On the effect of accreted interstellar matter on the terrestrial environment

Discussion is made of the accretion of interstellar hydrogen and dust on to the earth during the passage of the Solar System through a giant molecular cloud. For cloud densities above $$10^3-10^4 \enspace \text {cm}^{-3}$$ hydrogen remains neutral down to 1 AU and can be captured (along with dust) by the Earth's atmosphere, while for densities above about $$10^4 \enspace \text {cm}^{-3}$$ the hydrogen reacts to form water in the atmosphere before it can re-evaporate. The dust suspended in the upper atmosphere can reduce the global temperature, while water vapor from accreted hydrogen will condense as snow on the grains, increasing this effect. We suggested that the K/T boundary was an extreme event due to the passage of the solar system through the core of a giant molecular cloud, and may thus have been qualitatively different from other extinction boundaries, as evidenced by the anomolous Ir deposits.