Internal Activity of the Past and Present

The idea that the lunar crust is the product of differentiation in an ancient magma ocean is supported to some extent by compositional data, which show that lightweight rocks, containing such minerals as plagioclase, rose while denser materials, such as pyroxene and olivine, sank to become the source regions for the later radioactive heating episode that resulted in the outflows of mare basalts. Whether or not there ever was a uniform global ocean of molten rock, it is clear that the Moon's history is one of much heating and melting in a complex series of events that would have driven off volatiles (if any were present) and erased the record of earlier mineral compositions.

At present all evidence points to the Moon as a body in which, given its small size, all heat-driven internal processes have run down. Its heat flow near the surface, as measured at two sites by Apollo instruments, appears to be less than half that of Earth. Seismic activity is probably far less than that of Earth, though this conclusion needs to be verified by longer-running observations than Apollo provided. Many of the moonquakes detected seem to be only small "creaks" during the Moon's continual adjustment to gravity gradients in its eccentric orbit, while others are due to meteorite impacts or thermal effects. Quakes of truly tectonic origin seem to be uncommon. The small quakes that do occur demonstrate distinct differences from Earth in the way seismic waves are transmitted, both in the regolith and in deeper layers. The seismic data suggest that impacts have fragmented and mixed the upper part of the lunar crust in a manner that left a high proportion of void space. At depths beyond tens of kilometres, the crust behaves as consolidated dry rock.

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