Most of the knowledge about the lunar interior has come from the Apollo missions and from robotic spacecraft, including Galileo, Clementine, and Lunar Prospector, which observed the Moon in the 1990s. Combining all available data, scientists have created a picture of the Moon as a layered body comprising a low-density crust, which ranges from 60 to 100 km (40 to 60 miles) in thickness, overlying a denser mantle, which constitutes the great majority of the Moon's volume. At the centre there probably is a small iron-rich metallic core with a radius of about 350 km (250 miles) at most. At one time, shortly after the Moon's formation, the core had an electromagnetic dynamo like that of Earth, which accounts for the remanent magnetism observed in some lunar rocks, but it appears that such internal activity has long ceased on the Moon.
Despite these gains in knowledge, important uncertainties remain. For example, there seems to be no generally accepted explanation for the evidence that the crust is asymmetrical: thicker on the Moon's far side, with the maria predominantly on the near side. Examination of naturally excavated samples from large impact basins may help to resolve this and other questions in lunar history.
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