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It is expected that as the Sun travels through the interstellar medium (ISM),there will be different filtration of Galactic Cosmic Rays (GCR) that affectEarth. The effect of GCR on Earth's atmosphere and climate is still uncertain.Although the interaction with molecular clouds was previously considered, theterrestrial impact of compact cold clouds was neglected. There is overwhelminggeological evidence from 60Fe and 244Pu isotopes that Earth was in directcontact with the ISM 2 million years ago, and the local ISM is home to severalnearby cold clouds. Here we show, with a state-of the art simulation thatincorporate all the current knowledge about the heliosphere that if the solarsystem passed through a cloud such as Local Leo Cold Cloud, then theheliosphere which protects the solar system from interstellar particles, musthave shrunk to a scale smaller than the Earth's orbit around the Sun (0.22).Using a magnetohydrodynamic simulation that includes charge exchange betweenneutral atoms and ions, we show that during the heliosphere shrinkage, Earthwas exposed to a neutral hydrogen density of up to 3000cm-3. This could havehad drastic effects on Earth's climate and potentially on human evolution atthat time, as suggested by existing data.

Terrestrial Impact from the Passage of the Solar System through a Cold Cloud a Few Million Years Ago