Decades of research shows that Earth was once entirely or almost entirely frozen. The episode is known as Snowball Earth , and though its occurrence is widely accepted, many of its details remain hypothetical. Snowball Earth took place in Earth's Cryogenian Period , which spanned from about 720–635 million years ago during the Neoproterozoic Era. The Cryogenian had two major global ice ages that are commonly referred to as Snowball Earth. One is the Sturtian glaciation and the other is the Marinoan glaciation. Researchers have puzzled over these events. Research shows that the Sturtian lasted 56 million years, but climate models have struggled to explain how it could've endured for so long. Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) have a new explanation for the Sturtian glaciation's length. They're presenting it in new research published in the Proceedings of the National Academy of Sciences.…