Petrichor and the perfume of coming rain

I was raised in country NSW—not quite the outback of Australia but nevertheless a small rural town that relied on the annual rain to keep the grass growing to feed the sheep and cattle.

I remember how dry the summer could be when we were in drought and the sheer joy of smelling the coming of the rain. That faint smell of damp mixed with the scent of the dry aromatic earth is almost impossible to describe. So I was quite a bit excited to find out recently that this smell has a name and that it was created in Australia to describe almost exactly what I have just described.

In 1964 two scientists, Joy Bear and Richard Thomas, working at the CSIRO Division of Mineral Chemistry tried testing a range of rocks and minerals to see what their capacity was to produce, what is scientifically known as, argillaceous odour. Argillaceous odour is the odour produced by soils and clays when they are moistened. They found that

“the majority of silicate minerals and rocks, largely irrespective of kaolization or porosity, exhibit the phenomenon.”

This means that rocks with low and high clay content and with small or fine grains also produced the smell. They suggested that the smell be called petrichor.

Petrichor is not the smell of the minerals or rocks themselves. It is the environmental oils and aromatics that the rocks have absorbed and then released when in contact with moisture. Subsequent studies have shown that rain drops trap air bubbles when they hit porous rock. It is the air bubbles that absorb the aromatic oils. When the air bubbles burst out of the water droplet they create an aerosol which releases the oils into the air to create the petrichor.

The scientists were using a lab technique which is also used by traditional Indian perfumerers. Indian perfumerers expose clay discs to the environment during the heat of their summer. They then distil the absorbed compounds by dissolving the compounds in sandalwood oil to form matti ka attar, which translates as perfume of the earth.

Bear and Thomas created the word petrichor by combining the Greek word petra for stone with –ichor from Greek mythology to create a reference to the blood of the gods. They were characterising the smell to be the blood of the stone.

So as the Bureau of Meteorology are predicting a “Godzilla” El Nino this summer (2015-16), meaning a very bad drought year, lets hope that the country folk get to smell some petrichor this season.