Inventory, morphologies and ages of impact basins on Mercury based on image-, altimetry-, and gravity data from the MESSENGER mission

Kurzfassung

Mercury, the smallest and innermost planet in the solar system, offers unique opportunities to study planetary formation and evolution due to its exceptional position, composition and thermal regime. Its proximity to the Sun and extreme environmental conditions provide critical insights into the processes that shape terrestrial planets. Despite being the focus of two dedicated missions, Mariner 10 and MESSENGER, Mercury remains one of the least studied planets in our solar system. While these missions provided valuable surface images and initial datasets, many aspects of its geological history and ongoing processes remain unresolved. One of Mercury's most prominent surface features are its widespread impact structures. However, the formation mechanisms and post-impact evolutionary processes of these structures are not yet fully understood. To address these gaps, this work presents an updated and extended catalog of over 347 impact structures, incorporating detailed morphological classifications, morphometric and gravity anomaly measurements. This inventory was developed using high-resolution Digital Terrain Models (DTMs) and gravity field data, with Bouguer anomaly serving as a supporting dataset. The results reveal drastic density variations in Mercury's subsurface, as well as insights into the viscoelastic relaxation of basins. During impact basin formation, the elevated crustal temperatures facilitate rapid crustal flow, resulting in material redistribution towards the basin's center. Bouguer anomalies effectively capture post-impact density variations driven by mantle uplift and isostatic adjustments. Consequently, Bouguer anomaly contrasts, that are defined as the difference between the anomaly at the basin center and its rim, are employed as proxies for basin relaxation and investigated within Mercury’s thermal regime. Contrary to expectations, the Bouguer anomaly contrasts do not align with Mercury's current surface temperature patterns. Further analysis of other surface features, including compressional tectonic faults, volcanic smooth plains, crustal thickness variations and crater preservation, highlight a pronounced thermal influence on the eastern hemisphere, while the western hemisphere appears less affected. These findings suggest that the eastern hemisphere may have experienced prolonged exposure to elevated temperatures, potentially caused by Mercury’s past orbital dynamics and primordial spin-orbit resonance configurations. Mercury's thermal asymmetry may result from a past 1:1 (or 2:1) spin-orbit resonance, that has been already under discussion in the scientific community. Such a configuration, wherein the eastern hemisphere faced prolonged exposure to the Sun, would have led to significant heat transfer from the surface propagating to the subsurface, influencing crustal temperatures and driving the observed geological asymmetries. This work represents a substantial contribution to planetary science by delivering an extended and detailed database of Mercury's impact structures and advancing the understanding of its geodynamic evolution. The findings not only enhance our knowledge of Mercury’s geological processes, that are related to impact structures, but also provide a framework for interpreting thermal and orbital influences on other terrestrial planets.