According to the giant impact theory, the Moon was formed by accretion of the debris disk that resulted from the collision between Theia and the proto-Earth. Although this theory accounts for most characteristics of the Earth-Moon system, numerical simulations of impacts between a planetary embryo and the accreting proto-Earth indicate that more than 40 percent of the material in the circum-terrestrial disk generated by such an impact originates from the impactor. This poses a challenge for the giant impact theory in explaining the Moon’s Earth-like isotopic composition, a discrepancy known as the lunar isotopic crisis. Since terrestrial planets were melted one or more times during accretionary processes, magma ocean on the surface of a growing planet would appear. Small terrestrial planets with magma ocean cool faster than large ones, resulting that the viscosity of small terrestrial planets is larger than that of large terrestrial planets still covered by magma ocean. Here, it shows that giant impact between a high-viscosity Theia and a low-viscosity proto-Earth could produce a circum-terrestrial debris disk predominantly composed of material from the proto-Earth without violating the angular momentum constraint of modern Earth-Moon system. The theory proposed here may provide a natural way of explaining the lunar isotopic crisis.
Keywords: Earth-Moon system — The Moon — Lunar origin — Lunar science 1. INTRODUCTION
et al. 2016; Zhang et al. 2012), or the impactor itself must have had an isotopic composition identical to that of Earth (Mastrobuono-Battisti et al. 2015). Nonetheless, none of these models have fully accounted for all geochemical observations, highlighting the need for further research into the complex processes that shaped the Earth-Moon system.
According to the giant impact theory (Hartmann & Davis 1975; Cameron & Ward 1976), the Moon formed from material accreted from a circum-terrestrial debris disk, which was generated after a planet-sized body collided with the proto-Earth. Numerical simulations based on this theory have shown that this disk from which the Moon originated contains a substantial fraction of material derived from the impactor (Canup & Asphaug 2001; Canup 2004a, 2008). Since different bodies in the Solar System have distinct compositions, the canonical giant impact model (Canup & Asphaug 2001) would predict a lunar composition significantly different from that of Earth. However, isotopic comparisons between Earth and Moon samples reveal a remarkable similarity (Ringwood 1986; Lugmair & Shukolyukov 1998; Wiechert et al. 2001; Touboul et al. 2007; Zhang et al. 2012; Herwartz et al. 2014), a discrepancy known as the lunar isotopic crisis. To explain this isotopic similarity, either the Moon-forming material must have equilibrated with, or originated from, Earth’s mantle after the impact (Young
´Cuk & Stewart (2012) introduced a variant of the giant impact hypothesis involving a rapidly rotating proto-Earth. In this scenario, a larger portion of proto-Earth material can be transferred to the circum-terrestrial debris disk, which becomes dominated by proto-Earth matter. This disk is massive enough to potentially form the Moon following a collision between a fast spinning proto-Earth and a body slightly smaller than Mars. While this idea has the potential to explain the lunar isotopic composition, it yields an angular momentum exceeding that of the present Earth-Moon system. Other high-energy, high-angular momentum models, such as the Synestia model (Lock & Stewart 2017; Lock et al. 2018) and the collision of two comparably massive bodies (Canup 2012) also produce excessive angular momentum, which would need to be reduced by some feasible mechanism, still a matter of debate. The high-energy, high-angular momentum model ( ´Cuk & Stewart 2012) is designed to produce a circum-terrestrial debris disk primarily composed of
Corresponding author: Wenshuai Liu 674602871@qq.com 2 to represent the materials.