[♪ INTRO] We’ve been looking up at the Moon for as  long as there have been people to look.

And because the natural satellite  is tidally locked to the Earth, it only ever shows the same  face to us. And for a long time, we knew absolutely nothing  about its so-called dark side.

But its secrets have gradually been  revealed over the last fifty years, and scientists probing the far side  of the Moon have recently discovered something new and unexpected. There’s a chunk of metal buried  deep beneath an even bigger crater. And they hope that studying  it will help solve some of the outstanding mysteries around  our solar system’s formation.

We first got a good look at the  Moon's far side about 60 years ago, when various US and Soviet  probes returned the first images. These pictures revealed a huge  crater near the South Pole, which is now known as the South Pole-Aitken basin. At about 2,500 kilometers  across and 12 kilometers deep, it’s the biggest and oldest crater  we’ve found in the entire solar system.

And scientists think that  around 4 billion years ago, an asteroid of about 170 kilometers wide  impacted the Moon, creating this crater.   That impact would have been  about 1000 times more explosive than the asteroid that brought  about the end of the dinosaurs. But its incredible size isn’t the only remarkable  thing about the South Pole-Aitken crater. There seems to be something  surprising hidden beneath the surface.

Modern surveys with new  instruments and technologies have revealed the structure of  the Moon in unprecedented detail. For instance, the Lunar Orbiter  Laser Altimeter, or LOLA, onboard the Lunar Reconnaissance Orbiter, has given us precise measurements  of the Moon’s topography. And NASA’s Gravity Recovery  and Interior Laboratory, or GRAIL mission, has measured the  strength of gravity all over the Moon.

Combining these two data sets revealed  an incredibly high gravity reading in the floor of the South Pole-Aitken basin. And since extra gravity must come from extra mass, that means there’s something huge and  very dense hidden there under the surface. This mass concentration seems to  extend several hundred kilometers beneath the lunar surface.

And its density  is closer to that of metal than rock. And there’s a lot of it. Scientists estimate that it’s about five  times the size of the big island of Hawaiʻi, amounting to about three and half  million cubic kilometers of metal.

What’s more, the extra  gravity is enough to pull down the middle of the South Pole Aitken  crater by another 800 meters! Other than the dip, there’s nothing else  on the crater bottom to suggest what it is, but the lunar scientists from Baylor  University in Texas have a couple of ideas. They believe that the signal could  be coming from the asteroid’s core that collided with the Moon to  make the South Pole Aitken crater.

The theory goes that a big enough asteroid  would have been able to differentiate, which is when things like planets, or in this case an asteroid melt and the stuff in them separates from one another   forming layers Just like what happened to Earth, back in the day. So when the asteroid impacted the Moon, it shattered into bits of  rocks, but the metal core would have plowed through  and into the Moon’s mantle, leaving a metallic core of iron and  nickel surrounded by a rocky shell. And because the mantle was already  partly solidified, it just kind of… stuck there, and stayed there to this day.

There is another interpretation  for this extra mass, which doesn’t call for interplanetary ballistics. It’s possible that the extra density comes  from a concentration of heavy metal oxides, which formed near the end of  the mantle solidification. But these kinds of processes  aren’t very well understood, and the fact that the metal is right beneath the biggest known impact  crater in the solar system is a big clue that the two are probably connected.

Ultimately, impact sites like  the South Pole-Aitken basin are important natural laboratories for  understanding planetary formation processes. By studying this crater and the metal underneath, scientists hope to learn more about the  cooling and solidification of the Moon, the differentiation of asteroids, and the timing of collisions  in the early solar system. For example, we still don’t know exactly  when the South Pole-Aitken basin formed.

Getting a handle on that would help us  date other events in the solar system, for which we currently only have relative  ages from things like overlapping craters. So scientists are currently making  a case for sample return missions to the far side of the Moon. They won’t be able to access the massive  chunk of metal buried beneath the surface, but physical samples of the surface  material would tell us a lot.

Like, the state of the asteroid and the  Moon during the South Pole-Aitken impact. Scientists can also pinpoint  which radioactive elements got trapped within the melted rocks to potentially  determine the precise age of the impact. Combining these and other data  will help researchers to figure out the true nature of the extra buried mass, and  decide whether it really is asteroid shrapnel, or something else.

So even as we look further afield to  uncover the secrets of the cosmos, there are still plenty of mysteries still to  be solved in our own planetary neighborhood! Thanks for watching this episode of SciShow Space! If you would like to help support the  channel, check out Patreon.com/SciShowSpace to learn more. [ ♪ OUTRO ♪ ]