In the month of June, when the sky turns grey, the air gets humid and the breeze turns cool, you know that monsoon has arrived in India. With monsoon, comes a sweet, earthy scent, especially after the first rain.
This scent of rain has a name: Petrichor (In Greek, ‘petra’ is stone and ‘ichor’ is blood of Gods).
As the air becomes humid, the soil gains moisture from the air and wakes up soil-dwelling bacteria called actinomycetes from their summer rest. As a result, these bacteria actively start breaking down organic materials in the soil and produce a chemical called geosmin in this process.
When slow raindrops hit the ground, they capture tiny air bubbles from pores in the soil. These bubbles travel to the water surface and burst, thus spraying out geosmin along with certain plant oils present in the soil (watch it here). The combination of these fragrant chemical compounds makes up the petrichor.
It is more common to experience petrichor after a light rain rather than a heavy downpour as fast-moving raindrops during a heavy rainfall reduce the chance for bubbles to form and release the fragrant compounds.
Petrichor is refreshing for us, but what use is geosmin to the actinomycetes?
The geosmin produced by these bacteria attract hungry, tiny bugs that live in the soil. The bugs eat up the actinomycetes but at the same time become carriers of bacterial spores, which are then spread far and develop into more bacteria.
Since many years before petrichor was scientifically defined in 1964, perfume makers in Kannauj, India have been bottling the wet earth scent using extractions from parched clay.
So, the next time you miss monsoon in the middle of summer, you can probably try this mitti attar (Scent of Rain or Petrichor essential oil).
Sources and further reading:
Image: Rain by Musonda Kabwe, for Tone's Big Drop written by Edna Gicovi , published by Book Dash (©Book Dash, 2017) under a CC BY 4.0 license on StoryWeaver.
Meet the Molecules; Geosmin (https://www.jic.ac.uk/blog/meet-the-molecules-geosmin/)
Rainfall can release aerosols, study finds (https://news.mit.edu/2015/rainfall-can-release-aerosols-0114)
Love the Smell of Rain? There's an Ulterior Motive Behind The Lure of Petrichor (https://www.sciencealert.com/that-rain-smell-turns-out-we-re-smelling-the-circle-of-life)
Nature of Argillaceous Odour (https://www.nature.com/articles/201993a0)
Making Perfume From the Rain (https://www.theatlantic.com/international/archive/2015/04/making-perfume-from-the-rain/391011/)
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