The “snowball” Earth theory describes the complete glaciation of the Earth 600 to 800 million years ago. To explain this cataclysm, Alexander Pavlov, of the University of Colorado Boulder, and his colleagues propose in the Geophysical Research Letters a new hypothesis. Researchers suggest that less than a billion years ago, our solar system passed through a moderately dense interstellar cloud for approximately 500 years, which resulted in an increase in the flow of anomalous cosmic rays. or ACR (Anomalous Cosmic Ray).
These ACRs are ions, resulting from photoionization or exchange of charges on the neutral gases of the interstellar cloud and subjected to an acceleration as a result of the collisions which occur when they release the solar wind. However, according to the computer models of the authors of the study, the increase in the flux of ACRs over a million years could have been enough to completely disrupt the Earth's stratosphere.
During this period, it is indeed possible that a reversal of the Earth's magnetic poles favored the atmospheric penetration of cosmic rays in greater quantity, rays which would in turn have contributed to the formation of more nitrogen oxides (NOx ). Concentrations of these gases multiplied by 100 between 20 and 40 km above sea level could have destroyed 40% of the protective ozone layer (this figure rising to 80% in the polar regions).
Therefore, the combination of low luminosity due to the interstellar cloud and a very reduced ozone layer could explain a total glaciation of the Earth's surface. To validate this theory, the researchers will now focus on analyzing the levels of uranium 235 in rocks from this distant time (U235 is not naturally produced on Earth but is present in stellar clouds).

LAT 05 / 03 / 05 (Massive cloud-May-have frozen the earth)
http://www.agu.org/pubs/crossref/2005/2004GL021890.shtml
http://www.agu.org/pubs/crossref/2005/2004GL021601.shtml
http://www.nasa.gov/home/hqnews/2005/mar/HQ_05066_giant_clouds.html