It was late into May – the peak of the scorching hot Indian summer. An unforgiving heat wave has been blazing the landscape for the past few days like an angry dragon. It did not stop at sucking up the last trace of moisture from the land; it drained the life out of its inhabitants. Wells dried up and ponds exposed their muddy bottom. All life on land and water yearned for some rain.
Manu, a farmer, would look up at the sky from time to time. His eyes ardently searching the horizon for the tiniest sign of rain clouds. He clearly remembered previous night’s weather report speak of a possible thunderstorm in the coming days. If it rained that day it would be the season’s first showers, he thought. He had planted his crop in the middle of March that year. A prolonged heat wave and delayed rains meant devastation – irrigation cannot hold his crops for long if nature doesn’t favour. He looked with gloomy eyes at a stream of wind blow up some stray soil particles into a whirlpool of dust.
A cool spell prevailed in the afternoon before bands of grey clouds rolled in from the horizon blanketing the glaring summer sky. The first drop then the second and the third – all turned into smoke making a sizzling sound like water splashed on a hot pan. A lightning strike suddenly electrified the dark sky. The rain came pouring down. It had finally arrived – a long-awaited respite from the scorching heat of the fiery Indian summer.
Manu stood near his field soaked in rain smiling to himself. A small boy ran out of the house and prostrated himself onto the ground. He sniffed the wet soil briefly then waved both his palms towards his nose to direct more of the sweet fragrance to his nostrils, inhaling heavily. Sitting up he asked his father, “Baba, why does the ground smell so sweet after the rains?”
Manu, slightly shocked from such an unexpected question gazed blankly at his son. He didn’t have an answer to his son’s question who instead was looking up at his father, his wide innocent eyes eagerly waiting to receive an answer. Manu didn’t want to leave his son without an answer so he made up one. With a smile he replied, “Because Mother earth is happy with the rains she sends up a sweet aroma up in the air to let the clouds know that she is contended.”
The child laughed and said, “So the Earth and the sky are talking. It must be so much fun to talk in scents.”
“It is fun, my son. Indeed, it is”, replied Manu. He seemed pleased to have amused his young son with his answer.
But little did Manu know that the same question that his son asked – to which he made up an amusing answer – had intrigued both the ordinary and the extraordinary, the child and the adult, the poet and the prosaic, the artist and the scientist alike.
“Why does the ground smell so sweet after the first rain?”
Almost all of us on this planet who have seen seasons change, probably with the exception of the Inuit living in the Arctic or the Pygmies living in the Congo or the Atacameño of the Atacama desert, have experienced the fresh fragrance emanating from the ground during the first rains after a fairly long dry period and we have asked the above question at least once in our lives.
But did you know that there is a word for this rainy fragrance?
A new guest knocks on the dictionary door – it’s Petrichor!
It took scientists several decades of observation and analysis to decode the mystery behind this characteristic earthy fragrance intimately associated with the first summer rains. Several theories were proposed to explain the source of this odour.
However, it wasn’t until 1964 that a separate term to define this characteristic rain fragrance was proposed. Isabel Joy Bear and Richard Grenfell Thomas, two researchers at CSIRO coined the term “petrichor” to define the smell of rain. They explained that the term “argillaceous odour”, which was previously used to define the scent meant that ‘the phenomenon is restricted to clay or argillaceous materials’ alone. Whereas the new term “petrichor” is inclusive of the ‘diverse nature of the host materials’ from which they had successfully extracted the scent in their studies. The term “petrichor” originates from two Greek words – ‘petra’ and ‘ichor’ – ‘petra’ meaning stone and ‘ichor’ being used to define the ‘ethereal fluid that flows like blood in the veins of the gods’. In other word petrichor is blood of the stone.
Bear and Thomas had learnt about the perfume industry in Kanauj, India and the traditional perfume making practices still used to this day to prepare natural fragrances.
What drew the attention of these two researchers was the practice of extracting the earthy fragrance from baked clay discs dried under the summer sun by steam distillation and further absorption of the vapours containing the odour in sandalwood oil – the final product being “mitti ka attar” or earth perfume.
They figured out that the criteria for obtaining the petrichor was to allow the clay or rock to dry in hot air for a period of time, which then on moistening with water emitted the odour they were searching for. They employed a similar tactic but before doing that they heated the test substances – which ranged from clay to several categories of mineral rocks – in a muffle furnace at 600°C to get rid of any organic material and microbial contamination. After enough sun exposure of the processed materials, they were steam distilled to obtain a condensate which was yellow in colour and had oil globules floating on the water surface. On solvent extraction of this aqueous distillate petrichor was obtained which was basically an oil. They found that the amount of oil extracted was proportional to the area of exposed surfaces in the material and the duration for which it was exposed to the sun – a relation we shall explore in the later part of this article. All extracts obtained from the various types of substances had a characteristic infrared spectrum which confirmed a uniform composition. This might help explain why rain has an identifiable smell no matter in which part of the world we are in.
An earth scent called Geosmin
A year later in 1965, Gerber and Lechevalier, identified and successfully extracted a compound they called “geosmin” (Greek: ‘geo’ meaning earth and ‘osme’ meaning odour) from several strains of the bacterial genus Streptomyces. Geosmin, a metabolic product of actinomycetes, is the compound that imparts an earthy smell to soil. It is the same compound that give beetroots their earthy flavour. The researchers couldn’t work out the structure of geosmin in their 1965 study. However, they found that geosmin was a neutral oil containing only carbon and hydrogen but no nitrogen. Later Nancy N. Gerber elucidated the structure of geosmin in 1968.
Studies found that human noses are very sensitive to the smell of geosmin at 5 parts per trillion, even beating a shark’s sensitivity to the smell of blood which stands at one part of blood per billion parts of water. The human olfactory system owes its ability to single out the smell of geosmin to the presence of a single unique receptor for the chemical compound. Not just humans, other organisms are also attracted or repelled by the smell of geosmin. Drosophila are repelled by the smell of geosmin which they relate to spoilt food whereas camels are attracted by the scent which they associate with water, an extremely rare but necessary resource in the dry desert environment.
Release of geosmin into the air after a light rain serves many evolutionary purposes. The spores of Streptomyces that produce geosmin in soil release the scent as a signal to the organisms who are to help in their distribution. Springtails are attracted by the scent of geosmin and flock in huge numbers. They eat the bacteria whereas the spores which carry the genetic material for future generations are passed out in their faeces or attach to their bodies and later sloughed off at some distance from where it was originally picked up.
Oil of Petrichor inhibits plant growth
The same year in 1965 back at their CSIRO lab, Thomas and Bear, the petrichor duo, studied the effect of the oil they had extracted from clay and other silicate rocks on plant seed germination. They found that the oil of petrichor had an inhibitory effect on plant growth and slowed seed germination significantly. When seeds were sown on moist soil that had been exposed to summerlike conditions (and hence had formed petrichor oil in it) the seeds were slow to germinate. Whereas seeds sown on soil that had not been exposed to the summerlike conditions (hence, no oil formed) or were steam distilled (oil expelled into air) showed higher growth rate. Why so? We shall get to that question soon but first let us understand the composition of this petrichor oil.
What makes Petrichor?
Geosmin, the compound released by actinomycetes and also some fungi, is the main component of the oil. Other fatty acids like palmitic acid and stearic acid – mostly found in plant cell membrane and released by the leaves after decomposition – are also a part of it. When exposed to drought stress plants secrete essential oils which are a class of secondary metabolites. Essential oils are aromatic and its components have low molecular weights which make them volatile by nature. These oils protect the plants and the seeds from evaporative water loss. The oils released also has an inhibitory effect on seed germination – an evolutionarily beneficial adaptation for seeds germinating during this time would surely not survive the harsh environmental condition. Some of the oils leaches into the surrounding soil and rock crevices and is locked in the porous matrix. All of the above goes into making petrichor.
There is evidence of geosmin emission from soil reaching the highest concentration at 6 hours and fading after 24 hours of rain. Another research data shows the concentration of geosmin emission from soils more than doubling after a simulated rainfall, hence proving that geosmin is a component of petrichor.
A study found geosmin to have an inhibitory effect on seed germination and the effect was higher under light (12-hour photoperiod) – a situation akin to summer when days are longer. This finding might help us understand why the petrichor oil which we now know contains geosmin prevented seeds from germinating in Thomas and Bear’s study.
What enhances petrichor?
Long and harsh droughts stimulate plants to take stronger precautionary actions to protect themselves and also ensure the survival of the future generations. One such measure is increased oil production to prevent dehydration of plant parts and seeds. During drought period plant growth is stunted and essential oils are released to cope with the stressful environment. Geosmin production by actinomycetes occurs at higher temperatures and most strains produce geosmin in the temperature range of 30°-35°C.
Porosity of the soil and rock formations enables fungi and actinomycetes often associated with rhizophores to form colonies in the hollow aerated spaces. The same capillary network of the porous matrix allows the oils secreted by the plants, fungi and bacteria to accumulate in the soil. Adhesion to soil and rock particles maybe a mechanism by which the released oils are stored in the substance of the clay. It is here that larger surface area comes in handy.
However, aerosols that bring the aroma into the air outside cannot form if the porous matrix is filled with liquids. A long dry period and sun exposure is thus needed to volatilize the oils and geosmin into their gaseous components which remains trapped in the air within the porous matrix of the clay. Large sun exposed soil surfaces absorb heat which speeds up the process of volatilization of organic compounds and also the oxidation of long chain fatty acids.
Longer drought like conditions and large surface areas of rocks and clay materials are the prerequisites for a stronger aroma after rains. In their study Thomas and Bear could extract more of their oil of petrichor from materials that had both longer sun exposure and large surface areas.
Increase in emission of geosmin after a simulated rainfall on soil clearly indicated that geosmin, a component of petrichor, is stored in soil and rock pore spaces. When water seeps into the same pores it displaces geosmin which makes its entry into the air above the soil surface, further carried by the wind to long distances giving rise to the fresh smell of rain that can be sensed from afar.
How does rain stimulate the release of petrichor from soil?
2015 saw a landmark study done by MIT researchers that showed for the first time how rain drops falling on dry porous surfaces like soils generate aerosols which release contents of the porous medium into the atmosphere. The team employed high speed cameras to capture the impact of water drops after hitting soil surfaces at velocities similar to raindrops. The video captured the generation of bubbles within the water droplet from air trapped right at the point of contact of the water droplet and the soil surface which then rises through the body of the water drop to be released at the upper surface of the droplet. The phenomenon is similar to air bubbles rising from the base and walls of a glass when water is poured into it really fast.
It is believed that this aerosol carries the odour of geosmin and the volatile oils trapped in the soil pores to the atmosphere when the bubbles burst open. As the water wets the soil and gradually fills its porous capillary network air is displaced which exits the body of the soil or clay aiding in further aerosol generation. The aerosols carry with them volatile oils and geosmin which gives rise to the fresh smell of rain we call petrichor today.
Ending thoughts
“It was raining. A fine rain, a gentle shower, was peppering the pavements and making them greasy. Was it worthwhile opening an umbrella, was it necessary to hail a hansom, people coming out from the theatres asked themselves, looking up at the mild, milky sky in which the stars were blunted. Where it fell on earth, on fields and gardens, it drew up the smell of earth. Here a drop poised on a grass-blade; there filled the cup of a wildflower, till the breeze stirred and the rain was split.”
Virginia Woolf in ‘The Years’
The beauty of nature can push artists and writers to reach the heights of creativity we never knew existed. But scientists pondering over the mysterious and the beautiful can go to even greater lengths sometimes uglier as well to find the mystery behind all that is beautiful.
Most of us don’t want to get entangled in all the intricate details and science behind something we find enjoyable. We believe it takes away the charm of it. Take for example the fresh fragrance of summer rains. This smell is intimately associated with our childhood and brings back nostalgic memories of the past – an emotional reflection of the self, better left undisturbed.
The inquisitive mind however has a different way of perceiving beauty. So, the next time you breathe in the sweet fragrance after the first rain of the season ponder a little over those curious minds that worked to get to the core of this beautiful natural phenomenon.