April Scent Profile: Petrichor

Petrichor is the distinctive, earthy scent produced when rain falls on dry soil. The term was coined in 1964 by two Australian researchers, Isabel Joy Bear and Richard Thomas, from the Greek words petros (stone) and ichor (the fluid that flows in the veins of the gods in Greek mythology). So the word literally means something like "blood of the stones."

The scent itself comes from a few sources. One major contributor is geosmin, a chemical compound produced by soil-dwelling bacteria called Actinomycetes. These bacteria release geosmin when they die or when rain disturbs the soil, and the human nose is extraordinarily sensitive to it; we can detect geosmin at concentrations as low as 5 parts per trillion. 

Geosmin has also found its way into perfumery; synthetic versions of the molecule are available from fragrance suppliers, giving perfumers a way to capture that unmistakable "fresh rain" earthiness in a bottle.

Another component of the petrichor scent involves plant oils. During dry periods, certain plants exude oils that accumulate on rocks and soil. When rain arrives, these oils are released into the air along with geosmin, creating that complex, beloved smell.

In 2015, MIT researchers used high-speed cameras to discover the actual mechanism: when a raindrop hits porous ground, it traps tiny air bubbles that shoot upward and burst, releasing aerosols carrying those aromatic compounds into the air. It works almost like one of our tiny car diffusers! 

A few more tidbits worth knowing: the smell tends to be strongest after a light rain following a prolonged dry spell, because more oils and geosmin have had time to accumulate. Heavier rain tends to wash the compounds away too quickly for the aerosol effect to work as well. Some scientists believe humans may have evolved a fondness for the scent because it signaled the arrival of water and the potential for plant growth; essentially, it was a survival cue.