![Water and soil splashed by the impact of a single raindrop](https://upload.wikimedia.org/wikipedia/commons/7/79/Water_and_soil_splashed_by_the_impact_of_a_single_raindrop.jpg)
Petrichor () is the earthy
scent
An odor (American English) or odour (Commonwealth English; see spelling differences) is caused by one or more volatilized chemical compounds that are generally found in low concentrations that humans and animals can perceive via their sense ...
produced when
rain
Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water f ...
falls on dry
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
. The word is constructed , the
ethereal fluid that is the blood of the gods in
Greek mythology
A major branch of classical mythology, Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and a genre of Ancient Greek folklore. These stories concern the Cosmogony, origin and Cosmology#Metaphysical co ...
.
Origins
Long before this phenomenon received its name in 1964, it had been noticed and discussed in scientific circles.
On 17 April 1891, a brief note on the phenomenon, by (1833–1908), appeared in ''The Chemical News''—it was re-published in its entirety, a month later, in ''
The Scientific American
''Scientific American'', informally abbreviated ''SciAm'' or sometimes ''SA'', is an American popular science magazine. Many famous scientists, including Albert Einstein and Nikola Tesla, have contributed articles to it. In print since 1845, it i ...
''—in which 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
onsieur G.Andre."
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 (1891) of ''Comptes Rendus'', entitled "Sur l'Odeur propre de la Terre" ("On the earth's own smell").
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."
The phenomenon was first scientifically described in a March 1964 paper by Australian researchers
Isabel Bear
Isabel 'Joy' Bear (4 January 1927 – 8 April 2021) was an Australian chemist who worked at CSIRO for over forty years. She was the first woman to be awarded the Royal Australian Chemical Institute Leighton Medal. She was inducted into the Vic ...
and
Dick Thomas, published in the journal ''
Nature
Nature, in the broadest sense, is the physics, physical world or universe. "Nature" can refer to the phenomenon, phenomena of the physical world, and also to life in general. The study of nature is a large, if not the only, part of science. ...
''.
[
] 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
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
s during dry periods, whereupon it is
absorbed by
clay
Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4).
Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...
-based soils and rocks. During rain, the oil is released into the air along with another compound,
geosmin
Geosmin ( ) is an irregular sesquiterpenoid, produced from the universal sesquiterpene precursor farnesyl pyrophosphate (also known as farnesyl diphosphate), in a two-step -dependent reaction. Geosmin, along with the irregular monoterpene 2-met ...
, a metabolic by-product of certain
actinobacteria
The ''Actinomycetota'' (or ''Actinobacteria'') are a phylum of all gram-positive bacteria. They can be terrestrial or aquatic. They are of great economic importance to humans because agriculture and forests depend on their contributions to so ...
, such as
streptomyces
''Streptomyces'' is the largest genus of Actinomycetota and the type genus of the family Streptomycetaceae. Over 500 species of ''Streptomyces'' bacteria have been described. As with the other Actinomycetota, streptomycetes are gram-positive, ...
,
which is emitted by wet soil, producing the distinctive scent;
ozone
Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...
may also be present if there is lightning.
In a follow-up paper, Bear and Thomas (1965) showed that the oil slows seed germination and early plant growth.
Mechanism
When a raindrop lands on a porous surface, air from the pores forms small bubbles, which float to the surface and release
aerosol
An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...
s.
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 petrichor is more common after light rains. Members of the
Actinomycetes
The Actinomycetales is an order of Actinomycetota. A member of the order is often called an actinomycete. Actinomycetales are generally gram-positive and anaerobic and have mycelia in a filamentous and branching growth pattern. Some actinomycete ...
, gram-positive bacteria, are responsible for producing these aerosols.
The
human nose
The human nose is the most protruding part of the face. It bears the nostrils and is the first organ of the respiratory system. It is also the principal organ in the olfactory system. The shape of the nose is determined by the nasal bones ...
is sensitive to
geosmin
Geosmin ( ) is an irregular sesquiterpenoid, produced from the universal sesquiterpene precursor farnesyl pyrophosphate (also known as farnesyl diphosphate), in a two-step -dependent reaction. Geosmin, along with the irregular monoterpene 2-met ...
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.
Camel
A camel (from: la, camelus and grc-gre, κάμηλος (''kamēlos'') from Hebrew or Phoenician: גָמָל ''gāmāl''.) is an even-toed ungulate in the genus ''Camelus'' that bears distinctive fatty deposits known as "humps" on its back. C ...
s in the desert also rely on petrichor to locate sources of water such as
oases
In ecology, an oasis (; ) is a fertile area of a desert or semi-desert environment'ksar''with its surrounding feeding source, the palm grove, within a relational and circulatory nomadic system.”
The location of oases has been of critical imp ...
.
See also
*
Dimethyl sulfide
Dimethyl sulfide (DMS) or methylthiomethane is an organosulfur compound with the formula (CH3)2S. Dimethyl sulfide is a flammable liquid that boils at and has a characteristic disagreeable odor. It is a component of the smell produced from cook ...
– One of the molecules responsible for the odour of the sea
*
Citations
General references
* Bear, I.J. & Thomas, R.G. (September 1966), "Genesis of Petrichor", ''Geochimica et Cosmochimica Acta'', Vol. 30, No. 9, pp. 869–879.
External links
"Petrichor"at A Word a Day
From the Oxford English Dictionary*
* {{citation , url= http://www.straightdope.com/columns/read/3023/what-s-that-smell-right-before-it-rains-plus , title= What's that smell right before it rains? , first= Cecil , last= Adams , author-link= Cecil Adams , date= 2011-11-18 , access-date= 2018-01-04 , website=
The Straight Dope
"The Straight Dope" was a question-and-answer newspaper column written under the pseudonym Cecil Adams. Contributions were made by multiple authors, and it was illustrated (also pseudonymously) by Slug Signorino. It was first published in 1973 in ...
, publisher=
Sun-Times Media Group
Sun-Times Media Group (formerly Hollinger International) is a Chicago-based newspaper publisher.
History
Sun-Times Media Group was founded in 1986 under the name ''American Publishing Company'', as a holding company for Hollinger Inc.'s Ameri ...
Petrichor, ''U. K. Met office''.Why is the Smell After it Rains So Appealing? The Petrichor phenomenonPetrichor – Why we Love the Smell of Rain
1964 neologisms
Olfaction
Precipitation
Soil
Rain
Foul-smelling chemicals