Neil deGrasse Tyson Explains Why Craters Are Round

[Music] yes good to see you always thanks for showing up for these these little explainer videos well thanks for doing them you know you know i you know how i come up with them there's like things that people know about but they don't know as much as they think they should know or might want to know after they learned all they could know that was very dr seuss no it's it's i think i know enough about this and then you realize this stuff you didn't know and oh my gosh i'm glad i learned it so i'm making a judgment there that there's nothing wrong with going in-depth it's in-depth total so this one is something very simple the shape of craters shape of craters yeah yeah okay a picture of the moon any photograph of the moon right the shape of the craters well they're all circle concave little circles they're perfect circles every single one of them the big ones the little ones the medium ones they are perfect circles okay so let's guess let me guess aliens just like on earth same way they make the crop circles they make moonshine that's how they make the moon circles moon circles there so yeah here's the thing it's interesting that as humans if there's something scientists don't understand people rapidly just go to god or aliens well that's why they got the explanation for white all right it turned out it was a mystery why all craters are perfect circles for the longest time it was a mystery until about 100 years ago okay and let me tell you why it was a mystery all right so your first thought is well the moon doesn't have an atmosphere maybe it's getting slammed by meteors okay but is every meteor coming straight down at a 90 degree angle so that the explode that ready hits it makes it per is that surely some of them are at an angle you'd expect some craters to be elongated right and you have all sort of shapes uh from circle to very uh very flattened circles you'd expect that some of them would be divots like a like the when you swing a golf you know it'd just be like a oh okay i mean it came in on an angle on the angle from and with the with the with the because you'd have a the slope would be different on one side than the other then on the opposite because it came slid in and slid in so you'd expect that and it is nowhere to be found on the moon so this energized the geologists because we know that the dark areas of the moon is where lava has once flowed okay i think they call it basalt it's dark and it's lava fields all right and by the way the apollo astronauts aimed for those places because they're flat if you're gonna go quarter million mile and land sideways on and fall over that's it all right so they found the flattest areas and those are called seas these are um before people knew that the moon didn't have weather like we have or oceans these large flat areas were called see the sea of tranquility okay that's where it gets its name nice um and all the c's were named after psychological states or physiological states so there's a sea of fecundity there's a c all right yeah yeah right so there's a anyway but so they're flat because lava in liquid state flattens out right there you go okay all right so wherever there was a crater it was round so the geologist said it's probably not these uh asteroids because they'd have to all come in perpendicular they're probably all volcanic craters because volcanic craters you punch up it makes a hole the hole is a circle right okay you know you can pull that off so that's that's how it stood for the longest while until people saw craters on top of places where it didn't look like lava had flowed okay so how are you gonna have a crater and no lava signature anywhere near it what's up with that and so this was a conundrum okay i have a book from 1890 and it says um believed by many to be volcanic in origin uh perhaps they're actually of uh asteroid impacts we are not sure okay it's by the way it was that way for most of the 20th century until computers came along and you could simulate impacts nice you want to simulate it okay so here you go you ready if you if you have an impactor that comes in uh straight into a surface it makes a circular crater if you send it at an angle it will make an elongated crater okay this will happen so they're saying no wait a minute let's send it in faster turns out there is a magic speed with which you can send in an asteroid and on collision it will make a perfect circle no matter the angle it hits no matter the angle it hits correct and that's speed okay this is this is what okay you ready let me guess let me guess 88 miles per hour precisely the same speed that it takes to propel you into the future because the flux capacitors 89 miles per hour all right so here's the so here's what's going on let's let's take a rock and you can ask why does the rock sort of hang together right why isn't it just decomposed into sand well because they're molecules that are attached all right it's molecularly attached within itself right right these are electromagnetic forces you can we know what they are okay um all right you can add up how much energy is contained in all of these molecular bonds okay and you come write down that number and you say this rock is held together by this much energy you can do that okay okay all right uh some rocks are held together with not much energy like if you take a sort of a semi a kind of solidy thing like a snowball or so i'm going to get to that in a minute right oh okay like a sand ball right right that if you just sort of punch it the whole thing just crumbles back into sand but it holds its shape together just for a little while not much energy is holding that together snowball another example not much energy is holding unless you like really pack it in and what you're doing is you're melting the snow and have it re-freeze and that holds it up better we had a hole we had a whole explainer on the freezing and melting of ice so what you're really doing is saying to the other person you're having a snowball fight with i hate you i hate you even more even more because this is going to be a nice little ice ball that i'm not going to get in the face with so that held and if held together only loosely you can't even throw it without the thing flying apart okay so you can write down how much energy that is calculate it and write it down now move the object through space or through the air there's a kinetic energy it has and this is the energy of motion it's called kinetic energy there's a formula it's one half times the mass of the object times the velocity squared okay all right okay now the moment the kinetic energy that the object has exceeds the binding energy of the molecules right you have to ask well if the thing was going at that speed and then after it hits it's going at zero speed then what happened to all that kinetic energy it got pumped back into the object right but that's more energy than what is holding the object together in the first place right so if you have more kinetic energy than the binding energy of the thing that has the kinetic energy it's going to explode on impact it explodes precisely wow so we call these high-speed collisions and it's not just oh because it's going fast no it is higher speed than the energy that's holding it together so on impact it's an explosion and explosions happen in all directions now is that because the moon has no atmosphere because that doesn't happen on earth so so so let's i'll get to that in a minute okay let me get to that just imagine there's the air is not a thing just like okay imagine the air is not energy talking about the energy okay okay okay all right so so there you have it that's the entire reason so in fact all the craters on the moon the damn near all the craters on them there might be one or two that were volcanic all the rest are asteroids uh meteors coming in at whatever angle they choose doesn't make a difference that doesn't make a difference they all explode on impact they explode on your pack because they're coming in at with at what we call a high speed collision now now let's look at snowballs you should try this okay um when you throw a snowball at a wall the winter is coming up okay right we're recording this in november so you throw it at a wall when it hits the wall it doesn't stay as a snowball no it explodes it it basically pops it goes everywhere it goes everywhere and except for one little white dot so you've done this before yeah okay this told all hawaii friends and other friends who who don't have snowball fights so try throwing it at different angles when winter comes you will see that every time it explodes on impact and you see the scatter all around in equal directions that's because even at your speed okay you're not going hypersonic speeds it's just the speed that you throw it that's more kinetic energy that is the binding energy of the snowball right so that's why a high speed collision you have to ask what's binding the object and how much energy does it have of motion and it's the the relationship of those two numbers that'll tell you if it's a high-speed collision so a snowball on someone's chest exploding is a high-speed collision nice that's super cool yeah yeah and so let's just hope though that you never throw a snowball hard enough to leave a crater in somebody we're gonna have a problem at that point that would be more energy than is necessary to explode the the snowball at the other side but correct yeah you don't want to be leaving craters all right so let's get back to the atmosphere very important and interesting question okay so now an asteroid comes and it sees the atmosphere right well the atmosphere is going to slow it down a bit isn't it okay it's going to slow it down if it slows it down enough so that its kinetic energy is less than the energy that binds it together it will survive on impact right we do find uh meteorites we add the ite after you that's on the ground meteor coming through the air meteorite we find there are meteorites that make it to the ground okay wow yeah right we have them they're there let's go they slowed down enough put the brakes on them put the brakes on and it did not completely explode now in the explosion there could be some pieces that remain intact just in the explosion so meteor crater arizona which used to be called barringer crater and p then geologists said there's got to be volcanic because it's a perfect circle but there's no volcanic activity in arizona right there's just no volcano for miles all right so that was a conundrum when we figured it out it got renamed meteor crater and those those computer simulations were abundant in the early 70s when we were able to program computers in the science laboratories so uh that that asteroid they were saying well where is the asteroid if an asteroid did it let's dig and find the big asteroid all right and so they dug and they didn't find anything because they didn't know that the sucker blew up okay and so so a person bought that land a mining company bought that land on the expectation that they could mine the metals of the asteroid that must be buried deep in it and it was nowhere ninety percent of it vaporized on impact yes their pieces are scattered around and and those pieces are now in collections we have some at the museum but most of it is gone for that reason and that's why craters around there you go okay another explainer you know we keep doing this we'll run out of stuff to explain that's like reaching the end of the internet which i i don't think it's gonna happen or the end of netflix movies right right so the next does not happen that's the that'll be the end of the world when that's right all right chuck we got to go thanks for having me always a pleasure neil degrasse tyson [Music]

Channel: StarTalk

Views: 195,049

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Keywords: startalk, star talk, startalk radio, neil degrasse tyson, neil tyson, science, space, astrophysics, astronomy, podcast, space podcast, science podcast, astronomy podcast, niel degrasse tyson, physics, chuck nice, craters, asteroids, shape of craters, volcanoes, high-speed collisions, kinetic energy, snowballs, Moon

Id: xRnr79Fa554

Length: 14min 26sec (866 seconds)

Published: Tue Dec 01 2020