Atmospheric Loss during High Angular Momentum Giant

Introduction. During the end stage of planet formation , terrestrial planets are expected to experience a number of giant impacts. Understanding how much of the planet's atmosphere is lost to space during such collisions is vital to be able to determine the origin and evolution of planetary atmospheres. Recently, geo-chemical observations [1, 2] and high angular momentum models for the Moon-forming impact [3, 4] have renewed interest in the topic of atmospheric loss during giant impacts. Impact-driven loss is proposed to be an important process in explaining the origin of the Earth's atmosphere and the variability of terrestrial planet atmospheres [1, 2, 5]. In the canonical models of the Moon-forming impact , a Mars-sized impactor obliquely hit the proto-Earth near the escape velocity [e.g. 6]. In such collisions , atmospheric loss can occur either from direct ejection near the impact site or due to the ground 'kick' provided by the impact shock wave. Genda and Abe [7] showed that the shock wave would not cause a significant loss of atmosphere during the canonical Moon-forming impact. They later demonstrated [8] that the presence of an ocean could enhance the loss of the atmosphere, particularly on the smaller body in the collision or over multiple impact events. However, at present it is widely believed that the Moon-forming giant impact event did not lead to significant atmospheric loss from the proto-Earth. The canonical Moon-formation scenario [e.g. 6] was constrained by the angular momentum of the present Earth-Moon system, in which the Earth had a ~5 hour spin period just after Moon formation. However, with this constraint, it is not possible to form a lunar disk with enough proto-Earth material in order to explain the identical isotopic composition of the Earth and Moon. Recently, however, Ćuk and Stewart [3] offered a solution to this problem by showing that the Earth-Moon system could have lost angular momentum in an orbital resonance after the impact. They then showed that it was possible that a smaller, high velocity impactor incident on a fast-rotating Earth (2 to 3 hr spin period) could generate the observed isotopic similarity. An isotopically similar disk could also be formed by a graze-and-merge impact between two approximately equal-sized bodies [4]. In these new models, it is likely that more of the atmosphere could be lost, compared to the canonical giant impact scenario, because the Earth would be near or exceeding its spin stability …