Influence of an insulating megaregolith on heat flow and crustal temperature structure of Mercury

Mercury is covered by a megaregolith layer, which constitutes a poor thermally conducting layer that must have an influence on the thermal state and evolution of the planet, although most thermal modeling or heat flow studies have overlooked it. In this work we have calculated surface heat flows and subsurface temperatures from the depth of thrust faults associated with several prominent lobate scarps on Mercury, valid for the time of the formation of these scarps, by solving the heat equation and taking into account the insulating effects of a megaregolith layer. We conclude that megaregolith insulation could have been an important factor limiting heat loss and therefore interior cooling and contraction of Mercury. As mercurian megaregolith properties are not very well known, we also analyze the influence of these properties on the results, and discuss the consequences of imposing the condition that the total radioactive heat production must be lower than the total surface heat loss (this is, the Urey ratio, Ur, must be lower than 1) in a cooling and thermally contracting planet such as Mercury at the time of scarp emplacement. Our results show that satisfying the condition of Ur < 1 implies that the average abundances of heat-producing elements silicate layer is 0.4 times or less the average surface value, placing an upper bound on the bulk content of heat producing elements in Mercury's interior.