In a recent review article in the journal Science Advances, Dr. Aiko Voigt from IMK-TRO and international colleagues summarize the state of research regarding the Snowball Earth hypothesis.
Geological evidence from the Neoproterozoic era (1000 - 541 Million years before present) indicates that Earth experienced two million-year long extreme ice ages during which ice and snow not only covered the mid-latitudes but in fact reached the equator. Land glaciers covered most if not all of the tropical continents, and the global ocean was covered by sea ice. If correct, this raises the question of how life could have survived such harsh conditions. Alternative theories therefore have suggested that a narrow strip of tropical ocean could have remained ice free. The question of global versus near-global ice cover is arguably the most interesting and most important question regarding the climate dynamics of Snowball Earth episodes dynamics, and has rich implications also for the potential habitability of extrasolar planets. The review paper is available here: here .
(C) Picture: MPI-M/DKRZ/Aiko Voigt