Internal StructureThe average density of the Moon, calculated from its size and mass, is 3.34 g per cu cm (208.51 lb per
cu ft). The range of densities of the lunar surface rocks brought back during the Apollo program did not vary greatly from this figure. This means that materials in the Moon's
interior probably do not vary much from those on the surface. The pressure at the center of a more or less uniform Moon would be about 47.1 kilobars. This value is well in excess of the crushing strength of typical lunar rocks. Thus, when the materials from which the Moon was formed began to accrete, the Moon would have formed into a globe even though the materials have remained solid throughout its mass.Seismometers installed on the Moon by Apollo astronauts provided further indications that the lunar interior is rigid, as well as showing that the Moon is seismically much quieter than the Earth. That is, the
records of the quakesÑwhose centers were located 600 to 900 km (375 to 560 mi) below the surfaceÑindicated that both pressure and shear elastic waves occurred during moonquakes. This would not have been the case if the waves had to pass through fluid or semifluid layers in the Moon's interior. In addition, the quake records implied, through the very long decay time of the disturbances, that the Moon's surface layers must be highly fractured.The rigidity of the lunar interior was further evidenced by the interaction of the Moon with the solar wind. Data indicate that the Moon exhibits an electrical conductivity consistent with that of silicate rocks cool enough to act as solids. Finally, the virtual absence of a dipole magnetic field on the Moon, attested to by many spacecraft, discloses that the Moon does not have a metallic core.