On the thermal history, thermal state, and related tectonism of a moon of fission origin

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Abstract
The thermal history of an initially totally molten moon of fission origin properly accounts for (1) the mare basalt epoch, in terms of its duration, the depth of the source region, and degrees of partial melting which produced the magmas; (2) the present‐day heat flow of 17–18 ergs cm−2 s−1; and (3) the current high temperatures of the lower mantle as deduced from magnetic and seismic data. The model moon has a radius decrease of 5.4 km (3.1 times;10−3R) during lunar history. This value is within the rather poorly defined limits for the maximum change of the lunar radius of 10−3–10−2 R. The tectonic response to this 5.4‐km radius decrease and the associated thermoelastic stresses, stresses which have been dominantly tensional throughout lunar history, might be found in the larger‐scale normal faults, grabens, and tensional fractures of the lunar grid or lineament system. However, the majority of the thermoelastic stresses produced by the cooling of the moon have been dissipated via aseismic creep in the upper parts of the lunar mantle, not via faulting activity. A lower limit of 1024 P for the viscosity of the mantle of the moon (at subsolidus temperatures) is suggested, based on the apparent absence of solid state convection in the moon at any time during its history. This value is 103 times larger than that for the terrestrial mantle, a difference which might be due to the anhydrous nature of the lunar mantle versus the ‘wet’ mantle of the earth. The calculated energy derived from the thermoelastic stresses in the type A moonquake zone is orders of magnitude smaller than the available tidal energy. Hence the thermoelastic stresses are not an important energy source for the tidal moonquakes. The thermoelastic stresses can easily supply the energy for the high‐frequency teleseismic (HFT) moonquakes. The relative rarity of HFT's is explained by the long times (108–109 years) needed to accumulate the energy required to initiate faulting in the predicted source regions. These regions are in the uppermost mantle (depths between 80 and 200 km), where tensional quakes can occur, and at 10‐km depths in the crust, where compressional quakes can occur. The consistency between our thermal history model results and the corresponding characteristics now known for the moon add further support for the fission model for the origin of the moon.