Asymmetric thermal evolution of the Moon : LUNAR EVOLUTION

Rheological properties of the upper mantle of the Earth play an important role in the dynamics of the lithosphere and asthenosphere. However, such fundamental issues as the dominant mechanisms of flow have not been well resolved. A synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the Earth's upper mantle. The hot and shallow upper mantle flows by dislocation creep, whereas cold and shallow or deep upper mantle may flow by diffusion creep. When the stress increases, grain size is reduced and the upper mantle near the transition between these two regimes is weakened. Consequently, deformation is localized and the upper mantle is decoupled mechanically near these depths. Show more

High-resolution gravity data obtained from the dual Gravity Recovery and Interior Laboratory (GRAIL) spacecraft show that the bulk density of the Moon's highlands crust is 2550 kilograms per cubic meter, substantially lower than generally assumed. When combined with remote sensing and sample data, this density implies an average crustal porosity of 12% to depths of at least a few kilometers. Lateral variations in crustal porosity correlate with the largest impact basins, whereas lateral variations in crustal density correlate with crustal composition. The low-bulk crustal density allows construction of a global crustal thickness model that satisfies the Apollo seismic constraints, and with an average crustal thickness between 34 and 43 kilometers, the bulk refractory element composition of the Moon is not required to be enriched with respect to that of Earth. Show more