A familiar scent fills the air after a dry stretch is broken by a summer rain. There is even a word for this fresh, earthy smell: petrichor. The term was coined in the 1960s when scientists reported that the smell came from chemicals in the surface that the rain landed on.

Somehow the rain brought out this smell, but the exact process wasn’t known until two M.I.T. scientists captured the process in high-speed video. They were working on another project, testing the absorption of drops by various artificial surfaces, when they expanded their work to include natural soils.

They found that at the right velocity on the right kind of soil (sandy clay works, but sand doesn’t) a falling water drop can trap tiny air bubbles under it. Those bubbles capture molecules in the soil. As the water drop deforms, the bubbles scoot up through the drop and jet out into the air, like champagne bubbles or spray from a crashing wave.

If the drop falls too slowly, it is absorbed; too fast, and it splatters without the bubbles emerging. “The sweet spot has to do with the velocity of the droplet and the qualities of the soil,” said Cullen R. Buie, an assistant professor of mechanical engineering. He and a postdoctoral researcher, Youngsoo Joung, reported on their work in Nature Communications.