Petrichor: the scent of the Earth after it rains

August 8, 2022 | Environment
Petrichor: the unique Earth fragrance that comes out of the soil after it rains | Photo: Shutterstock

Where does the universal smell of Earth come from? Here's everything you need to know about petrichor, the pleasant scent of soil after it rains.

Can you smell the pleasant fragrance created by rain?

Probably you have already, as almost everyone. It is one of the most familiar essences on the planet.

We can experience it in a garden, in a forest, in a grass field, in the mountains, or even in small urban farming systems.

The human nose can detect up to a trillion odors and scents. Most of them are connected to childhood memories and past emotions.

But Earth has a particular smell, especially before, during, and after a rainfall event or thunderstorm.

For instance, when the electrical charge of lightning occurs, it splits oxygen gas into separate atoms, and some of those can reform into ozone (O3) in the clouds.

Wind currents can then transport it and channel the smell of ozone into your nostrils. Then, another natural phenomenon takes place.

As the rain begins to fall and touch land, a new perfume emerges from the soil - petrichor.

Lightning: it splits oxygen gas into separate atoms, and some of those can reform into ozone (O3) in the clouds | Photo: Shutterstock

Petrichor: A Universal Fragrance

So, what exactly does petrichor mean?

Petrichor is the name given to the scent of the Earth after precipitation or - as we often refer to it - the smell of rain.

The word originates from the Greek "petra," meaning "stone," and "ichor," a reference to the golden fluid or blood that flows in the veins of the Gods and immortals in Greek mythology.

How is petrichor formed?

Many natural clay pots emit a fragrance similar to rain when we sprinkle water on them. This scent emission is a property of many types of dry clays and soils.

The smell-of-rain phenomenon has been a mystery for quite a while and has not yet been entirely resolved.

We know that decomposed organic material gets blown from dry soil and lands on rocks.

When air is dry and there's no rain, the chemical mixture of organic components and minerals remains in the soil and rocks.

This signals plants to prevent seeds from sprouting and roots from growing due to the lack of water.

We also now know why this unique scent is everywhere around the world, why things smell like rain pretty much the same everywhere around the world, and why the fragrance is familiar to everyone.

Nevertheless, part of the secret has only been unveiled recently.

Petrichor: the universal fragrance of the Earth after rain | Photo: Zrnzević/Creative Commons

Bacteria, Spores, and Aerosoles

Researchers from the Massachusetts Institute of Technology (MIT) discovered that when a raindrop hits a porous surface, it traps tiny air bubbles at the point of contact.

These air bubbles are released as aerosols and feature a mixture of minerals and decomposed organic material.

The smell of rain is predominantly observed in arid regions and was known to be associated with the first rains after draught.

Rocks and soil in such arid or semi-arid regions develop fragrant oils.

These fragrant oils are also caused by a bacteria called streptomyces - the largest genus of actinomycetota (or actinobacteria) - developed during warm and relatively damp conditions.

Streptomyces are an important bacteria we've learned to use in order to produce several different antibiotics that have been very actively used in medicine.

So, when the climate is dry, some bacteria produce spores in the rocks and soils.

These spores come out of the rocks sometime in, for example, pre-monsoon or storm days due to humidity.

The MIT research concluded that raindrops trap these spores inside them before they burst into the air.

The spores act as aerosols and carry the smell to our noses. That is why you could smell the scent of the Earth right from your window.

You may see a similar process in action when a fizzy drink is poured into a glass, and gas bubbles float up to the surface and pop.

The fragrant oil compound that is found in the rocks is known as geosmin and gives beets their unique earthy flavor.

Interestingly, geosmin stands for "earth smell."

The smell of rain: human beings can detect petrichor better than sharks smell blood | Photo: Rodeheaver/Creative Commons

Springtails: Spreading the Scent Worldwide for Millions of Years

The springtail is also part of the process.

This insect enjoys the smell of geosmin and, at the same time, eats streptomyces.

Although streptomyces is poisonous to many different species and will kill certain animals, the springtail is immune to the poison inside the bacteria.

When springtails absorb streptomyces bacteria and release it in their feces, it is basically spreading their spores around the planet.

And that is precisely what they've been doing for over 400 million years.

And that is precisely why the Earth's smell is prevalent everywhere around the globe.

These bacteria are everywhere in the ground and seem to release the smell when the environment gets wetter or when the rain comes out, forcing these bacteria to produce all these molecules.

Unanswered Mysteries

Petrichor is thought to attract camels to oases and signal river and lake fish that it's time to spawn.

But why are our noses so extremely sensitive to this molecule?

There's still research being carried out to try to understand why is it that humans smell this so well and why is it that most of us like it.

We do know that we can smell it better than sharks smell blood and that only a few molecules per trillion molecules of air are needed for us to be able to detect this smell.

So, petrichor is millions of times more potent than most other odors we can smell.

Hopefully, one day, scientists will be able to discover why we're so sensitive to it.

For now, we know where it's coming from and what's spreading petrichor everywhere around the planet.