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Earth likely acquired much of its inventory of volatile elements during the main stage of its formation. Some of Earth’s proto-atmosphere must therefore have survived the giant impacts, collisions between planet-sized bodies, that dominate the latter phases of accretion. Here, we use a suite of 1D hydrodynamic simulations and impedance-match calculations to quantify the effect that preimpact surface conditions (such as atmospheric pressure and the presence of an ocean) have on the efficiency of atmospheric and ocean loss from protoplanets during giant impacts. We find that—in the absence of an ocean—lighter, hotter, and lower-pressure atmospheres are more easily lost. The presence of an ocean can significantly increase the efficiency of atmospheric loss compared to the no-ocean case, with a rapid transition between low- and high-loss regimes as the mass ratio of atmosphere to ocean decreases. However, contrary to previous thinking, the presence of an ocean can also reduce atmospheric loss if the ocean is not sufficiently massive, typically less than a few times the atmospheric mass. Volatile loss due to giant impacts is thus highly sensitive to the surface conditions on the colliding bodies. To allow our results to be combined with 3D impact simulations, we have developed scaling laws that relate loss to the ground velocity and surface conditions. Our results demonstrate that the final volatile budgets of planets are critically dependent on the exact timing and sequence of impacts experienced by their precursor planetary embryos, making atmospheric properties a highly stochastic outcome of accretion.

A giant-impact origin for the Moon is generally accepted, but many aspects of lunar formation remain poorly understood and debated. Ćuk et al. proposed that an impact that left the Earth–Moon system with high obliquity and angular momentum could explain the Moon’s orbital inclination and isotopic similarity to Earth. In this scenario, instability during the Laplace Plane transition, when the Moon’s orbit transitions from the gravitational influence of Earth’s figure to that of the Sun, would both lower the system’s angular momentum to its present-day value and generate the Moon’s orbital inclination. Recently, Tian & Wisdom discovered new dynamical constraints on the Laplace Plane transition and concluded that the Earth–Moon system could not have evolved from an initial state with high obliquity. Here we demonstrate that the Earth–Moon system with an initially high obliquity can evolve into the present state, and we identify a spin–orbit secular resonance as a key dynamical mechanism in the later stages of the Laplace Plane transition. Some of the simulations by Tian & Wisdom did not encounter this late secular resonance, as their model suppressed obliquity tides and the resulting inclination damping. Our results demonstrate that a giant impact that left Earth with high angular momentum and high obliquity (θ> 61°) is a promising scenario for explaining many properties of the Earth–Moon system, including its angular momentum and obliquity, the geochemistry of Earth and the Moon, and the lunar inclination.