Short Stuff: Petrichor

You know that amazing smell when it rains? Kind of clean, kind of earthy, one of a kind? It turns out that a miracle of nature produces it.

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Episode Transcript

Speaker 1 (00:04):
Hello, and welcome to the short stuff. I'm Josh, and
there's Chuck and there's Jerry over there. We are reveling
in nature this morning. We're just loving life because it
has rained and it smells amazing outside. And it turns
out that that amazing smell is called petrochor. Chuck go, yeah,
And so we decided to have school outside today. Remember that?

(00:26):
Oh yeah, I loved that because it also meant not
only that it was a nice day out and that
your teacher had clearly taken acid that morning, but that
the the end of the school year was fast approaching.
Uh sure, because I guess that was usually in the spring, right,
especially in Toledo. Like if you if it, if it
was nice enough to go outside, you were coming up

(00:47):
on the end of the school year. So love that, yes,
But petro chor, like you said, if you ever go
outside after a rain, especially a light rain, which we'll
get to more so than a heavy rain. Uh And
you think, man, what is it about that smell that
I love so much? And we should also do one
on fresh cut grass? Okay, that's something similar, but that

(01:10):
has a name, and that is petrochor. P. E. T R.
I C h O R. And it is that earthy
sort of warm, steamy, earthy fragrance that we get. And
there is a story behind it and reasons for it
that is science based, yes, which makes it just amazingly wonderful. Yes.

(01:30):
But it came from Australia, right. The name it's did
Australia in the sixties. Actually, um, it's based on two
Greek words um Petro's I believe, which means stone, and
i corps, which means the fluid that flows in the
veins of the gods. Blood of the gods is a
simpler way to think about I also saw a life

(01:51):
force of the gods coming from a stone. They really
went all out, but they had great names. Well, at
least the woman researcher did Isabel joy Bear. I love
that name. It's just a wonderful little name, like it
should be a like Nickelodeon Sprout cartoon or something. And
that Isabelle joy Behar. That's different, right, That's like Kathleen

(02:14):
Turner overdrive. Anyway. The other guy was Richard Thomas, who
had a whole hum name. But the two of them
coined this term in ur in an article in the
journal Nature The article is called Nature of our Galacious
Odor with a you because they were in Australia and
they coined this term. But they also kind of got
down to what creates this and rather than it just

(02:36):
being one thing, it seems to be a combination of
three amazing things that all kind of come to the
four during a rainstorm, especially after a dry spell. The
first rain after a prolonged dry spell really stinks up
the place with beautiful odors. Yeah, And the first thing
that we're gonna talk about is a molecule that's made

(02:57):
by a certain kind of bacteria. And the molecule is
called geosmin g e O S M I N and
it's produced by the bacteria stripped to mices when it dies, right,
and it's all over the ground if it's you know,
healthy ground. And we figured out the stripto mices makes
a really good antibiotic. We use stripedo mices secure a

(03:18):
bunch of different stuff. But um, it's not stripped of mices.
Were smelling. It's this molecule that stripped of mices produces
as when it dies, and I believe as it's being
consumed by other bacteria. Right, So this geosman stuff that's
in the soil um has this earthy smell that the
the earthy component of petrocre is it comes from this molecule.

(03:42):
And they knew this starting back in the sixties with
Bear and Thomas, and they didn't know exactly how that
would happen though, like how does a molecule in the
soil get into the air to make it, you know,
the smell after a rain. And then finally some m
I T researchers proved it once and for all in
two thousand and fifteen, that it becomes aerosolized. Yeah, that's

(04:04):
really cool. Like just four years now as we record this,
we've known exactly how this happened because they used these
really high speed close up cameras to the ground and
they found out what the rain drops they trapped little
tiny air bubbles when they hit the ground, and then
those bubbles shoot up through the rain drop and kind
of pop an aerosol like uh, sort of when you

(04:26):
look at it when you pour a soda, that's stuff
that fizzes at the top. It just sort of aerosoli aerosolizes, yeah,
and it spreads by the wind, and that's why a
light rain makes the smell more like, if it's just
like pounding with rain, it's not gonna aerosize and aerosolies
and spread out as much, right, because it's diffused by

(04:47):
the wind. Like it pops up from the rain drops,
it pops through the rain drops into the air, and
then the wind kind of carries it. So if you've
ever noticed, especially before a storm, it's technically not smelling
before the storm. It's smells just at the very beginning
of the storm when the first droplets have hit the
ground and have begun to aristolize, but the wind is

(05:08):
really starting to pick up and carry it through. That's
where Geosman really comes into play. Yeah, and here's the
deal with Geosman. Why it's kind of a big deal
for us humans is that, more so than any animal
that I could find, human beings are really really sensitive
to the smell of Geosman. It's so bizarre, man, Like,
we're super sensitive. Yeah, I mean that means it's important, right,

(05:31):
it does. And they're not quite sure why it's important.
They think that maybe we evolved to be able to
find water through the scent of Geosman. It makes sense,
but um, so we're more sensitive Geosman than sharks are
to blood. Amazing. A shark can smell something like one
part of blood per billion parts of water, and we

(05:55):
can smell geosman at five parts per trillion. So we're
more sensitive to the smell of Geosman than a shark
is to the scent of blood in the water, right,
which I did some more research into that. Apparently sharks
have been overstated a bit. They can smell blood pretty well,
but it's not like those things where they can smell

(06:15):
it a mile away. That's all internet legend. Okay, sure,
but even still, I mean, the shark smells blood to
sustain itself with food, and we somehow evolved to smell
geosmen even better than a shark can. We're not known
for us since a smell. So there is a there's
a riddle there. There's a red flag evolutionarily speaking, that

(06:35):
we have yet to figure out. But it definitely is significant.
All Right, We're going to spend sixty seconds trying to
figure it out. You listen to this break and we'll
come back and figure it out and let you know. Alright,

(07:07):
so here's another cool thing the paper. But did we
figure it out? Now? We didn't figure it out. Okay,
here's the thing in that paper by Baron Thomas in
n s four. It's pretty cool. Uh. They found out
that this the scent was being captured and sold in
India as mettica atar and Geosman is also like becoming

(07:31):
a perfume ingredient, which is pretty amazing. Yeah, it has
been in India for a while, but the uh, I
guess the perfume industry in the United States and Europe
is finally catching up to that these days. Yeah, we
we love the smell, but we hate the taste as
as they say, yes, because Geosman also appears um in
other places, sometimes in the ter war of wine or

(07:53):
in mineral water. I don't know that, however, detected it
at least they didn't realize that. Yeah. I need to
know what it it is because I love beats, I
love mineral water. I certainly am made partially of wine.
So I need to know if, like all those things
that I like, if it's in there, then I probably
like to taste. So you do like the earthy taste
of beats, because that's what gives beats. It's earthy tastes

(08:16):
yours I do too, but I'm also like walking on
a razor's edge of enjoying beats every time I do
to where if I stopped and really thought about the taste,
I would hate it immediately and possibly forever. So I
just think about other stuff, like you know, baseball or
my old cabbage patch kid when I was younger. Interesting. Yeah,

(08:38):
I don't know about the second one, but yes, okay,
but that's not all, is it, Chuck. No, This this
next part is really cool to me, um, because if
you've ever noticed, if you've been in a rainstorm in
the country or the woods, it smells very different than
in the city. And that's not just because when it
rains in the city it's kicking up pee and poop
and garbage and stuff like that into the here. No,

(09:00):
because it's something. It technically smells cleaner in the city
than it does even in the countryside. Yeah, and that's
not because there's more to clean in the city, and
so you're smelling the offshoot of that. What it is
is ozone, that third ingredient that you smell when you
have that patricor effect? Is that a term? The the

(09:23):
effect of it effect up, Yeah, is lightning. So when
a thunderstorm comes around that what you're smelling with the lightning,
that clean, sort of crisp thing that you can't quite
put your finger on is ozone, Yeah, which is produced
naturally up in the atmosphere. But the electrical like bolt
of anger that is lightning also excites the oxygen molecules

(09:47):
in there so much that it combine them together into
oh three, that's ozone, and that does have a very
specific smell. It's weird. It's a weird smell. I remember
we had a listener one time that was, um, do
you remember this guy he was making O owned deodorant. No, yeah,
he sents. I think he sent it to me because
he knows that I have deodorant problems. Oh I want
to try that. Who if you're listening please years ago.

(10:10):
But if you're listening citizen some more. I didn't care
for it. Had a weird smell, and he was like, yeah,
some people ozone's kind of goes one way or the other.
I'll use your old stick if you're not using it anymore.
I don't think you want my four year old ozone
deodan Yeah, yeah, with like your armpit hair is still
stuck to the top of it. Oh gosh. Um. So
the reason why it smells cleaner in the city is

(10:32):
because there's less geosman, because there's typically less soil in
the city and geosman smells more earthy than ozone. Um.
So you get more of a prominent geosman smell in
the countryside, which means you get more of an ozone
smell in the city, which means it smells cleaner in
the city. Yeah, it's not like there's more ozone in
the city, is just more prominent because there's less of

(10:53):
the geosman, right, right, But there is another ingredient and
we skipped over it's technically the second ingredient. And they're
not sure of this, but it's just it makes so
much sense that they're almost certain that this is the case.
That the third component of petroc or you've got geosman,
ozone and then tirpenes, the things that give plants their
distinctive smells. They think that tirpenes are activated through a

(11:17):
number of different mechanisms um that make them produce a
much more fragrant smell right around a rainstorm. Uh, and
then that contributes to the smell of petrocre to right,
and if it's been like really dry for a long time,
that rain may be hard enough to where it like
breaks off dry plant material and stuff like that releases
that chemical uh, and they like in it. In this

(11:38):
article from the BBC, just like when you crush up
dried herbs, it releases that smell. Yeah, and it would
probably follow the same process as the geosman where it
just becomes airsolized by the raindrops as they hit them.
Very cool. This is a good one to know. Like,
next time you're in a rainstorm and someone comments about
why it smells, just say patricor or the next time

(12:00):
was somebody just spits out a bite full of food
in their napkin and says, I hate beats. You can
explain what petricor right, Well, that's it for petrocor, patricor
whatever you want to call it. Uh, it has been done.
So short stuff is out. Stuff you should know is

(12:21):
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