Revisiting the Rain of Cometary Snowballs

It's an intriguing idea: A steady rain of kiloton minicomets of loosely packed ice, pelting the top of the atmosphere at a rate of about a dozen per minute could, over geological time, account for most of the water in the oceans, and who knows what else. That's the picture suggested last year by Louis Frank and John Sigwarth at the University of Iowa, based on their observations with the Visual Imaging System (VIS), the Earth camera built by Frank and carried aboard NASA's Polar orbiter. (See PHYSICS TODAY, July 1997, page 18.)