Petrichor

It’s been raining all day today after a hot, dry spell. The garden and its newly planted seedlings and seeds is grateful. When the rain began, there was a distinctive smell. But how can rain have a scent when water is odorless and colorless fluid.

Petrichor is the earthy scent produced when rain falls on dry soil. The word is constructed from the Greek roots petra (mérpa), meaning “stone’and ichör (ixop), the fluid that flows in the veins of the gods in Greek mythology.

The phenomenon was first described in a March 1964 paper, in the journal Nature, by two Australian researchers, Isabel Bear and Dick Thomas. They coined the word petrichor to replace the term argillaceous odour. The scientists explained that the smell derives from an oil exuded by certain plants during dry periods, then absorbed by clay-based soils and rocks. During rain, this oil is released into the air along with another compound, geosmin, a metabolic by-product of certain actinobacteria, like strepomyces which is emitted by wet soil, producing the distinctive scent.

The name, geosmin, is derived from Greek roots γεω- (geō-), meaning “earth,” and ὀσμή (osmḗ), meaning “smell.” The human nose is extremely sensitive to geosmin and can detect it at concentrations as low as 400 parts per trillion. Desert camels in the desert also rely on petrichor to locate water sources such as oases.

Petrichor is an evocative scent. Such smells arouse memories. What do you recall when you smell the rain?

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