At last, the first rain after a long summer came, quenching the thirst of the ground. Soon enough, you will notice the rich and refreshing earthy smell. This unique odor is called petrichor. It is a term coined by Australian scientists who were then studying the source of the earthy scent in 1964. They researched the aroma produced by moist sediment, rock, and clay. Soon enough, there were other studies made later to discover the reasons behind petrichor.
The earthy scent is caused by different factors, such as the oils secreted by certain plants, the presence of ozone, and a bacteria called Actinomycetes.
Actinomycetes are bacteria that typically thrive in warm and damp soil. While these thread-like filament organisms can live other habitats, such as animal bodies or water environment, the ground is their main refuge.
After the extended dry summer, the actinomycetes living in the soil start to slow down their activities. However, as the land becomes damp again due to the rainfalls, these bacteria wake up and release a byproduct called geosmin in the process.
Actinomycetes are an essential part of the environment as they improve the nutrients in the soil, boosting plants’ growth. They aid in decomposing dead plants and animals and recreates nutrients that the Earth’s flora needs. Plus, they also help regulate the presence of harmful soil bacteria, keeping the soil environment sustainable and thriving.
Geosmin contributes to the luscious smell coming after a rainfall. It comes from the ancient Greek words “geo,” or Earth, and “osme,” or smell. Geosmin is also responsible for providing beetroot its earthy taste and is also used as an ingredient in fragrances. However, geosmin is also regarded as the off or strange we get in wine and water.
Actinomycetes, again, aren’t the sole culprits for the enticing smell after rainfall. Plants also bring the musky odor of petrichor. Various varieties of plants produce a particular mix of oil, often made of stearic or palmitic acids during the extended summer. They release them to inhibit the growth and minimize the competition over scarce water. The process also serves as a countermeasure to halt the other seeds on the ground from sprouting, incredibly regulating the population while the resources are still scanty. As the wind blows, these secreted oils are then blown from the plants to soil, rocks, and sands. These chemicals then blend with minerals which bring the fresh smell after a rain.
Another source of petrichor is the ozone. The same gas that is found in the protective shield of the atmosphere that absorbs most of the Sun’s harmful rays. Though it is only a mere type of oxygen molecule, it has a unique odor, close to that of chlorine. During rains brought by thunderstorms, the electrical charges split up the oxygen and nitrogen molecules. Individuals atoms then combine with other gases in the atmosphere, consequently producing ozone. It is quite unstable if situated lower in the atmosphere and typically found higher up. However, windy storms sweep ozone lower in the lower atmosphere to our nose level, allowing us to detect it, contributing to the distinct smell after rain.
It’s incredible how the earthy smell after rainfall is caused by different compounds starting to be produced by living organisms ahead of the rainy season. Moreover, ozone also plays a role in providing that distinct and fresh fragrance we love. So, the next time you experience the first downpour of the rainy season, make sure to appreciate all the intricacies that smell has.
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