North America was once home to mammoths, mastodons (above) and beavers the size of black bears. Barry Roal Carlsen/University of Wisconsin-Madison hide caption

itoggle caption Barry Roal Carlsen/University of Wisconsin-Madison

A close-up composite image of a dust particle. The green patches contain bromine, calcium and/or iron. Courtesy of American Chemical Society hide caption

itoggle caption Courtesy of American Chemical Society