Stuff You Should Know - Short Stuff: Petrichor

Episode Date: August 14, 2019

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. Learn more about your ad-choices at https://www.iheartpodcastnetwork.comSee omnystudio.com/listener for privacy information.

On the podcast, Hey Dude, the 90s called, David Lasher and Christine Taylor, stars of the cult classic show, Hey Dude, bring you back to the days of slip dresses and choker necklaces. We're gonna use Hey Dude as our jumping off point, but we are going to unpack and dive back into the decade of the 90s.

We lived it, and now we're calling on all of our friends to come back and relive it. Listen to Hey Dude, the 90s called on the iHeart radio app, Apple Podcasts, or wherever you get your podcasts. 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 Petrocourt Chuck Go. Yeah, and so we decided to have school outside today. Mm-hmm, remember that? Oh yeah, I loved that. Cause 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 end of the school year was fast approaching. Sure, cause I guess that was usually in the spring, right? Especially in Toledo. Like if it was nice enough to go outside, you were coming up on the end of the school year. So loved that. Yes.

But Petrocourt, 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, 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.

About something similar. Gotta be. But that has a name and that is Petrocourt, 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, but it came from Australia, right? The name? It did, Australia in the 60s, actually. It's based on two Greek words. Petros, I believe, which means stone. And icor, which means the fluid that flows in the veins of the gods. Blood of the gods is a simpler way to think about it.

That's one way to put it. We also saw a life 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 Baer. I love that name. It's just a wonderful little name.

Like it should be like Nickelodeon Sprout cartoon or something. And not Isabel Joy Behar. That's different. Right. That's like Kathleen Turner overdrive. Anyway, the other guy was Richard Thomas, who had a ho-hum name.

But the two of them coined this term in 1964 in an article in the journal Nature. The article was called Nature of Argolatius odor with a U 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 being one thing, it seems to be a combination of three amazing things

that all kind of come to the fore 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 going to talk about is a molecule that's made 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 Streptomyces when it dies. And it's all over the ground, if it's healthy ground. Yeah, and we figured out that Streptomyces makes a really good antibiotic. We use Streptomyces to cure a bunch of different stuff. But it's not Streptomyces we're smelling. It's this molecule that Streptomyces produces when it dies. And I believe as it's being consumed by other bacteria.

So this Geosmin stuff that's in the soil has this earthy smell. The earthy component of Petricor comes from this molecule. And they knew this starting back in the 60s with Bear and Thomas. And they didn't know exactly how that would happen. They were like, how does a molecule in the soil get into the air to make it this smell after a rain?

And then finally, some MIT researchers proved it once and for all in 2015 that it becomes aerosolized. Yeah, that's really cool. 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 that raindrops, they trapped little tiny air bubbles when they hit the ground. And then those bubbles shoot up through the raindrop and pop an aerosol like when you pour a soda, that stuff that fizzes at the top. It just aerosolizes. And it spreads by the wind.

And that's why a light rain makes the smell more. Like if it's just pounding with rain, it's not going to aerosolize and spread out as much. Right, because it's diffused by the wind. Like it pops up from the raindrops, it pops through the raindrops 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 smells just at the very beginning of the storm when the first droplets have hit the ground and have begun to aerosolize. But the wind is really starting to pick up and carry it through. That's where geosmen really comes into play. Yeah, and here's the deal with geosmen,

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 geosmen. It's so bizarre, man, like we're super sensitive. Yeah, I mean, that means it's important, right? It does, but 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 geosmen. That makes sense. But so we're more sensitive to geosmen than sharks are to blood. Amazing. A shark can smell something like one part of blood per billion parts of water. And we can smell geosmen at five parts per trillion.

So we're more sensitive to the smell of geosmen 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 it a mile away. That's all internet legend.

OK, sure, but even still, I mean, the shark smells blood to sustain itself with food. Sure. And we somehow evolved to smell geosmen even better than a shark can. We're not known for our sense of smell. So there is a riddle there. There's a red flag, evolutionarily speaking,

that we have yet to figure out. But it definitely is significant. All right, we're going to spend 60 seconds trying to figure it out. You listen to this break, and we'll come back and figure it out and let you know. We're going to use Hey Dude as our jumping off point, but we are going to unpack and dive back

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bye, bye. Listen to Frosted Tips with Lance Bass on the iHeart Radio app, Apple Podcasts, or wherever you listen to podcasts. All right, so here's another cool thing. The paper by- Wait, wait. Did we figure it out? No, we didn't figure it out. OK.

Here's the thing in that paper by Bear and Thomas in 1964. It's pretty cool. They found out that this, the scent was being captured and sold in India as Matika Atar. And Geosman is also becoming a perfume ingredient, which is pretty amazing. Yeah, it has been in India for a while, but I guess the perfume industry in the United States

and Europe is finally catching up to that these days. Yeah, we love the smell, but we hate the taste, as they say. Yes, because Geosman also appears in other places, sometimes in the terroir of wine or in mineral water. I don't know that I've ever detected it. At least they didn't realize it. Yeah, I need to know what it is, because I love beets. I love mineral water.

I certainly am made partially of wine. So I need to know if all those things that I like, if it's in there, then I probably like the taste. So you do like the earthy taste of beets, because that's what gives beets its earthy taste, is Geosman. I do too, but I'm also walking on a razor's edge of enjoying beets every time I do. Oh, really?

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 baseball or my old cabbage patch kid when I was younger. Interesting. Yeah, I don't know about the second one, but yes. OK. But that's not all, is it, Chuck?

No, this next part is really cool to me, 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 air. No, because 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 effect of it, I think you just made it up. Is lightning.

So when a thunderstorm comes around, 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 bolt of anger that is lightning also excites the oxygen molecules in the air so much that it can bind them together into O3, that's ozone,

and that does have a very specific smell. It's a weird smell. I remember we had a listener one time that was, do you remember this guy? He was making ozone deodorant. No. Yeah, I think he sent it to me because he knows that I have deodorant problems.

Oh, I wanna try that. If you're listening, please. Man, this is years ago. But if you're listening, send us in some more. I didn't care for it. It had a weird smell and he was like, yeah, some people, ozone 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 that's gonna happen. This is, you want my four-year-old ozone deodorant? Yeah, yeah, with like your armpit hair still stuck to the top of it. Oh, gosh. So the reason why it smells cleaner in the city is because there's less geosmin because there's typically less soil in the city

and geosmin smells more earthy than ozone. So you get more of a prominent geosmin 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. It's just more prominent because there's less of the geosmin.

Right, right. But there is another ingredient that we skipped over. It's technically the second ingredient. And they're not 100% sure of this, but it makes so much sense that they're almost certain that this is the case, that the third component of Petricor, you've got geosmin, ozone, and then terpenes,

the things that give plants their distinctive smells. They think that terpenes are activated through a number of different mechanisms that make them produce a much more fragrant smell right around a rainstorm. And then that contributes to the smell of Petricor too. Right, and if it's been really dry for a long time, that rain may be hard enough

to where it breaks off dry plant material and stuff like that, releases that chemical, and they liken it in this 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 geosmin, where it just becomes aerosolized 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 Petricor. Right, or the next time somebody just spits out a biteful of food in their napkin, it says, I hate beets, you can explain why. Petricor.

Right, well that's it for Petricor, Petricor, whatever you want to call it, it has been done. So short stuff is out. Stuff You Should Know is a production of iHeartRadio's How Stuff Works. For more podcasts from iHeartRadio, visit the iHeartRadio app. Apple podcasts are wherever you listen

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