Petrichor: A scent of rocks and rain

When I hug my son after a day of fall bouldering, his hair smells of the sun-warmed rock we’ve been climbing over. It’s a distinctive odor, evocative of gray ledges and golden light returning after rain, and yet it’s not the rock I’m smelling, but tell-tale traces of life.

People have written about – and appreciated – that odor since ancient times. Two thousand years ago, the Roman scholar Pliny described the scent that occurred after a drought when rain first soaked the ground: “Then it is that the earth exhales this divine odor, that is so peculiarly its own, and to which, imparted to it by the sun, there is no perfume, however sweet, that can possibly be compared.”

In 1964, Australian chemists Isabel Joy Bear and Richard Grenfell Thomas published a research article describing the origins of this scent and giving it a name: “petrichor.” (In Ancient Greek, petr refers to rock, and ichor is the “tenuous essence” that runs through the veins of the gods.) Although this name implies that petrichor is essential to the rock, Bear and Thomas’s research contradicts that implication.

Bear and Thomas conducted several experiments to isolate the source of petrichor within rocks. They sterilized rocks with various mineral compositions, wetted them down to verify that they were scentless, and left them in an open location sheltered from rain. After a variety of time intervals, they again wetted the samples and assessed them for scent. They observed that over time, many rocks, but especially those dominated by silica or iron oxide, developed scent again, producing a classic petrichor aroma.

This research strongly suggested that petrichor arose not from the rocks themselves but from organic material settling out of the atmosphere and sticking onto the rocks’ surface, which moisture later revived with a gust of scent. By steaming silicate-rich rocks, Bear and Thomas were able to extract oil with intense petrichor – a method, they noted, that is similar to a steam distillation process used by Indian perfumers, to produce a scent called matti kar attar or “earth perfume.” Chemical analysis of this oil showed it to be “organic in nature.”

Since this research, other scientists have confirmed that rock scent arises from the chemical processes of metabolism, respiration, and decomposition that occur around us all the time. Mixed with these leftover chemicals are other scents, which can give a particular place a unique smell (and become a component of petrichor). These include volatile oils from plants, compounds released by bacteria and fungi, and molecules from inorganic reactions such as ozone that form in the atmosphere during lightning strikes. While this scent’s exact source and chemical composition may vary, the collective effect is a recognizable “smell of rock.”

As Bear and Thomas demonstrated, a close-up sniff of dampened rocks reveals this scent. But how does it rise up in the air, all the way to our noses? And what does rain have to do with it?

In 2015, mechanical engineers Young Soo Joung and Cullen R. Buie identified a three-step process by which a raindrop can produce mist. When a water drop hits the soil surface, air is trapped, forming a bubble. As the raindrop continues to collapse onto the soil, the first bubble breaks into smaller bubbles, which rise toward the drop surface. They burst from the surface of the raindrop, releasing minute jets of water that become a fine mist. Joung and Buie confirmed that the most likely condition for this aerosolization process to occur is when light rain falls on dry sandy-clay to clay-type (silica-rich) soils.

Joung and Buie conducted a second experiment to determine whether this aerosolization process could also carry surface residue into the air. They added dye to soil samples, exposed the soil to water drops, and suspended a piece of glass above where the water drops hit. Colored specks formed on the glass, which confirmed dye molecules had moved out of the soil into the water drop and had become trapped in the escaping mist. The same process transports scent molecules off rock and soil surfaces into the air to appreciative human noses.

Rachel Sargent Mirus is a teaching artist and writer. Illustration by Adelaide Murphy Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: nhcf.org.