Sure can smell the rain

Fly, thought, on golden wings,
go alight on the cliffs, on the hills,
where the sweet airs of our
native soil smell soft and mild!

Chorus of the Hebrew slaves, Nabucco
Giuseppe Verdi

Have you ever noticed the smell of rain? Why does wet soil smell so good?

Bottled petrichor. Photo by Kevin O-Mara. Clinc the image to see the original picture at Flickr.

The smell of wet soil plants oils released into the soil during dry periods is due. These substances accumulate in the soil and mix with geosmin, produced and released by several groups of bacteria, including actinobacteria (eg, Streptomyces) and cyanobacteria. Geosmine (from Greek “geo”, earth, and “osmin”, smell) is a bicyclic alcohol derivative of decain, and was firstly described in the 1960s (Gerber and Lechevalier, 1965). When it rains, these chemicals are released into the atmosphere and cause a special smell which is known in English as petrichor (Greek “petros”, stone, and “ikhôr” liquid flowing through the veins of the gods).

Geosmin structural formulae.

Animals can detect extremely low concentrations of geosmin and other similar substances released by wet soil. It is well known that camels, for example, are able to find water in the desert from distances up to 80 km by smell.

Botanical garden in Coimbra (Portugal). Photo by Artemi Cerdà. Click the image to see the original picture and details at Imaggeo.

Apparently, the process is simple. You Sure? In fact, the mechanism by which these substances pass from the soil to the atmosphere was not known until now. Young Soo Joung and Cullen R. Buie from the Massachusetts Institute of Technology (MIT) have unveiled the process … and have recorded it on video!

For this they used high-speed cameras. They have observed that raindrops may behave differently if they impact a homogeneous and smooth or an irregular and porous surface (eg, soil). In the latter case, droplets can trap small bubbles of air at the point of impact. Within the drop, the bubbles move up and explode on the surface. Thus, each bubble releases a small cloud of soil particles into the atmosphere.

Joung and Buie (2015) have also studied that the amount of these substances which may be sprayed strongly depends on variables such as rainfall intensity and permeability and porosity of the contact surface. During their experiment, they observed and recorded for the first time the impact of simulated raindrops on different wettable surfaces (engineering materials or soil naturally) using high-speed cameras. At the time of impact, small bubbles form in the solid/liquid interface, cross up the water body and are released into the atmosphere. Immediately after, they have also watched wind carrying and subsequently dispersing the cloud of released substances. In total, the whole process lasts a few microseconds. They suggest that this process can also contribute to the transport of other substances and even microorganisms, explaining, for example, the spread of diseases such as E. coli.

Aerosol generation from droplets hitting soils and porous surfaces. Click the image to access the original image in the article.

References

• Gerber NN, Lechevalier HA. 1965. Geosmin, an earthly-smelling substance isolated from actinomycetes. Applied Microbiology 13: 935-938.
• Joung YS, Buie CR. 2015. Aerosol generation by raindrop impact on soil. Nature Communications 6: 6083.

This post has been simultaneously published in G-Soil.