Consequences of a past encounter of the Earth with an interstellar cloud

In its motion through the galaxy the sun may have suffered a number of encounters with dense interstellar clouds for which the number density of molecular hydrogen H 2 is > 10³ cm −3 . Several authors have shown that this is sufficient density to stop the solar wind inside the earth's orbit. The earth's atmosphere would be subjected to an interstellar H 2 flux of > 7×10 9 cm −2 s −1 for periods of ∼ 10 5 years. We have examined by means of simple scaling arguments several consequences for the earth's atmosphere: (1) the ionospheric F‐region would largely disappear; (2) the water vapor content of the middle atmosphere would be greatly enhanced, reducing the mesospheric ozone concentrations and thereby lowering the average temperature and altitude of the mesopause; (3) as a result of (2), widespread mesospheric ice clouds would occur, increasing the planetary albedo; (4) the resultant radiative cooling at the surface may have been sufficient to "trigger" an ice age.