Evolution thermique de la Lune: volcanisme et génération d’une dynamo
IPGP - Îlot Cuvier
Soutenances de thèses
Planétologie et sciences spatiales (PSS)
This thesis' aim was to study the thermochemical evolution of the Moon following the crystallisation of a magma ocean. Extrusive volcanism is highly asymmetric on the Moon and is strongly correlated with the surface distribution in heat sources (i.e., thorium, uranium, and potassium as mapped by the Lunar Prospector and Kaguya gamma-ray spectrometers). In particular, almost 99% of the Moon's lavas are found to have erupted on the hemisphere that faces Earth. We therefore used a 3-dimensional thermochemical convection code to investigate how an asymmetric distribution in heat sources would affect the Moon's geologic evolution. Our main result is that the observed distribution in extrusive volcanism is a natural consequence of an initial asymmetrical distribution of heat sources in the lower crust (Laneuville et al. 2012). This work has implications for several other observations and phenomena. First, a strong temperature anomaly is still present today below the heat source enrichment on the nearside hemisphere. Depending on several assumptions, this anomaly could affect crustal thickness modeling from gravity data, as well as interpretations of the Apollo seismic data. Another implication concerns impact processes. In particular, the warmer nearside crust gives rise to impact basins that are nearly twice as large as farside bains, consistent with gravity observations from the GRAIL spacecraft (Miljkovic et al. 2013). The peculiar mantle evolution induced by an asymmetric distribution of crustal heat sources distribution also has an influence on the lunar core. In the second part of this thesis, we studied the thermodynamical evolution of the core and how this related to dynamo generation. We have shown that once a dynamo is started by core crystallization, such a dynamo is hard to stop. Therefore, the existence of an inner core (if confirmed by further seismic analyses) offers tight constraints on the core's initial temperature and sulfur content. If no inner core is present, alternative and more exotic models would need to be considered, such as those with an initially stably stratified density profile in the mantle.