Soutenance de thèse d’Emma CAMINITI le vendredi 27 septembre 2024

Titre de la thèse

"The volcanic and space-weathered surface of Mercury : spectral properties and geological evolution."

La thèse est dirigée par Alain DORESSOUNDIRAM (LESIA) et Sébastien BESSE (ESA).

La soutenance sera également accessible en direct sur la chaîne Youtube du LESIA :

https://www.youtube.com/channel/UCzPLngWE_6JVuJ4szh8U-RQ.

Abstract

Observations from the MESSENGER mission have revealed various spectral, compositional, and geological units on Mercury’s surface, whose origins and relations remain uncertain. My research aims to gain a better understanding of Mercury’s spectral properties and geological history, considering both the volcanic character of the surface and the intense space weathering it experiences. First, I focused on mostly volcanic smooth plains associated with major impact basins, especially the Caloris basin, to better understand the planet’s geological history. Using MESSENGER-MASCS-VIRS data I developed a method to classify footprints according to spectral units defined after MESSENGER. This method enabled the identification of different volcanic infills, their relative timing, and consequently, the reconstruction of the geological history of the basins. Notably, I showed that the exterior plains of the Caloris basin formed before the interior plains and that changes in the conditions of partial melting at depth could explain the spectral and compositional differences on the surface. My comparative study of major impact basins revealed that the basin’s depth and/or induced thermal anomaly is the primary factor influencing the properties of volcanic infills. Furthermore, I showed that lateral heterogeneities in the mantle are not necessary to explain the spectral properties of volcanic plains associated with impact basins on the surface. While vertical heterogeneities are also not required, it remains to be proven that they do not contribute to some spectral properties. Since composition is not the only parameter responsible for the different spectral units, I investigated the role of space weathering, specifically the effect of solar wind ion irradiation on the alteration of spectral properties. I conducted ion irradiation experiments using the SIDONIE electromagnetic isotope separator interfaced with the INGMAR vacuum chamber through which I performed visible to near-infrared measurements (IAS, IJCLab, France). Mid-infrared measurements were performed ex-situ at Synchrotron SOLEIL (France). I showed that ion irradiation alters the spectral properties of Mercury analogues, leading to an exponential darkening, a reddening and a flattening of spectra before a putative saturation in the visible to near-infrared. Moreover, mid-infrared features are shifted due to ion irradiation. I found that the alteration of spectroscopic features is closely influenced by the composition, likely contributing to the origin of spectral heterogeneities on Mercury. Finally, ion irradiation may not be the primary process responsible for reducing spectral band signatures on Mercury. My combined work on MESSENGER data and laboratory analyses highlights the importance of interdisciplinary and innovative studies that merge different techniques to explore Mercury, in preparation for the BepiColombo mission. My work will be particularly relevant for future analyses of SIMBIOS-SYS and MERTIS data.