You know that smell, right?
The phenomenon was first scientifically described in a 1964 paper by Australian researchers Isabel Bear and Dick Thomas, published in the journal Nature. In a moment of naming and branding genius, Thomas even managed to coin the term "petrichor" to refer to what had previously been known as "argillaceous odour” (not nearly as cool).
Petrichor, as it is now known, is the earthy scent produced when rain falls on dry soil. The word is constructed from Ancient Greek πέτρα (pétra) 'rock', or πέτρος (pétros) 'stone', and ἰχώρ (ikhṓr), the ethereal fluid that is the blood of the gods in Greek mythology. Nice work Dick.
Long before this phenomenon received its new name, it had been noticed and discussed in scientific circles.
On 17 April 1891, a brief note on the phenomenon, by scientist and voilinist(!) Thomas Lambe Phipson (1833–1908), appeared in The Chemical News. In the note he theorises that the odour "... was due to the presence of organic substances closely related to the essential oils of plants ..." and that these substances consist of "... the fragrance emitted by thousands of flowers ..." absorbed into the pores of the soil, and only released when displaced by rain. After attempts to isolate it, he found that it "... appeared to be very similar to, if not identical with, bromo-cedren derived from essence of cedar".
Bear and Thomas working at what was then the Australia’s CSIRO Division of Mineral Chemistry in Melbourne, were determined to identify and describe the origin of this distinctive smell. By steam distilling rocks that had been exposed to warm, dry conditions in the open, they discovered a yellowish oil – trapped in rocks and soil but released by moisture, that was responsible for the smell. The smell itself comes about when increased humidity – a pre-cursor to rain – fills the pores of stones (rocks, soil, etc) with tiny amounts of water.
While it's only a minuscule amount, it is enough to flush the oil from the stone, along with another compound, geosmin, a metabolic by-product of certain actinobacteria … and release petrichor into the air. This is further accelerated when actual rain arrives and makes contact with the earth, spreading the scent into the wind.
It’s a beautiful sequence of events, but one that may be hard to visualise. Thankfully, in a testament to the ongoing scientific fascination with this finding, a team of scientists at the Massachusetts Institute of Technology have released a super slow motion video of the petrichor process in motion. Using high-speed cameras, the researchers observed that when a raindrop hits a porous surface, it traps tiny air bubbles at the point of contact. As in a glass of champagne, the bubbles then shoot upward, ultimately bursting from the drop in a fizz of aerosols. Watch the video HERE.
As it happens, the human nose is super sensitive to geosmin and is able to detect it at concentrations as low as 0.4 parts per billion. Some scientists believe that humans appreciate the rain scent because ancestors may have relied on rainy weather for survival, and camels in the desert also rely on petrichor to locate sources of water such as oases.
The smell had actually been described long before it was addressed by science. In the perfume capital of India — Kannauj in Uttar Pradesh — a centuries-old process is still used to recreate the loamy smell of the first shower on dry ground, as an attar. They called it “mitti attar” or “earth’s perfume”, and it is made using clay extracted from the topsoil and baked in a kiln.
“The process takes about 15 working days”, says Akhilesh Pathak, a fourth generation perfumer who has inherited one of Kannauj’s oldest attar-manufacturing companies, Munna Lal Sons.
Due to its relatively recent fame, mitti attar has become increasingly popular throughout India and the Middle East. “I have been hoarding my 100 ml bottle for about four years now because I love how amazing it smells”, says Suman Bolar, a freelance writer from Bangalore for a 2016 article in the Hindustan Times. “I use it for various purposes. I put it in the washing machine when I wash my bedsheets, often put a few drops on my pillow before I sleep and sometimes just dab it behind my ears to feel good.”
Story Idea: Melanie Giuffré
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References
wikipedia.org/wiki/Petrichor
theconversation.com/the-smell-of-rain-how-csiro-invented-a-new-word
nature.com/articles/201993a0
hindustantimes.com/more-lifestyle/mitti-attar-take-home-that-lush-smell-of-the-first-rain-trapped-in-a-bottle
Images
1. Soil and water being splashed by a raindrop. Source: US Department of Agriculture.
2. Richard Thomas with Joy Bear studying petrichor. Source: CSIRO.
3. The Chemical News, 1891. Note by Thomas Lambe Phipson on page 179.
4. Video: Rainfall can release aerosols, high-speed video shows, Massachusetts Institute of Technology (MIT)
5. Dromedary camel in outback Australia, near Silverton, NSW. Photo: Jjron
6. Mitti attar made using clay extracted from the topsoil and baked in kilns
