| Institution(s) supplémentaire(s) | (2) Aurora Technology B.V., European Space Astronomy Centre/ESA, Madrid, Spain 3 Institut d’Astrophysique Spatial, CNRS/Université Paris Sud, Orsay, Franc |
Abstract | The study of geological processes such as cratering and volcanism provides information on the evolution of Mercury's surface and its interior since its formation. The Caloris basin is one of the most important geological features on Mercury giving an overview of morphological, chemical and spectral units (Fassett et al. 2009; Nittler et al. 2020) as well as deep material (Klima et al. 2018). It is commonly accepted that the basin is associated with two smooth plains, one in its interior and one surrounding the basin (Rothery et al. 2017). These spectrally different smooth plains appear to be both volcanic in origin and emplaced after the formation of the basin, certainly partially related with the long-term outcome of the basin formation (Roberts et al. 2012). The exterior plain is used to define the Low reflectance Blue Plains (LBP) while the interior plain together with the northern plains form the High reflectance Red Plains (HRP). The formation of large impact craters brings to the surface deep material such as the ancient graphite crust of Low Reflectance Material (LRM) buried under the secondary magmatic crust.
We present in this work a new spectral study of the Caloris Basin in order to improve our understanding of the basin emplacement and the avent of volcanic smooth plains in interaction with the underlying layers including LRM. Our method was to make use of the home-built Mercury Surface Spectroscopy (MeSS) database (this conference, Munoz et al. 2022 and Cornet et al. 2022). MeSS provide useful spectral parameters extracted from the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) instrument, onboard MESSENGER. Thanks to a rigorous statistical approach, we were able to characterize uniquely the interior HRP and exterior LBP within the Caloris basin, as well the LRM material.
Our approach allows to distinguish and map the interior plain from the exterior plain of the Caloris basin. Furthermore, we also show evidence of LRM excavated from craters inside the basin, supporting the idea that a layer of LRM was excavated by the emplacement of the main basin and then overlaid by at least one HRP volcanic layer. However, we also observe the presence of LRM concentrated in the western part of the exterior plain. These deposits are located in a geochemically different area from the overall outer plains. This result suggests that it is possible that the formation process of the exterior plain is not homogeneous all around the basin and could depend on different processes. The question of the age and timing of these interior and exterior smooth plains remain uncertain but our study is promising in order to better understand their interaction.
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