The Smell of the Earth After the Rain: Geosmin, Petrichor & Other Wonders

The first heavy drops of rain fell on the thirsty ground yesterday after a hot, hot summer that scorched our conscience. It seemed like release, like tears falling after a gigantic pressure mounting inside that had overflown, ready to burst.

"With the first drop of rain the summer was killed. Soaked were the words which starlight had born. Words that were meant just for you." writes Odysseas Elytis. 

This sweet melancholy of autumn is inextricably tied to the pit pat of the raindrops on the window pane, much as it sounds corny. Like many, I adore the ambience after the rain; when everything seems washed, purged, the green leaves and flowers shiny fresh with droplets hanging onto them refracted into myriads of rainbows in the emerging light; with the distinct smell of the earth that has soaked the water and brought out a scent at once musty and refreshing, a scent that is ancient and at the same time of the moment, galvanizing, a scent of the divine and the pagan. But what makes that delicious scent, popular enough to be first unapologetically encapsulated into a fragrance to sell by Christopher Brosius in his Fragrance Library for Demeter by the eerie name "Dirt"? The answer is more enjoyably lyrical.

via http://lifeofpri.blogspot.com

Petrichor is the name of the scent of rain on dry earth, which aided by the compound geosmin contributes to that delectable ambience of upturned earth and musty deliciousness which walkers of the woods have been known to enjoy with all their might. Due to poetic justice, I suppose, and because everything is paid upon at the cashier eventually in this world, both words have a Greek origin: "Petrichor" literally means the fluid in the veins of the gods hitting stone (from the words πέτρα i.e. stone and ιχώρ i.e. the mythical lifeblood of the Gods). "Geosmin" is simpler, more to the point: from the Greek word for earth, γαία (deriv. γεο-) and οσμήν i.e. smell; simply put, "the smell of the earth". The term "petrichor" was coined in 1964 by two Australian researchers, Bear and Thomas, for an article in the scientific journal Nature. In their article, it is argued that certain plants exude an oil during dry periods which is then absorbed by clay-based soils and by rock. The hitting of the ground during the rain releases this oil alongside geosmin, a germacranoid sesquiterpene or a trans-1,10-dimethyl-trans-9-decalol for the more chemically minded ~or simply a by-product of microorganisms, which acts as a metabolite.

Geosmin is produced by a number of microorganisms amongst which the mycelial soil bacteria Streptomyces. Geosmin is exactly that distinct smell that soil gives off when disturbed or just rained upon and its human detection threshold is so low (allowing almost all to savor it) and so pleasant, it is used to confer an earthy scent to perfumes! But careful: in flavor, by contrast, geosmin can turn a glass of water or wine (or fruits or vegetables) musty and unpleasant for consumption. Even lightning contributes to the scent of the earth after the rain nevertheless; the presence of ozone is electrifying, producing that energetic, come what may, putting on boots and clutching a cane walk in the woods mood one doesn't know they had in them until it actually happens. This is the magic turn of the screw that makes fragrances such as Creed's classic Green Irish Tweed (with the infinitely matching name to this fervent desire) and the quirky enchanted forest of Ormonde Woman work so well.

Several perfumers try to recreate that complex smell of the forest floor; upturned, a little decayed, yet at the same time fresh, cooling, a sort of clean all the same. Great vetiver fragrances, such as Guerlain's classic Vetiver and Vetiver pour Elle, Route du Vetiver by Maitre Parfumeur et Gantier , Chanel's Sycomore, Lalique Encre Noire or F.Malle's Vetiver Extraordinaire mingle that particular freshness that the exotic grass root of vetiver possesses alongside a smokier, mustier background which brings on a crepuscular tinge. Some patchouli fragrances, notably Voleur de Roses by L'Artisan Parfumeur utilize the more wine-like facets of the rose and the Indian leaf material to render a scent that approximates well a garden after the rain. Herbal accents and iris notes alongside grassy-musty ingredients (such as vetiver, angelica, wormwood or oakmoss) also produce this effect as in Roadster by Cartier, L'Artisan Parfumeur Timbuktu, the stupendous Derby by Guerlain, Angeliques sous la Pluie (F.Malle) and Dior Homme.

It's interesting to note that geosmin doesn't just contribute to the smelly landscape but could be a way of promoting sporulation as it occurs where humidity is involved. "Camels may well smell out an oasis by sniffing the air for traces of the fragrant metabolite. While camels quench their thirst, hordes of Streptomyces spores will be supped with the water or will find a way to stick onto the animals’ hides. In this way, spores can be carried for miles. In the same way, some cacti flowers may also use the geosmin scent to fool insects, in a sort of fragrant mimicry. Indeed, insects are attracted to the plants in the hope of a little refreshment, and in their quest for water, they actually serve as pollinators!"[source]

As with everything involving smell, there's more than meets the nose...

 
Music clip "With the First Drop of the Rain", lyrics by Greek nobelist poet Odysseas Elytis, set to music by Manos Hadjidakis and sung by Dimitris Psarianos.