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Soil and water being splashed by a raindrop

Petrichor (/ˈpɛtrɪkɔːr/) is the earthy scent produced when rain falls on dry soil. The word is constructed from Greek petra (πέτρα), meaning "stone", and īchōr (ἰχώρ), the fluid that flows in the veins of the gods in Greek mythology.

OriginsEdit

The term was coined in 1964 by two Australian CSIRO researchers, Isabel Joy Bear and Richard G. Thomas, for an article in the journal Nature.[1][2][3] 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.[4] In a follow-up paper, Bear and Thomas (1965) showed that the oil retards seed germination and early plant growth.[5]

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[6] in which he refers to the subject. He writes, "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 23rd April 1891, and printed in Volume 112 (1891) of Comptes Rendus, entitled "Sur l'Odeur propre de la Terre".[7]

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 goes on to explain his theory, 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."

MechanismEdit

In 2015, scientists from the Massachusetts Institute of Technology (MIT) used high-speed cameras to record how the scent moves into the air.[8] The tests involved approximately 600 experiments on 28 different surfaces, including engineered materials and soil samples.[9] When a raindrop lands on a porous surface, air from the pores forms small bubbles, which float to the surface and release aerosols.[8] Such aerosols carry the scent, as well as bacteria and viruses from the soil.[8] 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.[8]

The human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion.[10] Some scientists believe that humans appreciate the rain scent because ancestors may have relied on rainy weather for survival.[11]

See alsoEdit

  • Geosmin, the substance responsible for the odour of earth.
  • Dimethyl sulfide, one of the molecules responsible for the odour of the sea.

FootnotesEdit

  1. ^ Bear, Isabel Joy; Thomas, Richard G. (March 1964). "Nature of argillaceous odour". Nature. 201 (4923): 993–995. Bibcode:1964Natur.201..993B. doi:10.1038/201993a0.
  2. ^ Garg, Anu (2007). The Dord, the Diglot, and an Avocado Or Two: The Hidden Lives and Strange Origins of Words. Penguin. p. 399. ISBN 9780452288614.
  3. ^ The Conversation. "The smell of rain: how CSIRO invented a new word". March 31, 2015 2.02am BST https://theconversation.com/the-smell-of-rain-how-csiro-invented-a-new-word-39231
  4. ^ Yuhas, Daisy (July 18, 2012). "Storm Scents: It's True, You Can Smell Oncoming Summer Rain: Researchers have teased out the aromas associated with a rainstorm and deciphered the olfactory messages they convey". Scientific American. Retrieved July 20, 2012.
  5. ^ Bear, Isabel Joy; Thomas, Richard G. (September 1965). "Petrichor and plant growth". Nature. 207 (5005): 1415–1416. Bibcode:1965Natur.207.1415B. doi:10.1038/2071415a0.
  6. ^ Scientific American Vol. 64, No. 20 (MAY 16, 1891), p. 308 https://www.jstor.org/stable/26100386
  7. ^ Comptes rendus hebdomadaires des séances de l'Académie des sciences, 1891/01 (Tome 112), pages 598-599, https://gallica.bnf.fr/ark:/12148/bpt6k3068q/f598.image
  8. ^ a b c d Cooper-White, Macrina (15 January 2015). "Here's Why Rain Has That Distinctive Smell". The Huffington Post. Retrieved 17 January 2015.
  9. ^ Chu, Jennifer (14 January 2015). "Rainfall can release aerosols, study finds". MIT News. Retrieved 17 January 2015.
  10. ^ Polak, E.H.; Provasi, J. (1992). "Odor sensitivity to geosmin enantiomers". Chemical Senses. 17: 23. doi:10.1093/chemse/17.1.23.
  11. ^ Palermo, Elizabeth (21 June 2013). "Why Does Rain Smell Good?". LiveScience.com. Retrieved 17 January 2015.

ReferencesEdit

  • Bear, I.J. & Thomas, R.G., "Genesis of Petrichor", Geochimica et Cosmochimica Acta, Vol.30, No.9, (September 1966), pp.869-879.

External linksEdit