You know that unmistakable smell that spreads after a storm? If you like this perfume too, you can exclaim: “What a good petricore!”
They are not the protagonists of a fairy tale or even the lines of a poem, but the scientific names of some of the components that determine the typical ” scent of rain ” that we feel after a downpour. That precise odor is not in fact due to the water itself, but to a mixture of chemical substances : ozone, oils and resins, petricore and geosmin which, combined with each other, create this unmistakable aroma.
Petricore: the sap of the gods
The discovery of petricore was made public in the well-known scientific journal Nature about fifty years ago thanks to the studies of Australian chemists Isabella Bear and RG Thomas . The two, to find out what were the causes of the scent of the rain, tried to dry some clay and then extract the oils found inside. The result was a yellowish substance with a smell that was reminiscent of rain. Bear and Thomas were the ones who also devised the term “petricore” to indicate this particular perfume: it is the union of two Greek words ” petros ” (stone) and ” ichor ” (blood of the gods).
In 2015, thanks to a team of scholars from the Massachusetts Institute of Technology (MIT) , the mechanism by which petricore is released into the air was also discovered . With the arrival of the rain, in fact, scholars have seen how the drops of water falling on the ground are able to make the petricore retained by some plants and the clay of the ground spread in the air creating that characteristic odor.
Thanks to high-frequency cameras , the MIT scientists studied the impact of raindrops on some porous surfaces and managed to capture the tiny particles of air and water (aerosols) as they rise from the ground shortly after the rain fall, that is, when they release the petricore which, thanks to the wind, spreads into the air.
Geosmina: irresistible earthy fragrance
The smell of damp earth , on the other hand, even more intense when the rain occurs after a long period of drought, is due to a chemical compound called geosmin . Geosmin is produced by different classes of microorganisms including cyanobacteria (blue algae -green) and actinomycetes ( especially Gram positive aerobic bacteria of the genus Streptomyces ): it is released when the microorganisms themselves die. The earthy odor caused by geosmin can sometimes mix with that of the oils and resins produced by plants which, with the humidity of the rain, tend to dissolve even more in the air, giving it an aroma. sweetish .
The name “geosmina” also derives from the Greek for a combination of the terms ” gea ” (earth) and ” osmé ” (smell).
The human body is very sensitive to this fragrance and is able to detect it even from very far away. Several perfumes on sale are formulated with solutions containing geosmin precisely because it is particularly pleasant for the human sense of smell.
Camels in the desert are also able to perceive this substance. In fact, they would be able to hear it even at a distance of 10-15 kilometers in order to chase it to get to the oasis that emanates it and where, of course, they can drink. Furthermore, during watering, the spores of streptomycetes (producers of geosmin) remain on the animals that carry them throughout the desert.
Feeling that it is about to rain thanks to the ozone
It happens that some people are able to “smell in the air” the arrival of a storm . This is possible thanks to another little mystery easily revealed thanks to chemistry. In fact, the ozone molecules released by lightning in the upper layers of the atmosphere are transported by currents at low altitudes where they give the air that particularly pungent and garlicky smell that can lead one to assume the arrival of a downpour.
