Origin of the Lunar Formations

Abstract

The preceding chapter of our survey contained but a general description of the more common types of formations encountered on the lunar surface. What can we say at present about the origin of such a relief — and, in particular, of its characteristically dominant feature, the so-called craters? A glance at the almost bewildering array of such formations of all sizes — as shown on many photographs accompanying this chapter — makes it perhaps unlikely that all of them originated in the same way, or at the same time; and a more detailed analysis of their features suggests that a suspicion of different origin is probably well founded. In fact, the most reasonable approach to this problem can be made if we ask ourselves the following question: what are all the processes which could have conceivably cooperated in shaping up the surface of our satellite? And once we thus formulate our problem, we find ourselves facing two principal contending theories of crater origin: namely, the external theory — invoking the effects produced by impacts of other celestial bodies (asteroids, meteorites, or comets) on the lunar surface — and the alternative theory relying on the internal processes connected with convection, gradual defluidization and degassing of the lunar globe consequent upon its build-up of internal heat due to spontaneous decay of radioactive elements, or any other activity which could be loosely termed as ‘volcanic’. In point of fact, the entire surface of the Moon must be regarded as the outer ‘boundary condition’ of all internal processes which may have been going on in lunar interior since the origin of our satellite, as well as an ‘impact counter’ of external events which may have visited it from outside. In no other sense can an interpretation of the lunar surface possess any physical meaning.