Origin of the Moon and Lunar Water

Abstract

Three principal concepts regarding lunar formation have been examined: the accretion hypothesis, the mega-impact theory, and the multi-impact model. The multi-impact model amalgamates the salient facets of the mega-impact theory and the accretion hypothesis. As per this model, fragments of the terrestrial crust are ejected into space during collisions with numerous planetesimals (proto-asteroids) with diameters around 10-100 kilometers. In the vicinity of Earth’s orbit, this ejecta interacts with the accretion disk, augmenting its mass. Numerical computations demonstrate that particles within the prograde-rotating accretion disk effectively capture prograde-rotating ejecta while shedding retrograde-rotating ejecta onto the planetary body. The multi-impact theory provides an explanation for the creation of not just the Moon and Charon but also the satellites of asteroids. Several predictions regarding the parameters of asteroid satellites posited by this theory have already been validated through statistical analysis of binary asteroids. Different models of lunar formation yield varied conclusions regarding the quantity of lunar water, its subsurface distribution, and isotopic composition. The mega-impact theory postulates the genesis of a largely desiccated Moon. Consequently, the modest lunar water content may arise from comets, solar wind, or the transport of water from Earth’s atmosphere. These mechanisms for lunar water formation imply its superficial occurrence in polar regions and substantial deviation of the deuterium-to-hydrogen (D/H) ratio from terrestrial norms. Conversely, the multi-impact theory posits lunar water’s origin from planetesimals, akin to terrestrial water. Thus, a significant quantity of lunar water is inferred, expected to be pervasive across the lunar surface at varying depths, and possessing isotopic composition analogous to terrestrial water. Geomorphological structures in the lunar polar regions (smoothed craters, landslides, regular patterns) suggest the presence of a substantial permafrost layer with an approximate thickness of a kilometer.