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

Earth and Venus part company The Mariner and Venera probes sent to Venus in the 1960s and 1970s revealed many differences between the venusian atmosphere and that on Earth. One of the hardest to account for is the preponderance of noble gases on Venus, in particular an argon-36 level 50 times higher than on Earth. A new theory tracks the cause of this difference to around 4.5 billion years ago, when Earth and Venus are thought to have grown as a result of collisions between several Mars-sized planets. Numerical simulations show that when a giant impact occurs, the presence of an ocean drastically increases the rate at which atmosphere is lost. On Earth, almost all the proto-atmosphere accrued during planet formation would have been stripped away during collisions. Venus, nearer the Sun, is unlikely to have had a major ocean, and its proto-atmosphere would have survived. The atmospheric compositions of Venus and Earth differ significantly, with the venusian atmosphere containing about 50 times as much 36 Ar as the atmosphere on Earth 1 . 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 composition 2 , 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.