In this episode we learn about Mars’ ancient past, meet an architect hoping to build cities there and we hear from Mars itself, thanks to the planet’s video blog, of course. Plus: Our Moment of Um answers a question about money with the help of Kai Ryssdal and Molly Wood, from the Marketplace podcast, Make Me Smart.

Do you ever dream of visiting Mars?

How about living there? It could happen.

Scientists are currently studying the planet for clues about its past and this information might help us make Mars a little more like Earth. One day, we may even be able to build cities there.

Of all the planets in the solar system, Mars is the most like our own. It’s about half the size of Earth and has about one third the gravity. That means you’d be able to jump higher and farther there, but you’d need a really warm space suit. Today Mars is a frozen desert where temperatures can drop to -100 degrees Fahrenheit at night. Brrrrr! There’s no breathable air or liquid water on the surface. However, it may have been a quite different place 3 to 4 billion years ago.

“It looks like Mars long ago once had lakes, it had rivers,” said Bethany Ehlmann, a planetary scientist with Caltech.

She studies information gathered by the NASA rovers about the rocks and dirt on the Red Planet. With this data, Ehlmann and other scientists are discovering that 3.5 billion years ago Mars might have been a warmer, wetter place with “plenty of water to potentially support life,” she explained.

But, things change. Mars once had a decent blanket of gas surrounding the planet, something called an atmosphere. Earth has one too and they help trap heat from the sun, keeping a planet warm. Over time, Mars lost most of its atmosphere, turning it into the cold place we know today. Earth on the other hand, kept a relatively thick atmosphere.

“We’re trying to figure out why do some planets continue to as great places for life while others maybe become a little bit worse over time,” Ehlmann added.

Still, if we wanted to make Mars more like Earth, Ehlmann says we’d have to try to bring back that gas blanket, perhaps by evaporating the planet’s ice caps or sending plants there that could help make more oxygen. For now these ideas are in the realm of science fiction.

Tristan Bassingthwaighte thinks we might be able to find another way to live on Mars.

He’s an architect and was part of an experiment called HI-SEAS, or the Hawai’i Space Exploration Analog and Simulation. It’s a project where people live in a fake Mars base on the side of a volcano. They must act like they are really on an alien planet at all times, Bassingthwaighte explains.

“So the idea of maybe just going outside for a walk or to enjoy the weather does not exist… You are in a suit, you don’t really feel the wind or the sun, it’s quite hot and uncomfortable.”

Still he enjoyed the experience enough to want to live on the real Mars. In fact he’s thought a lot about what a Mars city would look like.

“The first Mars colonies, sort of out of necessity are actually going to be underground,” he said.

That’s because Mars is constantly bombarded with harmful cosmic rays and solar radiation. Earth has a strong magnetic field which shields people from this stuff, but Mars doesn’t. The rays and radiation on Mars could make humans very sick and even kill them over time. Fortunately, dirt can block this stuff!

“So until we get better suits and better protection for the surface you probably want your main living area be underground,” said Bassingthwaighte.

He imagines one day whole cities could be built in Martian caves. They could be sealed off and filled with air to breathe. They could have ceilings with digital images of clouds and fake sunlight to make you feel like you were outside. They could have homes and neighborhoods and downtown areas.

Bassingthwaighte admits, this would take a lot of planning, money and time. The people of Earth would have to work together and really focus if they wanted to pull off a plan this extreme. Still, he thinks it could happen if we set our minds to it.

“This is definitely something that could happen sooner than people realize,” he added. “I have every intention of retiring to Mars.”

Audio Transcript

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NARRATOR: You are listening to Brains On! where we're serious about being curious.

BOY: Brains On! is supported in part by a grant from the National Science Foundation.

MOLLY BLOOM: How would you like to vacation someplace beautiful, someplace exotic, someplace where you could leap three times higher than on Earth?

MALE: 10, 9, 8, 7--

HAYDEN: How would you like to vacation on Mars and maybe do research there? Or one day, live there?

MALE: --2, 1, liftoff.


Have a safe flight. And bring me back a souvenir, would you? I never get to go anywhere cool. Why do the astronauts get all the best jobs?

MOLLY BLOOM: Right now, Mars is not a great place for life as we know it.

HAYDEN: But it might have been once. And maybe it will be again.

MOLLY BLOOM: Strap on your spacesuits, cadets, today we're exploring the red planet.

HAYDEN: Keep listening.


MOLLY BLOOM: You're attuned to Brains On from American Public Media. I'm Molly Bloom, and my copilot on this journey is 12-year-old Hayden from Neenah, Wisconsin. Hi, Hayden.


MOLLY BLOOM: You got in touch with us to ask us about life on Mars. So what got you thinking about that?

HAYDEN: It was actually another Brains On episode. I was like, hey, maybe it'd be fun to ask a question.

MOLLY BLOOM: I'm glad you asked the question because it has inspired this episode. And so, do you think you should try to live on Mars?

HAYDEN: If they really want to and it works out.

MOLLY BLOOM: Well, at the moment Mars is a pretty lonely, desolate place.

HAYDEN: Oh, yeah. That's all Mars ever talks about.

MOLLY BLOOM: Excuse me?

HAYDEN: On his video blog.

MOLLY BLOOM: Mars has a video blog?

HAYDEN: Yeah, I'll show you. Check it out.


MARS: Hey, guys, it's me, Mars. I thought I'd do another video. Not much has changed this last time though. Still pretty cold, like negative 80 degrees Fahrenheit on average. It's really hard for me to hold on to heat because I don't have a lot of atmosphere.

And FYI, guys, an atmosphere is a thick blanket of gas that surrounds a planet and it can help trap heat. But mine is only about 1% as thick as Earth's atmosphere. So heat from the sun just radiates right off me back into space. Man, if I had a thick atmosphere like Earth, staying warm would be easy.

That planet is such a show off. It's like, oh, I'm Earth. Everybody wants to live here because I'm warm but I have liquid water. Let's make documentaries all about the rad animals that want to live on me. [SCOFFS] Whatever. Mars has rocks, and sand dunes, and frozen ice caps.

Oh, man, I'm so lonely. I tried talking to these NASA rovers again but they just ignore me. Snobs. Anyway, let's answer some emails, OK? All right. So spacefan99 asks, why are you red, Mars? You angry, bro? [SCOFFS]

What, no? I mean, I am angry no one invites me to their birthday parties or people don't follow me back on Twitter. It's just rude, guys. But does it make me red? No. I'm red because my surface is covered with iron oxide. It's the same stuff that makes rust red.

OK, another listener question. This one's from Venusrules. And they write, what's that giant pimple on your face? What are you talking about Venusrules? That's not a pimple, geez. That's only Olympus Mons, an actual volcano.

And hashtag, #humblebrag, it happens to be the largest volcano in the entire solar system. It's three times taller than Mount Everest. You hear that, Earth? I have the tallest pimple. I mean volcano. I mean volcano! [GROANS] You know what? I don't have to prove myself. Mars is an awesome planet. I'm just going to go inspire some science fiction writers to make most stories about me. So bye.


MOLLY BLOOM: Sounds like Mars has some issues to work out.

HAYDEN: Still it's a fascinating place. It wasn't always a frozen wasteland.

MOLLY BLOOM: That's right, you've been learning all about Mars for this show.

HAYDEN: Yeah. I interviewed Bethany Ehlmann. She's a professor at Caltech in Pasadena. And she studies Martian rocks.

MOLLY BLOOM: We should add, she doesn't collect the rocks herself. There are robots on Mars that find and analyze rocks and then beam the information back to scientists here on Earth.

HAYDEN: Well, when we talked, Bethany was in a studio in California and I was on the phone. I asked her what Mars was like billions of years ago.

BETHANY EHLMANN: It looks like Mars, long ago, once had lakes, it had rivers, it had a hydrothermal systems, it had aquifers and waters flowing underground, and it may even still have water underground today. But it's largely way colder and way drier than it once was early in its history.

HAYDEN: Yeah. Was it covered in jungles or a giant ocean or--

BETHANY EHLMANN: I don't know that there are any jungles because it would be really cool to find evidence of past trees on Mars or something. But we haven't found them yet. But if you think about-- what state are you in right now?

HAYDEN: I am in Wisconsin.

BETHANY EHLMANN: Mars in the past is probably a lot like the weather in Wisconsin in late fall transitioning into winter, right? So it's cold, a little bit snowy, maybe occasionally rains, maybe it's freezing rain. But there's plenty of water. And there's plenty of water to potentially support life and certainly things like lakes, and rivers, and groundwater, and things like that.

And the reason we know all of this is it's recorded in the rock record of Mars. So I'm a geologist by training and I study the texture of rocks, and their chemistry, and the minerals they're made of in order to figure out what the environments were like in the past.

So when we look back at the rock record of Mars, we see that Mars, a long time ago, like 3 and 1/2 billion years ago was once like Wisconsin in the winter in terms of its climate.

HAYDEN: Were there bugs there? I certainly hope there weren't mosquitoes and bees there. Then it would be very pesky to move in.

BETHANY EHLMANN: [CHUCKLES] I agree that mosquitoes on Mars would not be a plus. One of the things we're looking for with the NASA missions right now is to try to figure out the answer to the question, was there any kind of life on Mars? Or even is there any kind of life on Mars?

And you're right to maybe think and ask about bugs because we're not talking about little green men that walk around. We're pretty sure that they are not on Mars. But small bugs, or even microbes, little collections of cells that are alive, are certainly something that Mars could have supported in the past. The big question is, why did it change?

HAYDEN: That's actually what I was about to ask. What happened?

BETHANY EHLMANN: Well, we're not 100% sure. But this is something we're trying to figure out. We're trying to figure out why do some planets continue as great places for life. While others maybe become a little bit worse over time.

We think in the past that Mars had a thicker atmosphere. And the atmosphere acts as a blanket. So it can warm up the planet. It can trap heat and keep heat in. And that's why we think Mars once had all these liquid water.

But then Mars lost its atmosphere. One, because there's a solar wind that strips away, in small amounts, the atmospheres of all planets over time. Earth too loses pieces of its atmosphere. But Mars is smaller with less gravity to hold in its atmosphere so it may have lost some of its atmosphere to space.

But then it also has to do with the internal history of Mars. Mars's volcanoes started to shut off. And we don't think about this very much, but volcanoes on Earth, in addition to belching out lavas onto the surface, actually also belch out gases into the atmosphere that refresh the atmosphere and keep more gases in there even as Earth loses some of its gas to outer space.

But on Mars once the volcanoes shut off or became a lot less active, Mars lost more and more of its atmosphere to space. And as the atmosphere got thinner, the planet got colder.

HAYDEN: So what is Mars like now?

BETHANY EHLMANN: So let's put you-- first of all, if you were standing on the equator of Mars, so you'd see the sun but it would be about 40% fainter than it is from Earth.

You would have to be in a spacesuit to breathe because the atmospheric pressure on Mars is so low that your saliva and your tears in your eyes would start to boil if you were outside of your spacesuit. So you would be standing on the equator at your spacesuit looking at a 40% fainter sun. You'd see rocks and sand dunes and blowing dust.

HAYDEN: All right. I do have one final question.


HAYDEN: What would it take to make Mars like Earth again?

BETHANY EHLMANN: Yeah, that's a great question. We would have to make its temperature warmer somehow, right, to get some liquid water. The main way to do that is to put gas into the atmosphere.

One thing we know is if somehow Mars's polar caps, it has caps of H2O, ice water, ice and CO2 ice. If they were to completely evaporate into the atmosphere, the atmospheric pressure could double.

But other than that, it's a little bit in the realm of science fiction but people have talked about bringing colonies of algae, plants can take in carbon dioxide and make oxygen, right? And if we had a whole colony of microbes doing this, well maybe we could make the atmosphere of Mars thicker. So that's one idea.

HAYDEN: That would take a lot of stuff.

BETHANY EHLMANN: It would take a lot of stuff. It would take a lot of time too. So this is one of the reasons I hope that we go and explore Mars. But I'm content to do it in space suits to begin with. And then if we decide we want to change it, we can.

HAYDEN: Well, I really appreciated talking to you.

BETHANY EHLMANN: Thank you. It has been a pleasure talking about Mars.


MOLLY BLOOM: Hey, we're giving away another book. This time, it's a very charming novel about a boy who loves rockets, space, and his dog, Carl Sagan. He's making a recording about what it's like here on Earth so that life, if there is any life in the universe, can better understand our pale blue dot.

It's called See you in cosmos by Jack Cheng and I highly recommend it. It's surprising, and hopeful, and funny. Enter a chance to win the book at You have until midnight on October 3 to enter. That's


You're listening to Brains On from American Public Media. I'm Molly Bloom.

HAYDEN: And I'm Hayden. Today, we are talking about Mars.

MOLLY BLOOM: And before we get back to that fourth planet from the sun, a little detour for the--


GIRL: Mystery sound.

MOLLY BLOOM: Here it comes.


Do you have a guess, Hayden?

HAYDEN: Maybe it's a Martian robot rattling on Mars, driving.

MOLLY BLOOM: Excellent guess. And I will say that this sound does not necessarily have anything to do with the episode. So does that change your guess at all?

HAYDEN: Maybe. Maybe a waterfall.

MOLLY BLOOM: Excellent. We'll have the answer in a moment. But first--


--do you have a mystery sound you'd like to share with us, or a question, or a drawing? Email it to us at That's how Harper and Riley sent us this head scratcher.

GIRL: Why is money valuable? It's just paper. Can you explain?

MOLLY BLOOM: We'll answer that at the end of the show. Plus, we'll welcome the latest group of Brains Honor Rollies, so stick around. Now let's get back to that mystery sound, shall we? Here it is one more time.


All right, Hayden. Do you want to change your guess?

HAYDEN: Yes. Somebody blowing bubbles in their chocolate milk or water.

MOLLY BLOOM: Wow. That is an amazing guess. Here is Nico with the answer.

NICO: That was the sound of me blowing bubbles in a bottle of chocolate milk.

MOLLY BLOOM: OK. Hayden, how did you do that?


HAYDEN: I can't believe I actually guessed it. Has it ever done before?

MOLLY BLOOM: I think it's been guessed but not with that extreme level of detail. How did you-- how did you guess chocolate milk?

HAYDEN: [LAUGHS] It's because that's-- it's because that's what people at my school used to do. I was like, wait, bubbles, as I heard the second time.

MOLLY BLOOM: That is awesome. Well, you did a great job. Did you ever blow bubbles in your chocolate milk?

HAYDEN: Not very often. If I did, I'd usually do it at home. I mean, I never actually drank the chocolate milk because it was served in cardboard cartons. I don't like drinking things out of cardboard cartons.

MOLLY BLOOM: [LAUGHS] Well, we may probably do an experiment later to see if bubbles blown in chocolate milk sound a little different than bubbles blown in regular milk, or juice, or water. That's an experiment to try at home. Well, I'm super impressed, Hayden. Good work.


KIDS: Radar!


MARS: What's up, people? It's me again, Mars. Shout out to all the new followers who found me after the last video. I got a great new batch of questions too. Here's one from redplanetfever. They say, hey, Mars, can I come live on you? I hear you've got less gravity than Earth and I'm working on my dunks.

Yeah, it's true. I'm half the size of Earth and I only have about a third of the gravity. So you could totally dunk here. You bring the ball, I'll help with the bounce. OK, here's another message I got. This was from-- I love this username, Earthisoverrated22. [CHUCKLES] That is awesome. I totally agree.

OK. So they write, hi, Mars. My planet is too crowded and I want some peace and quiet, let's hang. Guys, I'm really touched. This seriously makes my day. I know I'm always red, kind of a blessing right now. That's sweet. Yeah, let's totally hang, guys.

But OK, I should warn you, there a couple of things about me you should probably know. First, like I mentioned, I'm pretty cold most of the time. I mean, some days, at noon on the equator, I could be like 70 degrees Fahrenheit. It's not bad. But you might want to bundle up before nightfall because I can drop to negative 100 degrees, which I'll admit is a little brisk.

Also, I don't have a breathable atmosphere really, so you probably want to pack your own-- it's like BYOA, Bring Your Own Atmosphere. Oh, and I don't know if this is a deal breaker, but I'm constantly bombarded with cosmic rays and solar radiation that could make you sick or even kill you. Not right away. But over time, kill you.

Space is full of the stuff too though, so it's not just me. I'd love to have a strong magnetic field that deflects the solar rays and cosmic radiation. I've been told I used to have one but I lost it somehow. It's like totally weird. How does that even happen? I had my-- I had my keys. Where'd they go? I had a magnetic field. Where did it go? I don't know.

Anyway, scientists are still trying to figure all that out. I know Earth has a powerful magnetic field and it shields all you people from this harmful stuff, which is cool. Be safe, earthlings. But if you come hang with me, yeah, you might have to worry about cold temperatures, no air to breathe, and that whole deadly race thing.

If you're down with that stuff, come on, let's do this. Let's hang out. And hey, subscribe to my channel if you haven't already. Next week, I'll be posting a lipsync I did to a Bruno Mars song, because I'm also Mars, and I think we may be related. Anyway, you don't want to miss that. OK, that's all for now. Later, everybody.


HAYDEN: Hey, Mars, I think I may know someone who used to cat sit for Bruno Mars, or his cousin, something like that. Anyway, I can probably hook you up. (SINGING) Don't believe me just watch.


MOLLY BLOOM: Do you have a question for Mars to answer on its video blog? Send it to us at We'll post a new video of the red planet answering some of your questions on our YouTube channel.

They can be scientific, or silly, or you can just say hi to Mars. Again, send Mars mail at and keep an eye out for a new video. Speaking of Mars-related questions, let's hear another.

HENRY: Hi, my name is Henry from Oakland, California and I'm nine years old. I was wondering, could people ever live on Mars?

MOLLY BLOOM: Great question, Henry. People with the University of Hawaii are looking into this right now.

HAYDEN: They run a NASA-funded project called HI-SEAS.


HAYDEN: Which stands for the Hawaii Space Exploration Analog And Simulation.

MOLLY BLOOM: The goal is to find out how to keep a crew of people happy and healthy on a long-term space mission. So the participants live in a small domed base on the side of a volcano in Hawaii.

HAYDEN: They pretend it's base on Mars. The people say for anywhere from a few months to a year.

TRISTAN BASSINGTHWAIGHTE: And we simply live far room from society my name is Tristan Bassingthwaighte and I was the crew architect for HI-SEAS4.

HAYDEN: Tristan says the dome is on an old volcano because the surroundings are actually a lot like Mars.

TRISTAN BASSINGTHWAIGHTE: You just have broken rock, lava flows that have solidified into goopy rivers of stone. And there's no plants, no forests, no roads. We have normal gravity. The sky is blue. You can hear the wind outside. You know there's not the radiation that exists on Mars. But the idea is to simulate the mental and social isolation from your families and daily life. So it approximates Mars in a lot of ways.

MOLLY BLOOM: When you live in this dome, you only have minimal contact with the outside world. You can send email but you can't call friends any time you want or get cable TV. There's no fresh food. And to make it really authentic, the participants can't even leave the base without wearing protective suits.

TRISTAN BASSINGTHWAIGHTE: So the idea of maybe just going outside for a walk or to enjoy the weather does not exist. It's all very structured. And you're in a suit. You don't really feel the wind or the sun. It's quite hot and uncomfortable. There's fans blowing in your ear to keep you cool. It's definitely not the same as going outside on Earth.

HAYDEN: A big part of the experiment is studying how people handle being cooped up with others for a long period of time. Imagine a road trip with six other people that lasts a year. [SCOFFS]

MOLLY BLOOM: The base is 1,300 square feet, sort of like an apartment or a small house in the US. Each person has a room about the size of a closet and the rest is common space.

TRISTAN BASSINGTHWAIGHTE: You can definitely get annoyed with each other. You can't really fight the way you might imagine siblings fight. I fought with my brothers growing up. You could go to your room, or go outside, or go to a friend's house and get away from them and things would get better.

But when you're in this dome, you can't go outside and it's not big enough to really get away from people. So if you have a fight, it's going to make things pretty tense for a long time. And you try to avoid it as much as you can.

MOLLY BLOOM: Researchers hope what they learn from HI-SEAS can help them plan for a real trip to Mars.

HAYDEN: That's something Tristan is interested in. He'd love to live on the planet. In fact, he's thought a lot about what a Mars colony would look like.

TRISTAN BASSINGTHWAIGHTE: The first Mars colonies, out of necessity, are actually going to be underground. Mars doesn't have a very strong magnetic field and has a very, very thin atmosphere. So what gives you a light sunburn on Earth would be very, very dangerous to your health on Mars. So until we get better suits and better protection for the surface, you probably want to have your main living area be underground.


But if you can imagine, say, a cave that is big enough to fit your entire neighborhood in, and you've got lights in the ceiling that simulate what the sky might look like, nice, bright blue lights. You pressurize and fill with atmosphere that entire cave, seal off the entrances, and then begin to introduce plant life, open water, that sort of a thing. And you'll eventually get these big, big, big pockets of what look like little slices of Earth.

You could have homes that are open to the air and walk around your neighborhood. And you'll be underground. But to the extent possible, it won't feel like it. This is definitely something that could happen sooner than people realize. I have every intention of retiring to Mars.

If you were to say, look at Mars in 100 years, it's not inconceivable that we could have a population of 50,000, 100,000 people living there or more. It really just depends on how well we set up the infrastructure and the basic requirements now.

MOLLY BLOOM: If you want to know more about how NASA would ship materials for a base to Mars, check out our Deep Sea Versus Outer Space episode.

HAYDEN: Retiring on Mars, wow, that sounds extreme.

MOLLY BLOOM: Yeah, I don't know if I'd want to do that. I'm kind of scared of outer space, to be honest. What about you, Hayden? Would you want to live underground on Mars?

HAYDEN: Probably not. You can't have a good dog or cat there. I would be very sad if I never had a pet.

MOLLY BLOOM: Well, we asked our listeners if they'd like to live on the red planet. And here's what they said.

PENNY: I would like to go to Mars because I like the color of the dirt.

BRENT: I wouldn't go to Mars because it'll actually take longer to get there than the amount of time I stay at Mars.

NATE: I don't want to go or live on Mars because, what if the spaceship explodes?

BRENNA: I think I would like to live on a Mars because you could float around.

ETHAN: If I ever went to Mars, I would do a lot of science experiments. And I would jump as high as I can because there's less gravity.

RAYONGE: I would like to live on Mars because I want to be a scientist. And I want to learn about the volcanoes and how the sand got red and the sandstone and the rocks.

NARRATOR: Lift-off. We have a lift-off.

PAIGE: I wouldn't want to live on Mars since there's no water there. And I wouldn't survive.

EMERSON: I really, really, really, really would like to go to Mars. If Daddy and I and my brother would go and Mama, we would have a lot of fun here.

GIDEON: I would really like to live on Mars because I would want to discover new things about Mars and search for life.

STELLA: I wouldn't want to live on Mars because when you blast off in the rocket, it would feel like a 380-pound gorilla sitting on you.



MOLLY BLOOM: That was Penny, Brent, Nate, Brenna, Ethan, Rayonge, Paige, Emerson, Gideon, and Stella. We want you to draw us a picture of what your Mars base would look like.

HAYDEN: Would it have special areas like an indoor farm or a soccer field? Would it be in a dome or underground?

MOLLY BLOOM: Email it to We'd love to see your designs. We'll share them on social media. Follow us there to see them. We're on Facebook. And we're @brains_on on Twitter and Instagram.


HAYDEN: Scientists think long ago, Mars was warmer and wetter, kind of like Earth.

MOLLY BLOOM: Today, though, it's a cold, dusty place with very little atmosphere and lots of harmful radiation.

HAYDEN: Humans would have trouble surviving there right now. But people are working on ways to set up bases and even whole cities on Mars.

MOLLY BLOOM: That's it for this episode. We had engineering help from Johnny Vince Evans, Eric Stromsted, Michael Osborne, and Veronica Rodriguez. Special thanks to Katy Salzman, Bruce Betts and the Planetary Society, and Jenny Josephson.

HAYDEN: Brains On is produced by Marc Sanchez, Sanden Totten, and Molly Bloom. The show's questions from listeners like you and me.

MOLLY BLOOM: The Brains On is funded in part by a grant from the National Science Foundation. Before we go, let's answer Harper and Riley's question, it's our Moment of Um.


HARPER: Hi. My name is Harper.

RILEY: And my name is Riley. And we live in Massachusetts.

HARPER: Why is money valuable? It's just paper. Can you explain?

MOLLY BLOOM: We've asked two very smart people to help us answer this question. They are Kai Ryssdal and Molly Wood, hosts of the Marketplace podcast Make Me Smart. So, Kai, why is money valuable if it's really just paper?

KAI RYSSDAL: The root reason is actually rooted in a word. And that word is "credit" or credere, to the Latin, which means to believe. We believe that $1 bill, or a 5 or a 10 or a 20, has value because we can get something for it. That's the deal. It used to be rooted in gold. It used to be rooted in silver. Now it's just rooted in this thing called the full faith and credit of the United States. But we believe it has value because the government says it has value. That's it. That's the whole deal.

MOLLY BLOOM: Wow. And that's the same all over the world?

KAI RYSSDAL: Yeah, pretty much. I mean, money is not rooted in gold anymore. It used to be that you could take $1 bill or a $100 bill and go to the bank or the Treasury Department and get $100 worth of gold. Can't do that anymore. But it's not rooted in something tangible anymore. It's rooted in belief.

MOLLY BLOOM: That is so interesting. So paper money has been around for a while. But there's also a relatively new kind of money we want to learn about. So, Molly, what is Bitcoin?

MOLLY WOOD: So it's money that is created by computer code. It is literally little chunks of code. And each Bitcoin doesn't actually exist. There is no coin that you can hold. There is no physical embodiment of this currency. But you can slice it up. You could get a part of a Bitcoin and use that to pay for something.

And the reason that it has value is basically the same reason that paper money has value, that people have agreed that they will use it in exchange for goods and services. But part of the reason that it works and doesn't totally fall apart, even though it's essentially made out of nothing and created out of air, if air is computer code, it works because there's this technology underneath it that is sort of a notebook.

It's a ledger. And it lets everybody see how many Bitcoin have been created and how they've ever been spent. It lets you see the entire history of a Bitcoin. So if you can imagine that for $1 bill, that would be remarkable. If you could see every time that dollar bill had ever changed hands, you'd be able to track its entire history, you would know for sure that it was not counterfeit.

And the people who use Bitcoin know how to make it. And they know that it's incredibly difficult. The way that bitcoins are created is through a process called mining. And it takes a super powerful computer and a really, really long time to make a new bitcoin. So that means that they're rare. And when something is rare, it's more valuable. So people have agreed that Bitcoin has value.

MOLLY BLOOM: Thank you so much for coming and making us smart today. Kai and Molly can both be heard on the public radio show Marketplace as well as their podcast Make Me Smart.

NARRATOR: Ba, ba, ba, ba, ba, ba, ba, ba, ba, Brains On.

MOLLY BLOOM: Creative, curious kids keep this show going. And to thank them for sending questions, drawings, mystery sounds, and high fives, we made the Brains Honor Roll. Here's the latest group to be added.



CREW: Brains Honor Roll. High fives.


MOLLY BLOOM: Phew, that's a wrap.

HAYDEN: Thanks for listening. And stay tuned for more answers to your questions. Bye!


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