You probably never thought that the particular scent produced when water falls on the soil had a name.
Well, the scent is the combination of two substances: petrichor and geosmin.
The word petrichor is formed putting together two Greek words: petros, meaning stone, and ikhôr, referring to the fluid that flows in the veins of the gods in Greek mythology.
The term was coined in the 1960s by two Australian geologists in the journal Nature, who described it as "the scent that derives from an oil exuded by certain plants during dry periods". When in contact with rain, this substance becomes an aerosol in combination with geosmin. Thanks to an MIT study from 2015, there are detailed images that explain the process.
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Research on petrichor has shown that this compound, which, for the time being, cannot be synthesised in a lab, slows seed germination, something that might point to a defence mechanism of plants during drought periods.
As for geosmin, its Greek root means "earthy scent", and it is a chemical substance produced by Streptomyces coelicolor bacteria and some types of cyanobacteria found in the soil when they react with water.
When there is a long period without rain, larger amounts of bacteria accumulate in the soil. Thus, this phenomenon is particularly intense in the summer, when you can perceive the "rain scent" more clearly.
Geosmin is important for desert vertebrates, especially for the survival of camels. They are able to smell the water from as far away as 80 kilometres, but they are not the only ones: some earthworms and insects can also follow the scent of these bacteria.
Next time you smell wet earth, you will know the scientific explanation.
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