Petrichor is the earthy scent produced when rain falls on dry soil. The word is constructed from Greek , "rock", or petros, "stone", and īchōr, the fluid that flows in the veins of the gods in Greek mythology.
Origins
The phenomenon was first scientifically described in a March 1964 paper by Australian researchers Isabel Bear and Dick Thomas, published in the journal Nature. Thomas coined the term "petrichor" to refer to what had previously been known as "argillaceous odour". In the article, the authors describe how the smell derives from an oil exuded by certain plants during dry periods, whereupon it is absorbed by clay-based soils and rocks. During rain, the oil is released into the air along with another compound, geosmin, a metabolic by-product of certain actinobacteria, which is emitted by wet soil, producing the distinctive scent; ozone may also be present if there is lightning. In a follow-up paper, Bear and Thomas showed that the oil slows seed germination and early plant growth. Long before this phenomenon received its name in 1964, it had been noticed and discussed in scientific circles. In May 1891 a brief note by TL Phipson appeared in The Scientific American refers to the subject. He wrote, "This subject, with which I was occupied more than twenty-five years ago, appears from a paragraph in a late number of the Chemical News to have recently attracted the attention of Professor Berthelot and M. Andre." No doubt, Phipson was referring to a short paper read by Berthelot and André at the meeting of the French Académie des Sciences on 23 April 1891, and printed in Volume 112 of Comptes Rendus, entitled "Sur l'Odeur propre de la Terre". Phipson continues, "I find, on referring to my old notes, which are dated 1865, that it is doubtful whether I ever published the results of these observations; and as the distinguished chemists I have just named have not quite solved the problem, I hasten to give the results I obtained so long ago." He then theorizes 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."
Mechanism
In 2015, scientists from the Massachusetts Institute of Technology used high-speed cameras to record how the scent moves into the air. The tests involved approximately 600 experiments on 28 different surfaces, including engineered materials and soil samples. When a raindrop lands on a porous surface, air from the pores forms small bubbles, which float to the surface and release aerosols. Such aerosols carry the scent, as well as bacteria and viruses from the soil. Raindrops that move at a slower rate tend to produce more aerosols; this serves as an explanation for why the petrichor is more common after light rains. Actinomycetes is the bacteria responsible for producing spores in soil. The human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion. Some scientists believe that humans appreciate the rain scent because ancestors may have relied on rainy weather for survival.
In the Discworld novel The Last Continent, there are frequent references to the Ecksian people being the only culture on the Disc to have a word meaning "that smell you get after rain".
Neil Gaiman frequently refers to the scent of earth after rain in his novel American Gods.
The word was featured in the Doctor Who episode "The Doctor's Wife", as part of a telepathic password to enter one of the TARDIS' old control rooms. To use it, Amy Pond had to imagine the smell of dust after rain. In "Closing Time", Amy is seen on an advertisement for a perfume with the same name; it shows a picture of her face and features a bottle of perfume and the phrase "Petrichor - For the girl who's tired of waiting".
In Barton Bishop's play Still the River Runs, brothers Jesse and Wyatt discuss the distinctive aroma that Wyatt identifies as petrichor.
