A complex Hermean volcanic history

Summary:  The volcanic history of Mercury is slowly being revealed as the Mercury aficionado community is analysing data returned by the MESSENGER mission. Although the presence of products of effusive and explosive volcanism is not disputable anymore (Byrne et al., 2018), their origin, evolution, and age is still a matter of debate. The description of Hermean volcanism is often the result of analysing orbital observations while their connection to the interior properties is still largely unanswered. Using MESSENGER/MASCS spectral scientific observations, we show that a heterogeneous mantle is not needed to explain the lateral variability of volcanic products seen in Mercury’s basins. Similarly, the support of deep learning techniques in analysing spectra from explosive volcanism highlights that this volcanic activity may have been long-lived on Mercury, likely younger than 1 Gy.

Do we need an heterogeneous mantle? The study of geological processes such as impact cratering and volcanism provides information on Mercury’s evolution. Volcanism has shaped Mercury's surface and considerably modified impact basins after their formation by the emplacement of younger volcanic infills. Caminiti et al. (2023) clarified the volcanic history of the Caloris basin by classifying MESSENGER MASCS footprints according to spectral units (Murchie et al., 2015). We applied this same spectral classification to the Rembrandt basin, improved the classification and characterized a new younger volcanic spectral unit (Helbert et al., 2013; Semenzato et al., 2020). These improvements allow us to distinguish low-reflectance material, high-reflectance red plains, low-reflectance blue plains and intermediate plains as well as to define the younger high-reflectance red plains. We investigated and compared major impact basins on Mercury: Caloris, Rembrandt, Tolstoj, Beethoven and Rachmaninoff. A detailed analysis of each basin allows us to clarify their geological histories including the number of volcanic infillings as well as to confirm that volcanic smooth plains are spectrally heterogeneous (Helbert et al., 2013). Similarities between spatially distributed basins highlight that spectral units associated with basin infills have no spatial and compositional dependence suggesting no lateral heterogeneity in the mantle. However, it depends on the size of the basin. This could be linked to vertical heterogeneities or different intensities of deep perturbations by impacts leading to different melt production and evolution through time. BepiColombo data are eagerly awaited to investigate the compositional variability of volcanic smooth plains and refine the definition of spectral and morphological units.

Is explosive volcanism extremely young? Explosive volcanic activity on Mercury extended after the end of the widespread effusive volcanism era. While prior research has recognized a prolonged period of explosive volcanic activity, the specific eruption timing for individual pyroclastic deposits remains unknown. We explored the evolution of explosive volcanism by examining the relationship between the morphological degradation of the vents and spectral changes in the associated deposits. We found a diverse range of spectral properties in pyroclastic deposits, which are typically characterized by increased brightness, a red spectral slope, and a higher curvature compared to the average surface. We observed a trend between the deposit spectra and the vent degradation characterized by a rapid initial darkening and flattening over time followed by stabilization. The oldest deposits reach a steady state with no further spectral changes. To explain these temporal variations in spectral properties, we propose three potential processes: space weathering, mixing with the background and changes in pyroclast size over time. We examine the implications of space weathering on spectral properties and discuss the eruption timeline for each scenario. This research aims to gain insight into Mercury’s volcanic history and the processes that have shaped its surface over time.