The 'snowball' fight over global deep freeze periods

A research team has picked a side in the 'Snowball Earth' debate over the possible cause of planet-wide deep freeze events that occurred in the distant past. According to a new study, these so-called 'Snowball' Earth periods, in which the planet's surface was covered in ice for thousands or even millions of years, could have been triggered abruptly by large asteroids that slammed into the Earth.