Petrichor, that fresh aroma you can smell after the rain, is caused by a little molecule called geosmin. The name literally means "earth smell." It's produced by soil bacteria and gives an earthy flavour to beets.

Humans are extremely sensitive to this scent โ€“ much, much more sensitive than sharks are to blood. Some think we evolved that way to find fresh drinking water after droughts.

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@InvaderXan I love your science tidbits and am heartened by the knowledge that there's something we're better at than sharks.

@lazykat I'm glad you enjoy them! I love sharing random bits of information.

Also, the thing with sharks surprised me when I found out. Unless I'm wrong, we're a few hundred thousand times more sensitive!

@InvaderXan Yes but how get I squeeze the Petrichor scent out of the soil during a drought?

@joamo Thinking about it, I'm fairly sure humidity does that. That's probably why evening air has a certain distinctive scent as the air starts to cool after sunset.

@InvaderXan new project; hook up humidifiers to wind turbines in my backyard

since we're on the subject, any ideas on low cost fog/mist reclamation? More the former than the latter, I'm more interesting in capturing stuff in the air before it hits the ground but I haven't been able to find specs on this sort of thing.

@joamo Someone on tumblr told me about a device used in South America called an atrapaniebla, for collecting water from fog. Apparently they're often used for sea mists in Chile.

In English, I think it's called a fog fence? Basically, a big sheet of netting which fog condenses onto. Perhaps that could be what you need?

(Image: Flickr/Nicole Saffie,

@InvaderXan Yeah! I read about that when some articles were published a while back, but I didn't see a name or specs for it. Is it literally just a net? IIRC the students designed some nifty features into it.

Answered my own question with first page googling: watersustainabilityandfogwater

Thanks for the search term :_earth:

@joamo Anytime! โ˜€๏ธ

As far as I can tell, it's a net with some drainpipe and a water butt to collect any water that condenses.

I'd guess something water repellant like nylon would be ideal. You'd probably need to suspend it carefully so the water runs down where you want it โ€“ though be careful if you're making one, as some types of cord shrink slightly when they're wet!

@InvaderXan Thatโ€™s the scent I follow to find fresh places for zapping!

@InvaderXan I know and love that smell for a long time. Finally I know where it comes from. Thanks! ๐Ÿ˜˜

@kat They're the best kind of facts to share around IMO! ๐Ÿ˜„

@ChristieMalry Haha! Not to worry, it's easy to sometimes miss a word when reading!

@ChristieMalry This is something I've wondered about social media before. It seems like it might confuse that part of your brain's wiring. Like, which word really was the first? The one I wrote? My username? The post above it in the timeline...?

In short, I wouldn't worry. ๐Ÿ˜ฌ

@InvaderXan Wow. This is one of those moments of where you learn on an intellectual level something you've always felt, but been left so curious as to what all is going on there. thank you for sharing ๐Ÿ’š

@screwjaw Well, the way it works in nature is that on a hot day, the soil gets hot and dry. This kills off the bacteria in it and the dead bacteria dry up and fall apart. Then when the soil gets wet, it releases that sweet, sweet geosmin aroma.

I'm not sure how well you could artificially replicate that though. ๐Ÿ˜‹

@screwjaw Well, you could use a beet if you want, but beets have all kinds of other flavour compounds. If you're just trying to get geosmin, a little dry mud would suffice.

@InvaderXan Are humans especially sensitive to geosmin, or are all/most mammals sensitive to it?

@Inskora Apparently it's something which humans are particularly sensitive to.

@InvaderXan It's produced by the bacteria I work with (Streptomyces). They also produce this when growing in incubators. Much nicer to work with than smelly E. coli or Bacillus. ๐Ÿ˜€

@kblin It must be so nice to work with fragrant bacteria! ๐Ÿ˜ฌ

@InvaderXan To be fair, we work with them because they're great sources many interesting molecules that we're hoping to turn into new antibiotics. ~70 % of the antibiotics in use today originate from Streptomyces bacteria or close relatives. The nice smell is welcome, but a side effect. ๐Ÿ˜‰

@kblin Oh interesting! Looking to find things which won't be affected by the impending antibiotic crisis, I take it? I didn't realise so many of them originated in so few actual organisms.

@InvaderXan Antibiotics and resistance will always be an arms race. Microorganisms have been playing this game forever, long before humans discovered antibiotics. But yes, we're trying to find something that works against some of the most dangerous multi-resistant bacteria, to keep up our side of the game.

@kblin Oh of course. I guess it's a sad fact that simply by developing new medicines we're breeding bacteria to resist them.

@InvaderXan You can find ancient bacterial samples (from permafrost soil and the like) that are resistant against antibiotics. As I said, bacteria have been playing the chemical warfare game for quite a while now. We're probably accelerating the resistance formation by using an antibiotic, that's true. But IMHO the cause of the upcoming crisis is that we declared victory in the 70s and stopped a lot of the R&D.

@kblin Do you think overuse of certain antibiotics is a factor too? Or is that a myth?

@InvaderXan It certainly is a factor, because it increases the selection pressure on the bad bacteria to become resistant. But resistance itself has always been there. Simply put, the soil bacteria that produce the antibiotic need to be resistant against it, or they'd be committing suicide. It's just that that kind of resistance doesn't matter because in general soil bacteria don't try to eat you.

@InvaderXan But of course e.g. taking an antibiotic if you've got a flu or other viral infection will suddenly make it very relevant for bacteria that live on or inside you to learn how to survive the antibiotic.

@InvaderXan anyway, bedtime for me. thanks for the opportunity to talk about the field I work in. ๐Ÿ‘

@kblin No problem. Always nice to hear about interesting science. Especially from fields I'm not involved in!

@InvaderXan In the last 40 years, we've identified only three completely new classes of antibiotics, everything else was just a remix of something we already knew.

@InvaderXan The most famous antibiotic (penicillin) was discovered in a mold fungus. But quite a bunch of the other ones come from a small family of bacteria that seem to have a very inventive biochemistry.
I'm biased, of course, but I think it's a super interesting research topic.

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