What do you think of this fact?
You know that smell when rain hits dry ground, or when you dig around in a garden. It feels like nature’s own air freshener. In reality, a big part of it is advertising: bacteria in the soil luring tiny animals so they’ll carry their offspring around.
The molecule behind the smell
The earthy note comes from a compound called geosmin. According to the John Innes Centre in England, it is made by soil bacteria of the genus Streptomyces, the same group that gives us many of the world’s most effective antibiotics. Science Notes adds that certain cyanobacteria produce it too, and that rain hitting the ground helps spread it into the air.
Your nose is extremely good at catching it. Depending on the source, people can detect geosmin at somewhere between 5 and 100 parts per trillion. The John Innes Centre says that makes us more sensitive to it than sharks are to blood.
A scent with a job
Every Streptomyces species studied makes geosmin, which hinted that the smell has to be useful. A 2020 study in the journal Nature Microbiology, led by researchers in Sweden with partners including the John Innes Centre, found out what for. The smell attracts springtails, millimetre-sized, six-legged soil animals.
The team showed this in several ways, as Lund University reports. Traps baited with Streptomyces caught springtails in the field. Electrodes on the animals’ antennae showed that they sense geosmin. And in Y-shaped tubes, the springtails chose the path that smelled of soil.
The deal works for both sides. The springtails eat the bacteria and, thanks to special enzymes, can handle their antibiotics. Meanwhile, spores stick to their waxy bodies and survive the trip through their gut, so the springtails spread the bacteria on their bodies and in their droppings. Fittingly, the bacteria only make geosmin when they form spores. Mark Buttner of the John Innes Centre compares it to birds eating fruit and spreading the seeds.
Bonus fact: The word “petrichor” for the smell of rain was coined in 1964 by two CSIRO scientists in Australia, Isabel Joy Bear and Richard Thomas. It combines the Greek “petra” (stone) with “ichor”, the ethereal blood of the gods in Greek mythology.
That cosy rain smell is basically a dinner invitation, just not addressed to you.