A possible direct exposure of the Earth to the cold dense interstellar medium 2–3 Myr ago
Cold, dense clouds in the interstellar medium of our Galaxy are 4–5 orders of magnitude denser than their diffuse counterparts. Our Solar System has most likely encountered at least one of these dense clouds during its lifetime. However, evidence for such an encounter has not been studied in detail yet. Here we derive the velocity field of the Local Ribbon of Cold Clouds (LRCC) by modelling the 21 cm data from the HI4PI survey, finding that the Solar System may have passed through the LRCC in the constellation Lynx 2–3 million years ago. Using a state-of-the-art simulation of the heliosphere, we show that during the passage, the heliosphere shrinks to a scale of 0.22 au, smaller than the Earth’s orbit around the Sun. This would have put the Earth in direct contact with the dense interstellar medium for a period of time and exposed it to a neutral hydrogen density above 3,000 cm−3. Such a scenario agrees with geological evidence from 60Fe and 244Pu isotopes. The encounter and related increased radiation from Galactic cosmic rays might have had a substantial impact on the Earth’s system and climate.
Most stars generate winds and move through the interstellar medium (ISM) that surrounds them. This motion creates a cocoon (astrosphere) that protects planets from the ISM. The Sun’s cocoon is the heliosphere. The Solar System has been inside the Local Bubble for at least the last 3 Myr, and possibly 10 Myr (ref. 1). The conditions near the Sun are not homogeneous, and several partially ionized clouds exist2. It is clear that the Solar System has traversed different regions of the local ISM during the past several million years, which has affected its heliosphere. Presently, the Solar System is traversing a local interstellar cloud (LIC) with a relative speed of 25 km s−1. The Solar System will be leaving the LIC in the next few thousands of years because of its proximity to its edge3.
