One big mudball (Image: Richard Roscoe/Visuals Unlimited, Inc./Getty)

THE idea that Earth was entirely frozen over about 700 million years ago – the so-called Snowball Earth hypothesis – poses one small problem: how did our planet thaw out? The conundrum could be explained if the Earth was more mudball than snowball.

Evidence for Snowball Earth comes from the discovery of glacial rock formations around the world that date back to this time. One proposed explanation for the subsequent thawing is that carbon dioxide levels soared during the freeze, warming the planet. But recent studies of oxygen isotopes suggest that the level of …