That Fresh Rain Smell? You're Actually Sniffing Bacteria

There is a particular smell that fills the air when rain falls on dry ground: rich, earthy and oddly comforting. Many people stop in their tracks to breathe it in. That smell has a proper scientific name, petrichor. According to Discover Magazine, the word was coined in a 1964 paper published in the journal Nature, combining “petro”, meaning stone, with “ichor”, a tenuous essence, to describe a tenuous essence derived from rock or stone. It is a poetic name for something wonderfully ordinary.

One of the most important ingredients in that smell has nothing to do with water. It comes from bacteria. The John Innes Centre, a research institute in the United Kingdom, describes geosmin as the soil-based compound that gives the evocative, earthy smell so characteristic of spring. It is made by soil bacteria called Streptomyces. And according to the centre, these bacteria all, without exception, produce geosmin, so wherever Streptomyces live, that earthy scent is part of the package.

Our noses are astonishingly good at picking it up. The John Innes Centre reports that the human nose is so sensitive to geosmin that it can be detected at one hundred parts per trillion. Discover Magazine gives the same figure. That is an extraordinarily tiny amount of a substance to notice, which helps explain why even a brief shower on a dusty path can fill the air so quickly with that unmistakable smell.

So how does the scent get from the soil to your nose? Researchers at the Massachusetts Institute of Technology found part of the answer in 2015. According to MIT News, when a raindrop hits a porous surface such as soil, it traps tiny air bubbles at the point of contact. Like the bubbles in a glass of champagne, they shoot upward and burst from the drop in a fizz of aerosols. Those aerosols, the researchers suggested, may carry aromatic elements, along with bacteria and viruses stored in soil, and more of them are released in light and moderate rain than in heavy rain.

Why would bacteria make this smell at all? A study reported by the John Innes Centre in 2020, published in the journal Nature Microbiology, offers an answer. The researchers found that springtails, tiny six-legged creatures that live in soil, are powerfully drawn to geosmin and to another earthy-smelling compound. The springtails then help to spread the bacteria’s spores in two ways: through their droppings and by carrying spores that stick to their bodies. The smell, in other words, works like an invitation.

So the next time you step outside after a light shower and take a deep breath of that fresh, earthy air, you can appreciate what is really going on. Raindrops are fizzing tiny scent-carrying aerosols up from the ground, and part of what you smell is a message sent by soil bacteria to tiny creatures under your feet. It may not sound romantic, but it is certainly remarkable. And it does not make the smell of rain one bit less lovely.