Why Craters Are Round, Size of Life, The Real Length of Day – TYTYK

Neil deGrasse Tyson (0:00)

Thanks for showing up for these. You know how I come up with them? They're 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.

Chuck Nice (0:13)

That was very Dr. Susie in that movie.

Neil deGrasse Tyson (0:20)

Welcome to StarTalk, your place in the universe where science and pop culture collide. StarTalk begins right now.

Something very simple, the shape of craters.

Chuck Nice (0:38)

Shape of craters?

Neil deGrasse Tyson (0:40)

Yeah. A picture of the moon, any photograph of the moon, what shape are the craters?

Chuck Nice (0:45)

Well, they're all circle, concave little circles.

Neil deGrasse Tyson (0:49)

They're perfect circles. Every single one of them. The big ones, the little ones, the medium ones, they are perfect circles. Okay?

So, this...

Chuck Nice (1:01)

Let me guess. Let me guess.

Aliens, just like on Earth, the same way they make the crop circles.

Neil deGrasse Tyson (1:11)

They make moon circles.

Chuck Nice (1:12)

That's how they make the moon circles.

Neil deGrasse Tyson (1:16)

Moon circles. 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. That's why they got the explanation for why. 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.

But is every meteor coming straight down at a 90-degree angle so that the explode, the red hits it makes a... Is that... Surely some of them are at an angle. You'd expect some craters to be elongated.

Chuck Nice (2:11)

Right.

Neil deGrasse Tyson (2:12)

And you'd have all sort of shapes from circle to very... Very flattened circles. You'd expect that.

Chuck Nice (2:20)

Some of them would be divots. Like when you swing a golf, you know, it would just be like a...

Neil deGrasse Tyson (2:27)

Oh, okay. I mean...

Chuck Nice (2:27)

Where it came in on the angle.

Neil deGrasse Tyson (2:28)

On the angle from... Because you'd have a... The slope would be different on one side than the other. Bend on the other. Because it came slid in.

Chuck Nice (2:36)

It slid in.

Neil deGrasse Tyson (2:37)

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. Oh, cool. I think they call it basalt. 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 a quarter million mile and land sideways and fall over, that's it. All right. So they found the flattest areas. And those are called seas. Before people knew that the moon didn't have weather like we have or oceans, these large flat areas were called sea, the Sea of Tranquility. Okay. That's where it gets its name.

Chuck Nice (3:24)

Nice.

Neil deGrasse Tyson (3:24)

And all the seas were named after psychological states or physiological states. So there's a sea of fecundity. There's a sea.

Chuck Nice (3:31)

Oh, all right.

Neil deGrasse Tyson (3:33)

Yeah, right. So there's a sea. 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 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. You know, you can pull that off.

So 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.

So how are you going to 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, believed by many to be volcanic in origin, perhaps they're actually of asteroid impacts. We are not sure. Okay. 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 have an impactor that comes in 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.