In the summer, after sunny days and hot asphalt, a storm arrives that downloads rain on the dry soil, perhaps after a period of drought, and we hear a particular smell. Not extremely pleasant, but a fresh, earthy aroma, for some familiar and pleasant motif. This smell has a precise name: Petriche (from the Greek Petros“Stone”, e Ichor“Lymph of the gods”). When the first drops of rain hit the dry soil, a sort of aerosol effect, a physical mechanism with which the drops of rain raise substances trapped in the ground in the air, and one is released mixture of substances produced by plants, bacteria and minerals. In reality, it is not a prerogative of summer, it also occurs in winter, but when the soil is drier, this physical process occurs in a more marked way.
Our nose capta with extraordinary precision this mix of drops of water and odorous molecules (Volatile Organics Compounds VOCS)among which the geosmina, produced by soil bacteria. In a nutshell, we smell the “essence” of the soil. We must not be surprised if we notice it, even if we do not associate it with anything in particular: the Petricor is a real message of nature that perhaps helped our ancestors to understand where there was waterlife, security. Today we still have to, as a memory that we have not lived, but that we feel ours.
Geosmina: the scent of the earth (and beets)
There geosmina It is a molecule produced by microorganisms soil, in particular from bacteria of the genre Streptomycesbelonging to Attachabacteria. It accumulates in the ground during dry periods and is freed in the air When the drops of water drag her out from the ground. He is the main responsible for the classic “Owned earth smell” after the rain.
Our nose is a very efficient sort of “molecular sensor”: it is able to perceive geosmina in tiny quantities, up to 5 parts for trilione (like a needle in a haystack to be clear). This means they are enough very few molecules to activate our olfactory receptors. Some studies have found how in some species to be able to recognize the smell of rain it was an evolutionary advantage and suppose, but it is theories that require confirmationthat even the hypersensitivity of our nose to geosmina could be an evolutionary effect due to the need for recognize humid placesso rich in drinking wateror avoid foods contaminated with mold and bacteria.
Geosmina is also the same molecule that gives to beetroot that “earthy” aftertaste. Not surprisingly, many wines or mineral waters that have unpleasant “cap” or “earth” hints are contaminated with traces of geosmina.
The aerosol effect: the drops that spray perfume
But how does this molecule do to reach our nostrils to the first shower of rain? The answer is very dynamic: the drops of rain, falling on Porose and dry surfaces Like a ground, they trap the air at the point of impact, creating bubbles that then go up and burst, freeing tiny particles in the air: a real Natural aerosol.
A study by the Massachusetts Institute of Technology (MIT) has shown that this phenomenon, called “aerosousization for impact “is more efficient on dry, wrinkled and porous surfaces, such as beaten or concrete. This is also why the scent of the paetricre is more intense at the beginning of the rainwhen the soil is still dry and all the Geosmina accumulated together with other substances is released.
The droplets transport with them geosmina, vegetable oils, volatile organic compounds (viocs) and, occasionally, spores of bacteria. It is not just an olfactory question: this process also has a ecological role, contributing to the cycle of substances in the soil and, in some cases, even to diffusion of microorganisms. The phenomenon is more marked in the summer: at the first rain, the dry soil releases all the substances accumulated in days of heat and humidity, releasing their smell tutto in a blow.
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