Super nice, especially after a period of drought: that sweet, fresh, earthy scent of the first raindrops. It is a unique and recognizable scent, which quickly dissipates after the start of a shower. What causes that smell, and why does it make us so happy?
It is not the drops themselves that smell so good, but the ground and leaves on which they fall. When they become wet again for the first time, they emit an odor for which there is even a special name: petrichor. That name has Greek roots: petros (stone, soil) and ichor (the blood of the immortals in Greek mythology). Australian chemists Isabel Bear and Dick Thomas coined the term in 1964 Nature.
They were the first to investigate the smell. Others had already speculated about it. For example, the British violinist and scientist Thomas Phipson wrote in 1891 Scientific American that vegetable oils that end up in the ground carry with them “the scent of thousands of flowers.” Bear and Thomas determined that the soil is indeed saturated with plant oils and that these are important in petrichor. They managed to distill those oils from soil and… did experiments with it.
For example, they saw that plant seeds germinate less well in soil with these oils. By excreting these substances, plants would prevent their seeds from germinating when it is too dry. During a rain shower, the oils are washed out of the ground and end up in the air as tiny droplets. Then their concentration in the soil decreases and seeds can germinate.
Bear and Thomas left unclear which components of those oils are important for petrichor. Botanists suggested later that it mainly concerns three general fatty acids: stearic acid, oleic acid and palmitic acid.
Spinach and mushrooms
The splashing of raindrops releases even more substances into the air. For example geosmine. This is an alcohol that is produced by many soil bacteria. And that substance is also important in petrichor, Bear and Thomas discovered. Geosmine smells earthy. Beetroot, spinach and mushrooms also contain geosmin. That’s why some people think it tastes like earth.
Human noses are particularly sensitive to geosmin. We can already detect the substance in a concentration of less than five molecules of geosmine per trillion other molecules in the air (a one with twelve zeros). Dutch, Japanese and German researchers wrote this in 2022 Environmental Microbiology: “That is equivalent to one teaspoon in two hundred Olympic swimming pools.” For comparison: sharks is said that they can smell a drop of blood at a ratio of one to a million – so 200,000 times less sensitive.
“We do not yet understand its exact biological function,” these authors continue. From geosmine, they mean. The biological function of our sensitivity to it is obvious. At least, according to some evolutionary anthropologists. For our distant ancestors – in fact for all animals – it was important that they could smell where it had recently rained. After all, they could expect fresh water, vegetation and perhaps also game there.
That also explains why we like that scent so much. We have traditionally associated this with abundance, with an end to hard times, with green and with happiness. Perfume makers in India They even seem to distill the scent from earth and sell it in expensive bottles.
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