Petrichor () is the earthy scent often associated with rain falling on dry soil or rock. The term was introduced by Australian researchers Isabel Joy Bear and Richard Grenfell Thomas in a 1964 paper in Nature. Bear and Thomas used the term for an odour released from rock or soil materials after dry conditions when they are exposed to moisture. The smell is associated with oils that accumulate in rocks and soil during dry periods and are released by moisture. Later experiments using high-speed cameras found that raindrops striking porous surfaces can trap air bubbles, which rise and burst as aerosols, providing a mechanism by which scent-carrying material can enter the air.
History
An early primary report on the phenomenon by Thomas Lambe Phipson (1833–1908) appeared in The Chemical News on 17 April 1891, which was re-published in its entirety, a month later, in The Scientific American. In the American presentation, he wrote, "This subject, with which I was occupied more than twenty-five years ago, appears... in... the Chemical News to have recently attracted the attention of Professor Berthelot and [Monsieur G.] Andre." Phipson refers to the short presentation of the two at a meeting of the French Académie des Sciences on 23 April 1891 (later printed in a French journal, as "Sur l'odeur propre de la terre" ("On the Earth's Own Smell"). Phipson continues, "I find... [from] my old notes... [of] 1865, that it is doubtful whether I ever published... these observations", going on to say, as the problem was not yet solved, he again offered his observations, theorizing that the odour "was due to the presence of organic substances closely related to the essential oils of plants", and that the substances consisted of "the fragrance emitted by thousands of flowers" that had been absorbed into the pores of the soil, only to be released by rain. Based on his attempts at isolation, he proposed it as a singular substance, "very similar to, if not identical with, bromo-cedren derived from essence of cedar."
A popular science summary of the phenomenon from this century states, rather, that the "[t]he smell is... a concoction of some 50 chemicals", plant-derived, and "trapped in the earth", awaiting release by rain, a perspective based on the mid-to-late 20th century research that would follow. In particular, the phenomenon was further 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". The authors describe how the scent derives from chemical compounds exuded by plants during dry periods, whereupon these are absorbed by clay-based soils and rocks. In a follow-up paper, Bear and Thomas suggested in 1965 that the chemical components giving rise to the "argillaceous odour of petrichor" also slow seed germination and early plant growth. Research in 2015 suggested that as the falling droplets of rain impact the surface of various wettable types of soil that are porous, aerosols are generated that "deliver elements of the porous medium to the environment", and suggest that this is the mechanism by which the petrichor scent is generated. Research in the 2020s has indicated that the volatile array of bacterial compounds, which include "the well-known...soil-smelling terpenoids geosmin and 2-methylisoborneol—compounds which animals, including humans, can "sense at extremely low concentrations" arise from evolutionarily conserved genes common to most all species of genus Streptomyces, and that these specific compounds mediate attraction of springtails (genus Collembola), an attraction that is conjectured to be mutually beneficial. Further research suggests that humans can detect imminent rain by the presence of ozone immediately before, coming from a combination of various soil components and lightning.
Mechanism
Preliminary research in 2015 suggests that 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 might carry the petrichor scent, as well as bacteria and viruses from the soil. This primary research report conjectured that raindrops that move slower might produce more aerosols, offering this as an explanation as to why petrichor is more commonly detected after light rains. Alongside plants, the Streptomyces genus of gram-positive bacteria, have, in part, been implicated in producing the chemicals that are found in these aerosols. With regard to specific chemicals, the human nose is sensitive, for instance, to geosmin, and can detect it at concentrations as low as 0.4 parts per billion. In popular reporting, speculation has been offered that camels in the desert may rely on the petrichor scent to locate sources of water such as in oases. Popular reporting has also suggested that some scientists believe humans might appreciate the scent of rain because ancestors relied on rainy weather for survival.
See also Geosmin – a compound contributing to the petrichor scent Dimethyl sulfide – A molecule contributing to the odour of the sea Mitti attar – a perfume recreating the loamy smell of rain
Citations
Further reading Bear, I. J.; Thomas, R. G. (September 1966). "Genesis of petrichor". Geochimica et Cosmochimica Acta. 30 (9): 869–879. doi:10.1016/0016-7037(66)90025-1.
External links
"Petrichor" at A Word a Day From the Oxford English Dictionary Hansen, Joe (2014-08-18), Where does the smell of rain come from? (3-min YouTube video), It's Okay To Be Smart, PBS Digital Studios, archived from the original on 2021-12-12, retrieved 2018-01-04 Adams, Cecil (2011-11-18), "What's that smell right before it rains?", The Straight Dope, Sun-Times Media Group, retrieved 2018-01-04 Petrichor, U.K. Met office. Why Is the Smell After It Rains So Appealing? The Petrichor phenomenon Petrichor – Why we Love the Smell of Rain Petrichor: why does rain smell so good?