Petrichor Potpourri

On summer days, after a light rain, a distinctive odor fills the air that’s almost universally pleasing to all who smell it. What is it? Why is it? When is it? Let’s get learnt.

Blood of the Stone God

While the particular scent has been around since rain and soil have coexisted, there has only been a name for it for about 50 years. It was the Australians who were responsible, specifically Isabel Joy Bear and Richard Thomas with the Commonwealth Scientific and Industrial Research Organisation (CISRO). In a 1964 article for the journal Nature (titled Nature of Argillaceous Odour), they coined the term by combining the Greek words for stone (petra) and blood of the gods (ichor).

Bear and Thomas weren’t the first to isolate the unique scent of petrichor. That distinction goes to a small region of India, specifically the city Kannauj in Uttar Pradesh, renowned in the region for its perfume industry. The scent is called Mitti Attar and means “the smell of earth.” To make it they bake clay, soak it in water, boil the mixture, then mix the condensate with sandalwood oil.

During dry periods certain plants excrete oils that are absorbed by rocks and certain soils. When it rains, particularly a light rain, these oils are released into the air as an aerosol. Aerosols are when tiny solid or liquid particles are suspended (floating around) in a gas. In this case the gas is the air. The Massachusetts Institute of Technology actually filmed this process with a high-speed camera.

One of the chemicals released in this pluvial aerosol is geosmin. It’s another word that comes to us from Greek: geo (earth) and osme (smell). Apparently, it tastes and smells like dirt and it’s responsible for the earthy scent of catfish. Our noses are so attuned to the smell of this stuff that it’s possible for us to detect it in concentrations as small as 5 parts per trillion, which helps explain why petrichor is such an evocative scent.