The Reason Behind Earth’s perfume!

“Notice the small things. The rewards are inversely proportional.” – Liz Vassey.

How often do we appreciate the little things in life?

All of us enjoy and love the aroma that lingers after a rain shower. Petrichor, as we know it, is a beautiful word. It was first coined in 1964 by 2 Australian scientists of Commonwealth Scientific and Industrial Research Organisation. 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. But it isn’t the beauty of the word that makes the scent so pleasing. There’s a lot of science behind it. Naturally, this topic has been chased by researchers and perfumers for years!

Now, why does Rain smell so good?

Well, there is not one single contributing factor that results in the smell that we adore. Compounds from bacteria and plants, splitting of atmospheric chemicals mostly play a major role in this regard.

The most widely cited compound whenever petrichor is discussed is Geosmin produced by a special class of soil bacteria called Actinomycetes. What’s special about this compound is that the complex chemical structure of geosmin makes it detectable to human nose even at extremely low concentrations. Our noses can detect just a few parts of geosmin per trillion of air molecules!

Apart from this, oils and fatty acids from plants are important contributors. During a dry spell, plants release a special mix of oils which inhibit growth and reduce competition for water. These oils remain in the soil and spaces in rocks. When rainfall occurs these oils are converted to smaller volatiles and the combination of these with Geosmin causes Petrichor! The geosmin and other petrichor compounds dissolved within the raindrop are released in aerosol form and carried by the wind to surrounding areas.

Another interesting thing is the pre-rain smell (or that calm before the storm smell). The splitting of oxygen molecules in the upper atmosphere due to lightning and other reasons results in formation of individual oxygen atoms which then combine with other oxygen molecules to form ozone. Ozone has a strong odour which can be detected. Although it is mostly unstable in the lower atmosphere, the winds during a storm can bring the Ozone down giving the ‘pre-rain’ smell.

Scientists still are actively researching on this topic. Interestingly, even though the phenomenon was characterized in 1960s, the mechanism was known just recently! A few years back, researchers at MIT have demonstrated using slow motion cameras the mechanism behind the phenomena! (Video) Another interesting field of study in the present time is how petrichor or the compounds involved in it affect early plant growth.

“Sometimes we should express our gratitude for the small and simple things like the scent of the rain, the taste of your favorite food, or the sound of a loved one’s voice.” – Joseph B. Wirthlin

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–Raibat Sarker

References and Further Reading:

• https://www.youtube.com/watch?time_continue=82&v=Waqmq_GTyjA
• https://www.compoundchem.com/2014/05/14/thesmellofrain/
• https://www.nature.com/articles/2071415a0

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