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Enhanced atmospheric loss on protoplanets at the giant impact phase in the presence of oceans

Abstract

The atmospheric compositions of Venus and Earth differ significantly, with the venusian atmosphere containing about 50 times as much 36Ar as the atmosphere on Earth1. The different effects of the solar wind on planet-forming materials for Earth and Venus have been proposed to account for some of this difference in atmospheric composition2,3, but the cause of the compositional difference has not yet been fully resolved. Here we propose that the absence or presence of an ocean at the surface of a protoplanet during the giant impact phase could have determined its subsequent atmospheric amount and composition. Using numerical simulations, we demonstrate that the presence of an ocean significantly enhances the loss of atmosphere during a giant impact owing to two effects: evaporation of the ocean, and lower shock impedance of the ocean compared to the ground. Protoplanets near Earth's orbit are expected to have had oceans, whereas those near Venus’ orbit are not, and we therefore suggest that remnants of the noble-gas rich proto-atmosphere survived on Venus, but not on Earth. Our proposed mechanism explains differences in the atmospheric contents of argon, krypton and xenon on Venus and Earth, but most of the neon must have escaped from both planets’ atmospheres later to yield the observed ratio of neon to argon.

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Figure 1: Changes in atmospheric mass during the stage of giant impacts.
Figure 2: The loss fractions of atmosphere (Xatm) and ocean (Xoce) induced by the global ground motion with various initial ground velocities (ug).
Figure 3: Changes in atmospheric mass for the cases with oceans.

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Acknowledgements

We thank E. Asphaug for comments and suggestions. This work was supported by a JSPS Research Fellowship and the 21st Century Earth Science COE Program (the University of Tokyo).

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Correspondence to Hidenori Genda.

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Supplementary information

Supplementary Figure 1

This figure shows typical numerical results of the motion of an atmosphere and ocean. The distributions of velocity, pressure, density, and temperature are shown. (DOC 182 kb)

Supplementary Figure 2

This figure shows particle velocity-pressure relations of the Hugoniot curves for olivine, water, and gas. (DOC 204 kb)

Supplementary Methods

This section provides the estimation of the mass of the gravitationally-attracted solar atmosphere. (DOC 39 kb)

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Genda, H., Abe, Y. Enhanced atmospheric loss on protoplanets at the giant impact phase in the presence of oceans. Nature 433, 842–844 (2005). https://doi.org/10.1038/nature03360

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