Origin of the surface features of the Moon

The current theories of the Moon are variants of Galileo’s hypothesis, according to which the smooth dark areas were oceans; since 1610 only the fluid of the oceans has been changed, from water to (subsequently solidified) lava. Recent high-resolution photographs, however, are incompatible with the Galilean hypothesis and its modem versions: the smooth dark surfaces, including the floors of the large dark craters, seem to be denuded areas on the once-molten lunar body, from which the ‘lunar soil’ has been blown away by explosions of comets. The lunar soil seems to be the impact-crushed ancient crust of the Moon, mixed with the debris of meteorites and comets; it is the material of the rugged highlands. The exposed areas of the lunar body are partly glazed with thin coats of lava, and covered with dust darkened at the surface. The multitudes of small craters, according to G. K. Gilbert (1893), are meteorite in­dentations; the large craters, however, have been created by the explosion of comets, as suggested by Kopal (1959). If the impact is weak, the explosion sweeps the soil, or, in bare marial areas, the comet-debris, to a circular or crescent-shaped wall; a more violent impact also indents the brittle lunar body in a manner than can be reproduced in many details by a steel-ball indentation of a block of glass. A simple calculation combining the Hertz theory of elastic contact with the Griffith biaxial theory of fracture explains the flatness of the crater floor, and the terraced wall; the polygonal shape of the indentation craters and the tangential rays around them are inertia effects due to the near-sonic velocity of crack propagation. Indentation leaves behind residual stresses which are partially relieved in moonquakes. The delay of the stress release may be analogous to the delayed fracture of glass caused by the reduction of the surface energy by adsorption or chemisorption. The surface-active agents are probably water and other volatiles injected by the comets into the indentation cracks; streams of such volatiles seem to have excavated the sinuous rilles. The usual calculations of high-velocity impact cannot be applied to the Moon because they are based on an approximation for calculations of armour-plate penetration: under hypervelocity impact, steel can be regarded as a heavy non-viscous liquid because its yield stress is roughly pressure-independent and relatively negligible. The debris of a brittle body, however, obeys Coulomb’s law of soil mechanics: its shear strength is approximately proportional to the pressure, instead of being independent of it.