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Can Rain Extract Blood from a Stone?

In a manner of speaking, yes, and we can smell it.

When the land has been dry and it starts to rain, there is a peculiar smell in the air, an earthy scent called petrichor. In the 1960s, two Australian scientists, Isabel Bear and Richard Thomas, began to steam-drill rocks exposed to dry conditions and they discovered the oil that serves as a precursor to petrichor. This oil is secreted by plants during the dry season and seeps into rocks and clay-based soils. When it finally rains down on the porous ground, small air bubbles containing this oil form and migrate to the surface of the soil and become air borne.

And that’s petrichor. You may think you need heavy rains to release this oil into the air but, actually, scientists found out that slower rain drops tend to create more aerosols, so light rains actually offer better conditions for this aromatic phenomenon.

Petrichor is made up of many substances, some of them secreted by bacteria, like the molecule geosmin to which our nose is very sensitive. When you eat beets and get that earthy taste in your mouth, that’s geosmin you’re noticing.

When Bear and Thomas described the oil released from the dry soil by the rain in their 1964 Nature paper, they had to come up with a name for its smell. They initially characterized it as an “argillaceous odour”, meaning a smell that comes from clay. But they needed something more specific. Which is why they coined the word “petrichor”. In Greek mythology, gods didn’t have blood flowing through their veins; they were animated by a divine liquid known as “ichor”.

Since “petro” means stone, petrichor is the blood of a stone. And all it takes to release it is a light rainfall.


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