(Thanks goes out to Xenus)
Several consequences of the passage of the solar system through dense interstellar molecular clouds are discussed. These clouds, dense (more than 100 cm−3), cold (10–50 K) and extended (larger than 1 pc), are characterized by a gas-to-dust mass ratio of about 100, by a specific power grain size spectrum (grain radii usually cover the range 0.001–3 micron) and by an average dust-to-gas number density ratio of about 10−12. Frequently these clouds contain small-scale (10–100AU) condensations with gas concentrations ranging up to 105 cm−3. At their casual passage over the solar system they exert pressures very much enhanced with respect to today’s standards. Under these conditions it will occur that the Earth is exposed directly to the interstellar flow.
It is shown first that even close to the Sun, at 1AU, the cloud’s matter is only partly ionized and should mainly interact with the solar wind by charge exchange processes. Dust particles of the cloud serve as a source of neutrals, generated by the solar UV irradiation of dust grains, causing the evaporation of icy materials. The release of neutral atoms from dust grains is then followed by strong influences on the solar wind plasma flow.
The possibility of solar encounters with dense interstellar clouds (IC) with particle concentrations about 10 − 1000 cm−3 and more, is of great interest in view of its possible effects upon the Earth. The question of whether dense IC would prevent the solar wind (SW) from reaching the Earth with the result of cloud material directly impacting the terrestrial atmosphere, as well as many other aspects of this complex problem were already discussed in the literature of the past (Fahr, 1968a, b; Talbot and Newman, 1977; Holzer, 1977; Fahr, 1980; Ripken and Fahr, 1981; Zank and Frisch, 1999; Scherer, 2000; Scherer et al. 2002, and references therein). Such a scenario is considered as possibly triggering global glaciations, depositions of a prebiotic material on the primordial Earth, possible ecological repercussions for the Earth due to an accretion of the cloud’s matter, and of course, bio-mass extinction (Yabushita and Allen, 1997). It should also be added that in fact the correlation between periods of glaciations and long-time variations in the accretion rate of interplanetary dust particles onto the Earth is revealed (Farley and Patterson, 1995).
Yabushita and Allen (1983) have argued that if the Sun should ever have passed through such dense core of GMC (nH = 105), then as much oxygen as is now present in the terrestrial atmosphere could have been removed, due to water formation.
We have also shown that large dust grains of the cloud, when present in countable abundance, serve as a source of neutrals, generated by the solar UV irradiation of dust particles, followed by a strong influence on the gas flow, by mass loading.