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Getting our hands on a few moon rocks radically changed our understanding of the solar system!

How to Date a Dead Thing:

Host: Reid Reimers
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When NASA’s New Horizons probe flew by Pluto, the star of the show was an enormous icy plain in the shape of a heart.

Six months later, we learned that the heart was no more than ten million years old, which is practically yesterday compared to Pluto’s four and a half billion year history. What’s even more amazing is that we only know this because astronauts brought a bunch of rocks home from the Moon almost fifty years ago.

In fact, any time we’ve figured out the age of a surface in the Solar System, it’s thanks to those rocks. Today, we know the Solar System is about 4.58 billion years old. We’d know that even if we’d never gone to the Moon, because we’ve calculated that age by dating meteorites that have landed on Earth.

What we wouldn’t know is how the Solar System developed since then. Just like on Earth, different parts of the Moon’s surface formed at different times. Some areas probably date all the way back to the early days of the Solar System, while others formed in the eons since.

If you want to piece together a rough history of the Moon, all you really need is a telescope and a method called relative dating, which despite the name has nothing to do with incest. Basically, you figure out how old things are by comparing them to each other. For instance, when one lava flow is covering another, that tells you that the top lava flow is younger.

And both lava flows must be younger than whatever’s under them. But if you want to know how many years ago those events happened, you need to switch to an absolute dating system. On Earth, you could just pick up a rock from the area and take it back to the lab to calculate its age.

On another planet, though, that’s way too much work to be practical. So scientists came up with another way to date parts of the Solar System, based on the fact that surfaces are generally pretty smooth when they form and only get all pockmarked after being exposed to impacts from space over millions or billions of years. That means the more craters an area has, the older it must be -- but how old?

That’s where the Moon rocks come in. Six crewed Apollo missions and three robotic Soviet missions landed on the surface of the Moon and brought back a bunch of rocks. These missions sampled all kinds of different regions on the Moon, each with their own number of craters.

When rocks from each location were brought back to Earth, researchers dated them and worked out a relationship between the number of craters in an area and its age. Using that relationship, planetary scientists can look at the density of craters in other regions to estimate their age. It really is an estimate, though, because there are a few factors that make the calculations less exact.

For one thing, we know that in general, the rate of impacts has slowed over time, as the debris in the Solar System gets used up. But we don’t know how exactly the rate has changed over time. A bigger problem crops up when we try to use this technique on other worlds.

When scientists used it to estimate the age of Pluto’s heart, they had to make assumptions about how the rate of impacts on Pluto compared to the rate on the Moon. Another issue is that crater counting doesn’t seem to work for surfaces that are more than about four billion years old. At that point, the surface reaches saturation, meaning that it has so many craters that each new crater just obliterates an old one.

Crater dating was just one of the breakthroughs that came from the Moon rocks that were brought back to Earth. They also gave us clues about where the Moon came from. Before the Apollo landings, there were two popular ideas about how we got the Moon.

Some astronomers thought that the Moon came from the same protoplanetary disk that formed Earth, which would make it like the large moons of Jupiter and Saturn. Others thought that it was captured later on, like Neptune’s moon Triton or Pluto’s moon Charon. Studying the rocks the astronauts brought back basically ruled out both possibilities.

Researchers found that chemical signatures in those samples matched rocks on Earth almost exactly, which would be super unlikely if the Moon was captured from somewhere else in the Solar System. And the Moon rocks also contained very little water compared to those on Earth, which doesn’t make sense if they formed in the same region of the protoplanetary disk. These problems eventually led to the development of what’s now the most widely-accepted model for where the Moon came from: it formed after a giant collision between Earth and a Mars-sized object.

So bringing back rocks from the surface of the Moon was more than just incredibly cool. It changed how we study the entire Solar System, and taught us a lot about how the Moon formed in the first place. Thanks for watching this episode of SciShow Space, and thanks especially to our patrons on Patreon who help make this show possible.

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