Two chemists built petrichor from rock and the fluid in the veins of the gods

The coinage hid in a paper about clay

Isabel Joy Bear and R. G. Thomas were mineral chemists at CSIRO, Australia's national science agency. Their 1964 paper ran under the dry title “Nature of Argillaceous Odour.” Argillaceous means clay-bearing, from the Latin argilla; the study set out to explain the musty smell clay releases when it is wetted.¹ The source turned out not to be the clay. As humidity climbs ahead of a storm, it fills the pores of soil and stone with water and drives out what was trapped there.² What was trapped is an oil that plants exude during dry spells and that the ground quietly absorbs and holds.³ A second paper the pair published in 1965 found the same oil slows the germination of seeds, a chemical brake on growth through drought.³

To fix a name to the smell, Bear and Thomas described it as an odour “which can be regarded as an ‘ichor’ or ‘tenuous essence’ derived from rock or stone,” and coined petrichor to hold the idea.¹ They built the word from two Greek roots: petra, meaning stone, and ichor, which in Greek myth is not blood but the fluid that runs in the veins of the gods.⁴ The name is exact about what the smell is: an essence the rain draws out of dry ground.

The word left the laboratory

For decades petrichor stayed a specialist's term, buried in the citation trail of soil chemistry. Popular science pulled it out. By 2012, the word had reached a general readership; writing in Scientific American, Daisy Yuhas noted that “there's a name popularly ascribed to this range of post-precipitation scents: petrichor.”⁵ The adverb does the work. A coinage from a mineralogy journal had become something people said. The following spring, Smithsonian retold the word's birth for a general audience, Joseph Stromberg noting that Bear and Thomas “coined the term petrichor to help explain the phenomenon, combining a pair of Greek roots: petra (stone) and ichor (the blood of gods in ancient myth).”⁶

From science writing the word crossed into fiction, where it no longer needs explaining at all. In Annie Darling's 2018 novel True Love at the Lonely Hearts Bookshop, a character can simply “smell the heavenly scent of petrichor rising up from the damp, hot”—the word spent on atmosphere the way any sensory noun would be.⁷ The dictionaries came last, as they tend to. Merriam-Webster now carries a full entry and dates the word's first known use to 1964, the year of the paper.⁴

Geosmin is one molecule inside the word

The word people use now carries something Bear and Thomas never named. Merriam-Webster's current definition has the odour arising “from a combination of volatile plant oils and geosmin released from the soil into the air and by ozone carried by downdrafts.”⁴ Geosmin is a single compound. Nathan Gerber and Hubert Lechevalier isolated and named it in 1965, one year after petrichor, drawing it in pure form from soil actinomycete bacteria. Its name means “earth smell”; the compound itself is a neutral oil that boils near 270 degrees Celsius.⁸ Bacteria of the genus Streptomyces make it, and the human nose is extraordinarily tuned to detect it, though the reported threshold varies by source. Smithsonian puts it as low as five parts per trillion;⁶ a standard encyclopedia entry gives 0.4 parts per billion.³ Both describe a sensitivity few smells can match.

Why the bacteria make it stayed unclear until recently. In 2020, a team led by Paul Becher showed in Nature Microbiology that geosmin has a job: sporulating colonies release it, alongside a related compound, to attract springtails, tiny soil arthropods that eat the colonies and carry the spores away on their bodies. The genes for it switch on only as the bacteria form spores.⁹ Geosmin is a signal a living thing emits to be found. Petrichor is the name of a smell. The chemists coined the narrower word, and common use widened it to hold geosmin, ozone, and the plant oils at once: a drift in the language, not a mistake in it.

What no older word held

English already had words for pieces of this. Musty belongs to wet clay. Earthy belongs to geosmin. What it lacked was a word for the event itself: the scent of rain landing on ground that has gone dry, the instant the first drops sink into warm soil and the smell lifts off it. That is what Bear and Thomas named, and there was no word for it before they wrote one down.¹ Petrichor has drifted well past the single plant oil they meant by it, but it still points where they pointed, at the smell that belongs to dry stone and the rain that ends the drought.