Friday, July 07, 2017

Perichor Smell of First Rain

Petrichor (/ˈpɛtrᵻkɔər/) is the earthy scent produced when rain falls on dry soil. The word is constructed from Greek πέτρα petra, meaning "stone", and ἰχώρ īchōr, the fluid that flows in the veins of the gods in Greek mythology.

The term was coined in 1964 by two Australian CSIRO researchers, Isabel Joy Bear and Richard G. Thomas, for an article in the journal Nature.[1][2] In the article, the authors describe how the smell derives from an oil exuded by certain plants during dry periods, whereupon it is absorbed by clay-based soils and rocks. During rain, the oil is released into the air along with another compound, geosmin, a metabolic by-product of certain actinobacteria, which is emitted by wet soil, producing the distinctive scent; ozone may also be present if there is lightning.[3] In a follow-up paper, Bear and Thomas (1965) showed that the oil retards seed germination and early plant growth.[4]

In 2015, MIT scientists used high-speed cameras to record how the scent moves into the air.[5] The tests involved approximately 600 experiments on 28 different surfaces, including engineered materials and soil samples.[6] When a raindrop lands on a porous surface, air from the pores forms small bubbles, which float to the surface and release aerosols.[5] Such aerosols carry the scent, as well as bacteria and viruses from the soil.[5] Raindrops that move at a slower rate tend to produce more aerosols; this serves as an explanation for why the petrichor is more common after light rains.[5]

Some scientists believe that humans appreciate the rain scent because ancestors may have relied on rainy weather for survival.[7]