Bear and Thomas named the odour rain draws from rock

The smell a dry spell finally releases

Petrichor is the name for the earthy scent that rises when rain first meets dry ground, sharpest after a long dry spell finally breaks. The word covers the whole event, not one ingredient in it: a mix of volatile plant oils and the bacterial compound geosmin, both released from the soil into the air, carried further by ozone that a storm's downdraft pushes ahead of the rain itself.¹ It is a young word for an old, near-universal experience, coined in 1964 by two chemists who needed a name for something their own fieldwork kept turning up and no existing word covered.²

What the chemists needed a name for

Isabel Joy Bear and Richard Grenfell Thomas were mineral chemists at CSIRO's Division of Mineral Chemistry in Melbourne. Certain rocks and clays held an odour that released itself, distinctly, the moment the material was wetted.¹ They wrote the work up as "Nature of Argillaceous Odour" for Nature, and inside the paper they gave the puzzle its own word.

They built the coinage from two Greek roots: petra, rock or stone, and ichor, the fluid the Greeks imagined ran in the veins of the gods in place of blood.¹ Petrichor, literally, is stone's ichor, the "tenuous essence" they judged the rock itself was releasing. Given what the odour smelled like, they reasoned it came from decaying plant and animal matter: oils, hydrocarbons, alcohols. These had settled onto mineral and clay surfaces during the dry spell. Water flushed them back into the air.³ The name kept working for its own coiners: eighteen months later Bear and Thomas returned to Nature with "Petrichor and Plant Growth."⁴

Petrichor is not geosmin

Twenty months after the coining, a different pair of scientists gave a single chemical compound its own name. At Rutgers, Nathan Gerber and Hubert Lechevalier isolated an earthy-smelling neutral oil from cultures of the soil bacterium Streptomyces griseus, extracting about a milligram of it from every liter of broth, characterized it chemically, and called it geosmin.⁵ Geosmin is one molecule, a neutral oil with a boiling point near 270°C, built from carbon and hydrogen and no nitrogen.⁵ Acid converts it into an odorless twin, argosmin, with a boiling point near 230°C. That is a reaction a single, precisely defined molecule can undergo, and a whole scent event cannot.⁵ Petrichor is the entire scent event that geosmin, together with plant oils and ozone, contributes to, exactly the hierarchy Merriam-Webster's own definition preserves.¹ The two words were coined twenty months apart, by research groups on opposite sides of the world who never worked together, for two different kinds of thing: a compound and a phenomenon.²⁵ Where the two get run together, it is rarely a source stating outright that petrichor is geosmin. It is a headline's shorthand read as a definition, while the sentence underneath usually keeps the two in their right order.

The word working outside the lab

The dictionary caught up first. Merriam-Webster lists 1964 as petrichor's first known use and defines it as the composite scent, with geosmin named as one ingredient among several rather than a synonym for the whole.¹ Reporting in 2015 on an MIT high-speed-video study of how raindrops fling scent into the air, the science writer Maris Fessenden used petrichor for the whole event and kept geosmin in its place as one contributor: "This smell, called geosmin, is produced by bacteria that make their homes in decaying matter and soil."³

What the word gives, over "earthy smell" or "the smell of rain," is the coining's own claim: that this is one distinctive thing, worth its own name, arising from more sources than either plain phrase implies. It misleads only where a sentence shrinks it to the single molecule underneath it.