Small droplets of glass reveal a rain of molten rock during the extinction of dinosaurs

Sixty-six million years ago, a gigantic asteroid (Chicxulub) collided with our planet. The impact triggered a series of global events that led to the extinction of 75% of species, including non-avian dinosaurs. During this planetary-scale event, known as the Cretaceous-Paleogene (K-Pg) boundary extinction, a crater nearly 200 km in diameter was formed, and millions of tons of molten rock, dust, and gas were expelled into the atmosphere. A significant volume of these materials, known as ejecta, is represented by tiny glass droplets (impact spherules) distributed around the globe. Our hypothesis suggest that the morphology and size of the impact spherules, and their fractions within the K-Pg bed, are related to their origin and transport mechanisms. To test this, 2000 spherules from Gorgonilla Island (Colombia) and 650 from Wahalak creek (Mississippi) were analyzed using standard stereo-microscopes. Our observations suggest that three levels within the K-Pg bed are related to three different transport mechanisms: 1) the lower layer records the accumulation of rotational molten rock droplets, which arrived very hot at the depositional site following ballistic trajectories. 2) the middle layer represents tiny molten rock droplets transported by the rapid expansion of a high-temperature cloud arriving hot at the depositional site. 3) the upper layer represents the accumulation of tiny droplets condensed from a vapor cloud. Elemental mapping analyses using scanning electron microscopy and synchrotron and the study of additional samples from Mexico and Spain support our conclusions and allow us better to understand the physical processes at work during asteroid impacts.