Effects of lunar cumulate mantle overturn and megaregolith on the expansion and contraction history of the Moon

Gravity anomalies derived from recent Gravity Recovery and Interior Laboratory data suggest the presence of early volume expansion of the Moon. The absence of identifiable thrust faults limits the total net contraction that has occurred. These observations provide constraints on the lunar thermal evolution which raise questions for giant impact origin of the Moon. To study the lunar expansion/contraction history, we perform 3-D thermochemical mantle evolution models, with solidifying core overlain by a layer of ilmenite-bearing cumulates (IBCs) resulting from mantle overturn after magma ocean solidification. Our models focus on the effects of the overturn-produced density stratification with a deep heat-producing element (HPE) distribution and a top insulating megaregolith layer. The deep HPE can cause an early expansion up to 1.5 km radius due to the heating of thedeep mantle. This HPE distribution also reduces the present-day contraction by similar to 7 km. Compared to the models without overturn, an end-member model with a stable IBC-rich layer on the core-mantle boundary shows an overall present-day contraction as small as 1.1 km. The low thermal conductivity of megaregolith also affects the present-day contraction, reducing it by similar to 3 km.