Late Impacts and the Origins of the Atmospheres on Venus, Earth, and Mars

Introduction. Diverse origins of terrestrial planet atmospheres are inferred from differences in the noble gas abundances and isotope ratios observed on Venus, Earth, and Mars [e.g., 1, 2]. Models for the origin of terrestrial atmospheres typically require an intricate sequence of events, including substantial loss and isotopic fractionation of solar nebula gases, outgassed mantle volatiles, and delivery of volatiles by late ac-creting planetesimals. Impact events, large or small, may add or remove volatiles depending on the specific impact parameters. The Moon-forming giant impact is thought to be the last major collision on the growing Earth. Recently, Ćuk and Stewart [3] proposed that a giant impact onto a fast-spinning and nearly fully-grown Earth can explain the identical isotopic composition of the Moon and Earth. Here we discuss the origin of Earth's early atmosphere in light of the high-spin model for Moon formation and new constraints from recent measurements of primordial noble gases in basalts from mid-ocean ridges and mantle plumes [4-6]. We propose that major differences in the noble gas signatures of terrestrial planetary atmospheres are a result of the different outcomes of late impact events on each planet. Earth. The end stage of Earth's accretion included multiple giant impacts with sufficient energy to generate multiple magma oceans of varying depths. Because protoplanets formed in the presence of the solar nebula, the atmosphere and mantle of the growing Earth should include a nebular component. Chondritic meteorites contain a noble gas signature that is distinct from nebu-lar, and accreting planetesimals should also add a chondritic component to the Earth. Building upon previous work, Mukhopadhyay et al. [4] find that Earth's atmosphere cannot be derived from any combination of fractionation of a nebular-derived atmosphere followed by outgassing of deep or shallow mantle volatiles. The primordial Xe isotopic composition of the whole mantle is distinct from air, mantle Xe cannot be residual to atmospheric Xe, and the Ar/Xe ratio in Earth's mantle is near chondritic. If a nebular or outgassed atmosphere existed on the early Earth, it has largely been lost (>~70% with larger loss fractions favored). Furthermore, more than one atmospheric loss event is inferred from the mantle 3 He/