Why Venus has No Moon

Venus does not have a moon. We argue that this is at least as surprising as the presence of Earth's moon and more surprising than the absence of a substantial moon for Mercury or Mars. We do not know if Venus ever had a moon. The accepted explanation for Earth's moon is a giant impact with an impactor on the order of one Mars mass. Given current theories of solar system formation, it is unlikely that Venus would have avoided such a large collision. Simulations suggest that most large collisions create a disk from which a moon forms. Moreover, the natural outcome is one where the sense of orbital motion and planetary spin are the same, leading to outward tidal evolution. Despite the smaller sphere of influence of Venus relative to Earth, and the larger solar tidal influence, only very large moons or very dissipative tides allow such a moon to escape. The alternative of inward evolution and coalescence cannot be explained without a second large impact that provides an angular momentum impulse of the opposite sense. A two large collision hypothesis is presented, and argued for. Since tidal evolution is primarily symmetric with respect to relative mean motion, the moon created by the first giant impact returns to Venus on roughly the same timescale as the time between giant impacts, 10^7 years. This hypothesis also allows Venus to eventually evolve to the current slow retrograde rotation state, an outcome that is otherwise difficult to explain quantitatively, notwithstanding the accepted current balance between solar thermal and solid body tides. The two giant impact hypothesis may have isotopic and possibly compositional consequences for Venus but the coalescence is unlikely to have left a clear geophysical or geological trace. We have not identified a clear observational test of this model.