Chemistry:Petrichor

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Petrichor (/ˈpɛtrɪkɔːr/ PET-rih-kor)[1] is the earthy scent from an array, in particular, of plant-derived chemicals, produced when rain falls on dry soil. The word was coined in 1964 by Australian scientist Richard Thomas,[2] in a paper co-authored with Isabel Bear which described the phenomenon.[3] The term was derived from from grc πέτρα (pétra) 'rock', or πέτρος (pétros) 'stone', and ἰχώρ (ikhṓr) 'ichor', the ethereal fluid that is the blood of the gods in Greek mythology.{{citation needed lead|date=May 2026}

History

An early primary report on the phenomenon by Thomas Lambe Phipson (fr) (1833–1908) appeared in The Chemical News on 17 April 1891,[4][non-primary source needed] which was re-published in its entirety, a month later, in The Scientific American.[5]Lua error: Internal error: The interpreter exited with status 1. 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."[5]Lua error: Internal error: The interpreter exited with status 1. Phipson refers to the short presentation of the two to 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").[6]Lua error: Internal error: The interpreter exited with status 1. 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."[5]Lua error: Internal error: The interpreter exited with status 1.

A popular science summary of the phenomonen 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.[7] 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.[8][9][3][7] Thomas coined the term "petrichor" to refer to what had previously been known as "argillaceous odour".[10] 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.Lua error: Internal error: The interpreter exited with status 1. 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.[11]Lua error: Internal error: The interpreter exited with status 1.

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.[12]

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.[13]Lua error: Internal error: The interpreter exited with status 1. Adding to contemporary studies of the decades-old subject of petrichor aerosols, further research suggests that detection of ozone by humans preceding rain—that chemical the product of various soil components and the action of lightning (when relevant)—adds to the phenomena of perichor sensing.[14]

Mechanism

A leaf with droplets on damp soil

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.[12][15] Such aerosols might carry the petrichor scent, as well as bacteria and viruses from the soil.[12] 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.[12] Alongside plants, the Streptomyces genus of gram-positive bacteria, have, in part, been implicated in producing the chemicals that are found in these aerosols.[13]Lua error: Internal error: The interpreter exited with status 1. 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.[16]Lua error: Internal error: The interpreter exited with status 1.

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.[17] Popular reporting has also suggested that some scientists believe humans might appreciate the scent of rain because ancestors relied on rainy weather for survival.[18]

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

  1. ↑ Lua error: Internal error: The interpreter exited with status 1. "petrichor". Dictionary.com Unabridged. Random House. https://www.dictionary.com/browse/Lua error: Internal error: The interpreter exited with status 1.. 
  2. ↑ Ward, Colin (11 April 2014). "Isabel 'Joy' Bear". CSIROpedia (CSIRO). https://csiropedia.csiro.au/bear-isabel-joy/. "Thomas gave the name 'petrichor' to this odour." 
  3. ↑ 3.0 3.1 Bear, Isabel Joy; Thomas, Richard G. (March 1964). "Nature of Argillaceous Odour". Nature 201 (4923): 993–995. doi:10.1038/201993a0. Bibcode: 1964Natur.201..993B. "The diverse nature of the host materials has led us to propose the name 'petrichor' for this apparently unique odour which can be regarded as an 'ichor' or 'tenuous essence' derived from rock or stone […] it does not imply that petrichor is necessarily a fixed chemical entity but rather it denotes an integral odour, variable within a certain easily recognizable osmic latitude.". Lua error: Internal error: The interpreter exited with status 1.
  4. ↑ Phipson, T.L. (17 April 1891). "Cause of the Odour Emitted by the Soil of a Garden after a Summer Shower". The Chemical News 63 (1638): 179. https://upload.wikimedia.org/wikipedia/commons/8/80/The_chemical_news._Volume_63%2C_January_-_June_1891._%28IA_s713id13691660%29.pdf. Retrieved 2026-05-10. Lua error: Internal error: The interpreter exited with status 1.
  5. ↑ 5.0 5.1 5.2 Phipson, T. L. (May 16, 1891). "The Odor of the Soil after a Shower". Scientific American 64 (20): 308. ISSN 0036-8733. Lua error: Internal error: The interpreter exited with status 1.
  6. ↑ Berthelot, M. & Andre, G. (January 1891). "Sur l'odeur propre de la terre". Comptes rendus de l'Académie des Sciences (Comptes Rendus) 112 (01): 598f. https://gallica.bnf.fr/ark:/12148/bpt6k3068q/f598.image. Lua error: Internal error: The interpreter exited with status 1.
  7. ↑ 7.0 7.1 Garg, Anu (2007). "There is Even a Word for That". The Dord, the Diglot, and an Avocado Or Two: The Hidden Lives and Strange Origins of Words. Penguin. p. 399. ISBN 9780452288614. https://books.google.com/books?id=SBlK4QrLmiIC&q=petrichor#v=snippet&q=petrichor&f=false. "The smell is said to be a concoction of some 50 chemicals from dry plants that are trapped in the earth. With the rains, they are released in the atmosphere." .
  8. ↑ Poynton, Howard (March 31, 2015). "The Smell of Rain: How CSIRO [scientists Invented a New Word"]. The Conversation. https://theconversation.com/the-smell-of-rain-how-csiro-invented-a-new-word-39231. Retrieved 2026-05-10. 
  9. ↑ Logan, Tim (August 27, 2018). "Why You Can Smell Rain" (in en-US). The Conversation. https://theconversation.com/why-you-can-smell-rain-101507. Retrieved July 14, 2020.  Note, Professor Logan makes no mention of research prior to that of the 1960s, and so this citation was moved from its appearance in the opening paragraph about work in the 1890s.
  10. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named ward
  11. ↑ Bear, Isabel Joy; Thomas, Richard G. (September 1965). "Petrichor and plant growth". Nature 207 (5005): 1415–1416. doi:10.1038/2071415a0. Bibcode: 1965Natur.207.1415B. Lua error: Internal error: The interpreter exited with status 1.
  12. ↑ 12.0 12.1 12.2 12.3 Chu, Jennifer (14 January 2015). "Rainfall Can Release Aerosols, Study Finds". https://newsoffice.mit.edu/2015/rainfall-can-release-aerosols-0114. 
  13. ↑ 13.0 13.1 Becher, Paul G.; Verschut, Vasiliki; Bibb, Maureen J.; Bush, Matthew J.; Molnár, Béla P.; Barane, Elisabeth; Al-Bassam, Mahmoud M.; Chandra, Govind et al. (2020-06-01). "Developmentally Regulated Volatiles Geosmin and 2-Methylisoborneol Attract a Soil Arthropod to Streptomyces Bacteria Promoting Spore Dispersal". Nature Microbiology 5 (6): 821–829. doi:10.1038/s41564-020-0697-x. ISSN 2058-5276. PMID 32251369. https://www.nature.com/articles/s41564-020-0697-x. Retrieved 2022-01-01. Lua error: Internal error: The interpreter exited with status 1.
  14. ↑ Yuhas, Daisy (July 18, 2012). "Storm Scents: It's True, You Can Smell Oncoming Summer Rain". Scientific American. https://www.scientificamerican.com/article/storm-scents-smell-rain/. 
  15. ↑ Joung, Y. & Buie, C. (14 January 2015). "Aerosol Generation by Raindrop Impact on Soil". Nature Communications 6 (6083). doi:10.1038/ncomms7083. https://doi.org/10.1038/ncomms7083. Retrieved 2025-05-10. Lua error: Internal error: The interpreter exited with status 1.
  16. ↑ Polak, E. H.; Provasi, J. (1992). "Odor Sensitivity to Geosmin Enantiomers" (in en). Chemical Senses 17 (1): 23–26. doi:10.1093/chemse/17.1.23. ISSN 0379-864X. Lua error: Internal error: The interpreter exited with status 1.
  17. ↑ Jordán , Antonio (2015-02-24). "Sure Can Smell the Rain" (in en-GB). European Geosciences Union (EGU) SSS Division blog. Munich, Germany: EGU Soil System Science (SSS) Division. https://blogs.egu.eu/divisions/sss/2015/02/24/sure-can-smell-the-rain/. 
  18. ↑ Palermo, Elizabeth (21 June 2013). "Why Does Rain Smell Good?". Live Science. LiveScience.com. http://www.livescience.com/37648-good-smells-rain-petrichor.html. 

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Further reading


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