The history of air

摘要

The Earth was born in violence. Modern scenarios of its origin suggest that, as the Earth grew, matter arrived in progressively larger chunks. The final crescendo included the impact of a Mars-size proto-planet that added mass and energy to the nascent Earth and, incidentally, created the Moon. So great was the energy delivered in this impact that the proto-Earth probably melted completely, and silicate vapour formed a fiery (although short-lived) envelope around the planet. Amidst such hostility, it seems hardly possible that the fragile envelope of atmospheric gases could survive. Planetary scientists have taken it virtually for granted that the primordial atmosphere of the proto-Earth would have been stripped by such a stupendous impact, and have looked for mechanisms that might have regenerated the gaseous envelope after the tumult subsided. But Genda and Abe have taken a closer look at the problem and, writing in Icarus, they show that Earth's atmosphere is not quite as fragile as it once seemed.