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The clean, earthy fragrance that fills the air following a long-awaited summer rain is known as petrichor. Fascinatingly, extensive research has been conducted on this scent since its discovery more than fifty years ago.
As explained in the PBS video series Reactions, petrichor was first identified in the 1960s by Australian scientists Isabel Bear and Richard Thomas, who discovered they could extract a yellowish oil from dry rocks, clay, and soil. This substance, primarily composed of fatty acids from plants, is the foundation of the scent.
Plants generate petrichor during dry periods, with the oil being absorbed by rocks and soil. Although the fatty acids themselves are almost odorless, they break down in the soil, releasing much stronger-smelling compounds. When rain falls, these compounds, along with geosmin—the chemical responsible for the earthy smell—are released into the air.
The reason why plants release petrichor during dry spells remains a mystery to scientists. Bear and Thomas theorized that it might assist in plant growth, but after applying the oil to plants, they found it actually caused them to wither. Another hypothesis suggests that plants produce petrichor during water shortages to inhibit the growth of competing vegetation.
Watch the full story behind this familiar scent in the video below.
