The tiny spheres, evidence of a long-ago asteroid strike, are smaller than grains of salt. (Credit: Mike Meyer/Meteoritics and Planetary Science)

In 2006, an undergraduate student from the University of
South Florida named Mike Meyer spent his summer collecting fossils from the
walls of a quarry. In typical intern glory, his job was to pry open fossilized
clams and wash away sediment, looking for shells of long-dead single-celled
organisms.

Instead, he found what appeared to be miniscule