Caption: 
An SwRI-led study updated bombardment models based on small glassy particles, known as impact spherules, that populate multiple thin, discrete layers in the Earth’s crust, ranging in age from about 2.4 to 3.5 billion years old. Spherule layers — such as the one shown in this 5-centimeter, 2.6-billion-year-old sample from Australia — are markers of ancient collisions. 