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Explaining the Inexplicable: Petrichor


It’s that inexplicable earthy smell that fills your nose right after a much-needed downpour, as though the rain has open a cracked in the street and unleashed the pure scent of the earth’s inner core. It smells wholesome, organic, like camping in the Adirondacks and fishing for trout. At least if you’re in the country. When you’re in the city, the smell—a phenomenon known as petrichor—can often amount to reliving yesterday’s garbage and the neighbor with ten dogs who use your front stoop as their personal restroom.

Named by Australian researchers J. Bear and R.G. Thomas in 1964, the two discovered that the smell was the result of fifty or so different compounds found in an oily essence emitted from rocks and soils and sent on a trip to your nose. Published in an article titled “Nature of Argillaceous Odor” in the UK’s Nature Magazine (described in great, wordy detail here), Bear and Thomas pinpointed the cause of one of the earth’s most mysterious scents.

During drier periods, plants secrete oils that seep into rocks and soil. The oils—which inhibit seed germination during times of drought, perhaps to regulate competition amongst the plants during times of scarce resources—are then released into the air after coming into contact with rain. Additionally, ozone—the molecule responsible for that sharp, chlorine-like scent that comes just before a storm—also contributes to the making of petrichor.

But it doesn’t stop there.

The airborne oils come together with other compounds to round out the smell of petrichor. Each particular environment has its own particular scent, due to the range of compounds in its respective terrior. Hence, densely forested areas like Big Sur smell like diving face first into a moist mound of dirt (on account of geosmin, a chemical created by the soil-bound bacteria known as actinomycetes), and cemented-over metropolises like Manhattan smell like… well, something else.

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