Kamen

Where the smell comes from

Layers of volatiles, for example those emitted by microbes and plants, can accumulate on stone surfaces and are modified by the atmosphere. Raindrops can drive them out of their stone surfaces and make them better noticeable. Experiments by Bear and Thomas strongly suggested that petrichor arises not from the rocks themselves, but from organic material settling out of the atmosphere and sticking onto the rocks' surface. The smell is associated with oils that accumulate in rocks and soil during dry periods and are released by moisture.

Other scents that become a component of petrichor 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. Raindrops striking porous surfaces can trap air bubbles, which rise and burst as aerosols, providing a mechanism by which scent-carrying material can enter the air.

Geosmin

Geosmin contributes to petrichor, the strong scent that occurs when rain falls after a spell of dry weather or when soil is disturbed. It is a bicyclic alcohol with the formula C₁₂H₂₂O. Geosmin is produced as a secondary metabolite by various blue-green algae (cyanobacteria), filamentous bacteria in the class Actinomyces, other prokaryotes, and various eukaryotes. Most streptomycetes, which are found predominantly in soil and decaying vegetation, have a distinct earthy odor that results from production of the volatile metabolite geosmin. Geosmin is produced from the sesquiterpene precursor farnesyl pyrophosphate in a two-step reaction that depends on Mg²⁺.

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