at the surface.

The Glinka crater region, in Mercury’s Beethoven quadrangle (H-07), is a particularly informative case because mature plains, crater-related deposits, vent-proximal bright materials, hollows, and contractional landforms occur within the same stratigraphic setting. Glinka’s bright deposits have previously been linked to explosive volcanism and facula-like materials on Mercury [2]. Here, we reassess the geological significance of bright and spectrally steep materials in and around Glinka, focusing on their stratigraphic relationships and implications for the timing of explosive volcanism.

2. Data and approach
We integrated MESSENGER MDIS Narrow Angle Camera (NAC) images, Wide Angle Camera (WAC) multispectral data [3], and the H-07 stereo-derived digital elevation model [4] to compare morphology, spectral behavior, and relative stratigraphy. Spectral variability was described using reflectance at 750 nm and continuum slopes across the MDIS wavelength range, following parameter-based approaches commonly applied to MDIS–WAC datasets [5] where diagnostic absorption bands are weak or absent. These parameters distinguish mature dark/red-sloped terrains, fresh bright/flat crater materials, and anomalous bright materials with steep spectral slopes. The interpretation is constrained by superposition and crosscutting relationships, crater-degradation states [6], and the spatial association between crater-related ejecta and vent-proximal deposits

3. Results
The mapped units define two contrasting spectral trends. The first follows the expected optical-maturity sequence, from fresh bright/flat crater material to intermediate ejecta and darker, spectrally steeper mature plains. The second departs from this sequence. Two crater-related units, Bright–Reddened ejecta (BR) and Spectrally Steep ejecta (SE), combine relatively high reflectance with steep VIS–NIR spectral slopes, a behavior not readily explained as a simple intermediate stage of ordinary space weathering.

BR and SE occupy a spectral domain comparable to the vent-related deposits exposed on the floor of Glinka crater, especially the bright central facies associated with the irregular depression interpreted as the volcanic vent area. They are therefore interpreted as crater-related exposures of a shallow bright and spectrally steep material, rather than merely as ejecta with different degrees of optical maturity. The lack of clear NAC-scale evidence for extensive smooth melt ponds or large impact-melt accumulations further supports this interpretation. Although a contribution from impact-generated glass or melt-bearing ejecta cannot be excluded, excavation and redistribution of a common pre-existing shallow component appear more likely than a purely impact-melt origin.

4. Interpretation
The stratigraphic implications are significant. BR is associated with a moderately degraded crater and is most compatible with a Calorian placement, whereas SE is linked to a fresher crater-related unit and is most compatible with a Mansurian placement. Because the anomalous material is sampled by BR, it must have been present in the shallow subsurface before the BR-forming impact. Its emplacement age cannot be directly constrained, but it is compatible with a pre-Calorian origin, possibly as early as the Tolstojan interval. If genetically related to explosive volcanism, this material may record an older pyroclastic episode whose primary surface expression was later buried, reworked, or erased, but whose spectral signature remained detectable where later impacts excavated it.

The vent-related deposits currently exposed on the Glinka floor represent a later stage. Because they overlie the floor of Glinka crater, whose degradation state is consistent with a Tolstojan placement, these deposits must postdate the host-crater-forming event. Their strong spectral contrast relative to the surrounding mature plains suggests incomplete optical equilibration and supports emplacement toward the younger part of the post-Tolstojan window, most plausibly in the Mansurian, although a late Calorian onset or limited extension into the early Kuiperian cannot be excluded. This is consistent with evidence that explosive volcanism on Mercury was long-lived and extended across multiple geological periods [7].

Structural relationships further refine this sequence. One lobate scarp is progressively subdued where it intersects the Glinka floor deposits, suggesting that contractional deformation had already begun before, or partly overlapped with, vent-related emplacement. Conversely, another scarp deforms both the NIR-steep and bright, spectrally steep Glinka floor deposits and locally offsets the vent area, indicating that part of the contractional activity postdates emplacement of the exposed vent-related materials.

5. Implications
The Glinka region records a multi-stage evolution involving formation and maturation of the regional substrate, emplacement or preservation of a shallow bright and spectrally steep component, excavation of this component by the BR and later SE impacts, younger vent-related deposition within Glinka crater, and continued contractional deformation. More broadly, these results suggest that some spectrally anomalous crater ejecta on Mercury may act as windows into buried volcanic or compositionally distinct materials, rather than simply marking variations in impact freshness. This has direct relevance for future BepiColombo observations  [8], which may test whether similar bright and spectrally steep crater-related anomalies elsewhere on Mercury record buried pyroclastic materials, localized crustal heterogeneity, or both.

Acknowledgement: G.M. and M.I. acknowledge support from the Italian Space Agency (2022-16-HH.1-2024).

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