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If you take a look at all the impact craters in our solar system, the vast majority are nice, neat circles. But why? Meteorites and asteroids strike planets and moons at all sorts of angles. Where are all the elliptical craters?

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As a SciShow viewer, you can keep building your STEM skills with a 30-day free trial and 20% off an annual premium subscription at The Solar System is a busy place, full of speedy space rocks that leave quite the mark when they smack into a planet or moon.

I’m talking about craters. But wait…why are all of these near perfect circles? Meteorites and asteroids fall from the sky at all sorts of angles.

Shouldn’t that make craters more…oblong? It turns out, they occasionally are. And finding the odd elliptical crater on a rocky body can teach us some important things about our solar system’s past. [♪ INTRO] It’s pretty easy to make your own elliptical crater by throwing a rock into some mud, but the stuff falling from space is traveling way faster than you could ever hope to throw anything, over 10 kilometers a second.

And when a super speedy meteor or asteroid strikes the ground, the collision releases so much energy it’s basically like a bomb going off. It creates a shockwave that clears the material from the crater. In other words, it isn’t the space rock itself that’s pushing the ground out of the way.

And since shockwaves radiate out in all directions from a single point, you get a circular shape, even if the rock isn’t falling straight down. But if you look closely at the Moon, or Venus, or Mars, you’ll see that around 5% of craters are somewhat elliptical. How is that possible?

Well, there’s debate over the specific mechanics, but if the rock comes in at a very shallow angle, different parts of the rock can hit different points on the surface, creating an elongated shockwave. According to computer simulations, the angle can vary, depending on the speed, size and make-up of the space rock, as well as what its target is made out of. But roughly, we’re looking at around a maximum of 15 degrees.

And because we know elliptical craters can only form from shallow impacts, they can give us hints about a planet’s history. For example, back in 2018, one team of researchers used elliptical craters on Mars to infer how much the planet’s tilt has changed over the past 3.5 billion years. Just like Earth, Mars’s spin axis isn’t perpendicular to its path around the Sun.

And the exact angle of its tilt changes over time. Now, the majority of space impactors come from the fairly flat asteroid belt, so if a planet’s tilt never changes, the number of shallow impacts for a given latitude should stay the same. But if the tilt changes, you’d see either more or fewer elliptical craters because the ground’s now at a different angle to where asteroids come from.

So the team ran some simulations to see how many skewed craters ended up at different spots for different tilt angles, and then compared them to what Mars’s surface actually looks like. And they concluded that over the past 3.5 billion years, Mars has spent most of its time wobbling between a 10 degree to 30 degree tilt. Which is very different from Earth, whose tilt only varies by a couple of degrees, likely because the tug of our large moon stabilizes any wobbling.

So there’s still a lot for astronomers to learn, both about and from craters and their shapes. It’s the kind of research that leaves a bigger impact than you might think. Thanks for watching this SciShow video, and thanks to Brilliant for supporting it!

The beautiful geometry of asteroid craters is exactly the kind of thing that Brilliant courses explain. Literally. They have a course called “Beautiful Geometry.” That course helps you understand the concave geometry of something like a crater, along with so much more.

It’s an engaging way to learn about geometry through origami folding and puzzles. That’s the kind of stuff you’ll find in Brilliant’s courses. They don’t just teach you what you want to know.

They do it the way you want to learn. So you can head to or click the link in the description down below to get started with a free 30-day trial and 20% off an annual premium Brilliant subscription. From craters to geometric principles, there’s beauty everywhere. [♪ OUTRO]