This article was originally published at Eos. The publication contributed the article to Space.com's Expert Voices: Op-Ed & Insights.
Earth froze over 717 million years ago. Ice crept down from the poles to the equator, and the dark subglacial seas suffocated without sunlight to power photosynthesis. Earth became an unrecognizable, alien world—a "snowball Earth," where even the water was colder than freezing.
