Ahh, don’t you just love that smell in the air when it rains for the first time after a bit of a dry spell?
Well that smell is known as ‘petrichor’, or the earthy smell of rain, and it’s created chemically by a terpene called Geosmin, which has a very earthy odor and can be detected by the human nose in concentrations as low as 5 parts per million.
Geosmin can be found in many different things that may have an earthy smell or taste, like Catfish, Trout, Spinach, and some Mushrooms, but in the case of petrichor, the geosmin that you’re smelling is being released from a tiny soil-dwelling bacteria called Streptomyces, which uses geosmin to attract organisms that disperse their spores.
See, after a long dry spell or some other environmental event, conditions in the soil may not be favorable for streptomyces to thrive, so they produce spores that they use to reproduce as well as geosmin. Then when it rains, the raindrops disturb and aerate the soil, stirring up these terpenes and sending them into the air, giving us that pleasant smell we enjoy so much, but also attracting small rodents, insects and critters who dig around in the soil to stir up the spores of Streptomyces, allowing them to drift or be carried away by said critter to new fertile soil where it can grow and thrive, continuing it’s cycle of life for the next generation.
It’s a pretty wild and awesome example of different species evolving and working with one another, playing off their strengths, weaknesses, desires and demands so that they all can keep on keeping on - who would’ve thought evolution could smell so good?!
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