Hey, everybody.
This week, let’s take it a little slower and watch this raindrop create the smell of
a summer storm.
The earthy smell of a gentle rain after a dry spell is so evocative that it has its
own word.
"It means, 'the smell of dust after rain.'"
"What does?"
"Petrichor."
The word comes from the Greek for the blood of stones.
It was coined by two poetically-minded Australian scientists searching for the source of the
scent.
Isabel Bear and Richard Thomas studied petrichor in the 1960s.
It’s a humble area of research -- but a universal experience.
And it has two main sources: plants, and bacteria living in the soil.
Bear and Thomas found that they could extract a yellow oil from warm, dry rocks, clay, and
soil.
That oil -- petrichor -- contained fatty acids from plants, mostly palmitic acid and stearic
acid, as well as smaller compounds.
These long fatty acids don’t smell like much.
But after some time in the soil, the researchers found they got broken down into a smorgasbord
of much smaller, much smellier molecules.
What isn’t clear is what these fatty acids do.
Petrichor is released from soil after long dry spells.
There’s often a surge of plant growth when it first rains after such a drought.
Bear and Thomas thought that petrichor might have something to do with it.
They gave some to plants, but it didn’t help them grow.
Plants given petrichor actually withered.
It’s been speculated that plants might secrete fatty acids during dry spells to slow other
plants down and cut out competition for water.
Which is pretty ruthless, for a plant.
Around the same time, another duo of scientists isolated a compound associated with both freshly
plowed earth and tainted fish.
They, too, coined a Classical name for their discovery: geosmin, meaning the smell of earth.
Geosmin is produced by a group of bacteria called actinomycetes, and no one is quite
sure why.
You may smell it in your garden in freshly turned soil or newly watered plants and, yes,
after a light rain.
Geosmin creates an unpalatable taste in water and any fish that live in it, and it imparts
an earthy flavor to beets.
The smell is nicer, and we now know it’s an important part of petrichor.
Our noses can detect geosmin at concentrations less than ten parts per trillion.
That’s around a teaspoonful in two hundred Olympic size swimming pools.
Since the 60s, few researchers have paid much attention to petrichor.
It wasn’t until 2015 that MIT scientists began looking at how the smell might actually
reach your nose.,
They recreated the effect of raindrops falling on a porous surface.
Their high-speed videos showed air getting trapped beneath the raindrop.
Those air bubbles then burst and spew tiny jets of water -- see them?
They contain water from the raindrops and chemicals from the soil.
They’re forming an aerosol -- tiny droplets suspended in the air, much smaller than the
raindrops.
Because the aerosol drops are so small, they can travel on the wind much more easily than
raindrops, and carry the smell of petrichor to our noses.
This is the first time anyone showed that rain could create aerosols, effectively filling
the air with much finer particles of water.
But only a light rain will do it.
Heavy rain doesn’t create those bubbles -- so a heavy downpour may not smell like
much instead of carrying the poetic aroma of petrichor.
Thanks for joining us this week, and a huge thanks to Youngsoo Joung and Cullen Buie from
MIT for letting us use their footage.
And to Neil Gaiman for teaching me the word petrichor.
What’s the most poetic scientific term you know?
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