The distinctive scent that fills the air when rain first hits dry ground has an official name: petrichor—from the Greek words ‘petra’ for ‘rock’ and ‘ichor’ meaning ‘divine blood.’ It’s generated by a combination of aromatic oils released from plants and a chemical called ‘geosmin’ produced by soil bacteria.
The scent draws six-legged arthropods known as springtails, which feed on the bacterial colonies that produce geosmin. This is turn helps the soil bacteria to disperse their spores and complete their lifecycle in an ancient symbiosis.
The exact scent of petrichor depends on the environment. In the desert, it has a spicy, leather-like pungency, while in Arizona’s pine forests, the terpenes from resin produce a cleaner, sharper aroma.
These plant compounds, along with geosmin, build up on the surface of leaves and the ground during dry weather. When the first raindrops strike, these compounds become encapsulated in tiny bubbles, which shoot upwards as if from a glass of champagne, then burst to release a fragrant aerosol into the air.
The human nose is so sensitive to petrichor that we can detect it in concentrations as low as one hundred parts per trillion; possibly an ancestral adaptation to the scent’s role in signaling the arrival of water.
So next time you catch the scent of petrichor, know that you’re experiencing an odor that has perfumed our planet for over 300 million years!
This Earth Note was written by Diane Hope and produced by KNAU and the Sustainable Communities Program at Northern Arizona University.
