Caption: 
The millimeter-scale gray circles are all formerly molten droplets ejected into space when an asteroid struck the Earth about 2.56 billion years ago. These droplets, known as impact spherules, returned to Earth and were concentrated at the base of the Reivilo layer in South Africa. The droplets originally consisted of glass and crystals formed in flight that have since been replaced by other minerals. The spherules still contain substantial extraterrestrial material based, for example, on their containing 176 parts per million iridium in bulk. The flat to irregular black masses were originally mud fragments ripped up by high-energy currents and/or waves during the deposition of the layer; the source of that energy may have been an asteroid striking the ocean.

Credit: 
Courtesy Bruce Simonson, Oberlin College and Conservatory