Smell of rain

Story time! Featuring two awesome women in science. The smell of rain is called petrichor. Originally though it was called “argillaceous odour” because people thought the smell came from clay. But scientists were stumped, if the smell came from clay, why did other things also smell earthy?

Scientists knew the smell came primarily from arid regions. But the interesting thing was in India they were able to produce this scent and use it in a perfume that they called “matti ka attar” which loosely means earth perfume. Despite this, the science behind the smell of rain was still unknown.

The year is now 1964! In comes Isabel Joy Bear. She’s an Australian chemist who was working at the CSIRO at the time. Her and Richard G. Thomas worked together and like many scientists before them, they saw that this early smell was coming from a variety of different rocks and minerals. They coined the term petrichor which is Greek for “petra”- rock or stone and “ichor”- which is the fluid that flows in the veins of gods. So petrichor is literally the blood of stones. 

Over the next year, Bear and Thomas extracted petrichor from a variety of sources and found that it is a yellow and oily substance. But this is where they started running into problems. They could tell that this oil wasn’t just one substance. It was a mixture of different things!

Unfortunately for Bear it was still 1964. Science had gotten far, but not far enough. Bear knew what equipment she needed to understand what makes up this oil. But the equipment was not advanced enough for her needs. If only she was able to get a mass-spectra that wasn’t so complicated.

Bear wasn’t about to let this stop her though. She turned to infrared spectroscopy (IR) and found that petrichor could be separated into 3 parts: a basic substance, an acidic substance that gives the yellow color, and a neutral substance which was the largest fraction.

Unfortunately, Bear and Thomas though that petrichor only formed by substances being present in the atmosphere at the right place and time. Therefore, a biological origin was improbable and they published their results as is.

A year later, Nancy Gerber, a biochemist, and Hubert A Lechevalier, a biologist, are studying streptomycetes. They noticed that freshly plowed soil has a distinctive “earthy” smell, which led them to think that the root of the smell were microbes!

At this time it was known that actinomycetes produced this earthy odor. But what was causing it? Other scientists had already grown strains of bacteria and concluded that the smell must be a combo of things, namely: acetic acid, acetaldehyde, ethyl alcohol, ammonia, etc.

Gerber and Lechevalier grew different strains of Streptomycetes and found the smell came from a substance they named geosmin from Greek “Ge”-earth and “some”-odor. How did they find this smell though? They used gas chromatography (GC) to separate and analyze bacterial brothes. They found that they could smell geosmin at the same time that the GC showed a sharp peak at 4.0 minutes!

But some samples didn’t have the smell!?! Through GC they were able to decipher these samples and see that other compounds could mask the earthy smell and not all bacteria produced the same amount of geosmin.

Now let’s flash forward 50 years! The year is now 2015. MIT scientists Young Soo Joung and Cullen R Buie decide to find out why this smell arises after rain. They believed that this aroma was caused by aerosols that were formed when raindrops hit soil.

They visualized the raindrows and found that aerosols are formed in a 3 step process the first being bubble formation, the second bubble growt, and the 3^rd is the bubble bursting. They were also able to test a variety of different impact velocities and surfaces to see how the aerosol formation would change. So how does this affect the smell of rain? Soil is a porous structure, so Joung and Buie tested how raindrops would affect porous substances ranging from soils to clays to titania. There results were startling! They found that a slower rain generates more aerosols. That’s why light mistings smell stronger than heavy rain. Why? Because raindrops need to form bubbles before they can make aerosols. These bubbles are formed when the rain fills up an air pocket or pore in the soil. When it’s raining fast these pores stay wet and the bubbles can’t form.

In a slow rain pores in the soil have more time to dry causing more bubbles and therform more aerosols to form!

Ok so now you might be asking, why do bacteria produce geosmin? What does rain have a smell? Well in 2020 scientists in Sweden wanted to answer these questions. They laid out a network of traps with streptomyces and waited to see what would be attracted to the bait. They found that springtails were captivated by the smell of geosmin. After reaching the geosmin the springtails will disperse the spores either by defecation or through the spores that stick to their bodies thus giving rise to a new generation of streptomyces.