The smell of rain on dry ground is called petrichor. Two Australian researchers, Isabel Joy Bear and Richard Grenfell Thomas, named it in 1964, from the Greek petra, stone, and ichor, the golden fluid said to run in the veins of the gods. Part of the smell is geosmin, a compound linked to soil bacteria of the genus Streptomyces, and the human nose can catch it at concentrations as low as 0.4 parts per billion. It reaches you by bubble: a raindrop landing on dry, porous ground traps tiny pockets of air, which rise, burst and throw a fine mist into the air.
A smell without a name
Everyone knows the smell. Few of us, until 1964, had a word for it. Chemists had been circling it for a long while; a note on the odour of a garden after a summer shower appeared in The Chemical News in 1891, and Bear and Thomas listed it among their references. In the literature it went by a rather dry label, “argillaceous odour”, meaning the smell of clay.
Bear and Thomas worked at the CSIRO Division of Mineral Chemistry in Melbourne. Their paper in Nature, published on 7 March 1964, kept the old label in its title, “Nature of Argillaceous Odour”, and gave the smell a new name. Petrichor: the blood of the gods, drawn from stone. Most chemical names are less romantic.
Geosmin: the smell’s signature
The scent is a mixture, and the story of petrichor is partly the story of what is in it. Part of it is put down to oils that collect in rocks and soil during dry spells and are released by moisture. One ingredient has a name of its own. In 1965 the American biochemist Nancy Gerber and the biologist Hubert Lechevalier named a compound they had isolated from soil bacteria: geosmin, from the Greek for earth and smell.
Geosmin is made by thread-like soil bacteria, chief among them the genus Streptomyces. It is also the reason beetroot tastes of earth. And our noses are absurdly good at it.
That is under half a part per billion. Picture a vast grid of grey dots with one coloured dot in it, as the video does, and you have the idea.
Why does rain smell? Follow the bubbles
A smell sitting in the soil does you no good. It has to get into the air and up your nose, and by the researchers’ own account, nobody had watched it happen. In 2015 Youngsoo Joung and Cullen Buie at the Massachusetts Institute of Technology filmed raindrops striking porous surfaces with high-speed cameras, in roughly 600 experiments on 28 types of surface, among them 16 soil samples.
They saw the drop trap tiny bubbles of air at the point of contact. The bubbles rose through the drop and burst at its surface, throwing out a fizz of aerosols, much as bubbles do in a glass of champagne. Those aerosols, the researchers suggested, may carry the aromatic elements of the soil into the air, where a breeze can take them.
The speed of the drop matters. Light or moderate rain on sandy or clay soils produced plenty of aerosols in the experiments, and slower, lighter rain may produce more of them. That could explain why the smell seems strongest after a gentle shower rather than a downpour. Could, may: the evidence points that way, and the file stays open.
An ordinary moment
A word from a 1964 chemistry paper turns out to name one of the most ordinary moments there is, and a surprising amount hides inside it: a molecule made by bacteria, a nose tuned to fractions of a part per billion, and a raindrop that turns itself into a tiny spray can. For another everyday wonder from the archive, see why gold is gold.
Stone, the blood of the gods and soil bacteria. That’s the smell of rain.
