Snowball Titan solves methane mystery?
Saturn's largest moon may once have been a giant snowball. Titan is already a frigid moon made mostly of ice. But methane gas in its atmosphere keeps the surface just warm enough for a scattering of lakes filled with liquid hydrocarbons. Scientists have puzzled over Titans atmospheric methane because the molecule is easily broken down by sunlight. Calculations suggest that all the methane Titan seems to possess should have been used up within tens of millions of years--a blip in the moon's roughly 4-billion-year lifetime. Adding to the mystery, the methane breakdown creates other compounds that rain over the surface, helping to fill the lakes. If used-up methane was replaced, this process would happen constantly, so Titan should be covered not by lakes, but by a global ocean hundreds of meters deep. Michael Wong at the California Institute of Technology says snowballs may be the missing piece. Scientists suspect Earth went through a snowball phase about 2 billion years ago, when the planet became covered in ice.