National Geographic: A massive release of methane, a potent greenhouse gas, may have triggered rapid melting of the last “snowball Earth” about 635 million years ago, a new study suggests. According to the snowball theory, ancient Earth experienced periods of global glaciation when ice sheets extended all the way to the Equator. Methane ice forms and stabilizes beneath glaciers under certain temperatures and pressures, noted lead study author Martin Kennedy, a geologist at the University …