Influence of Megaregolith on the Thermal Evolution of Mercury's Silicate Shell

A so-called megaregolith layer that is considered to be produced by continuous impacts in Mercury's early stages is integrated into the thermal evolution models of Mercury to study its influence on the thermal evolution of Mercury's silicate shell. This research first implements a one-dimensional parametric global thermal evolution model. Our results indicate that megaregolith directly affects the thermal evolution of Mercury's silicate shell by virtue of its good insulation performance. The way megaregolith exerts its influence is by prolonging the process of partial melting and reducing the heat loss, resulting in a thicker crust and thinner stagnant lid. As for the deep parts of the silicate shell, it is suggested that more energy is taken away from the mantle due to the longer partial melting, leading to lower temperatures below the crust compared with the case in the absence of megaregolith, which further helps to advance the formation time of the inner core and promote its final size. In addition, we also carry out a simplified two-dimensional mantle convection simulation as a supplement to the one-dimensional model. The two-dimensional simulation depicts a typical mantle plume fractional melting scenario. Our calculations indicate that megaregolith may be key to the long-term volcanic activities on Mercury. As far as the megaregolith itself is concerned, the thermal structure of this particular layer is more sensitive to thermal conductivity, suggesting that for such a highly fragmented structure, the thermal conductivity coefficient plays a key role in its evolution. Our work emphasizes the importance of megaregolith to the evolution of Mercury.