This video is supported by the SciShow pin!

You can find a cool new space pin every month at DFTBA.com/SciShow. There’s nothing quite as peaceful as gazing up at the Moon on a quiet, clear evening.

But that moon is actually a remnant of  the most catastrophic thing that has ever happened to our planet. Four and a half billion years ago, another planet the size of Mars, called Theia, slammed into the young Earth with such force that it sent a massive  amount of debris into orbit. That debris came together to form the Moon we know today.

And while this is the leading theory for how the moon formed, there are some big questions that remain. For instance, since the Moon is smaller than Theia was, where did the rest of it go? As it turns out, solving that mystery may explain a bizarre phenomenon that has puzzled scientists for decades. [Intro music] Before we can get to what happened to Theia, we need to talk about a different scientific mystery.

Deep within our planet, nearly 3000 kilometers underground, there are two huge blobs of strangely dense something. They are thousands of kilometers across, hundreds kilometers tall, and sit just above the boundary between Earth's mantle and core. There’s one under the Pacific Ocean and one under Africa.

And at the surface, both of these regions have a weirdly high number of hotspot volcanoes. So geologists think that those deep blobs might have something to do with causing plumes of magma to rise up and erupt. We know about these blobs because we can see them.

Of course, not with our eyes of course, but through seismic waves from earthquakes. See, the speed of a seismic wave depends on the physical properties of the material it’s passing through. So by looking at how fast a wave travels, as well as how it reflects and refracts through the structure of the Earth, we can piece together a picture  of the interior of our planet.

This is called seismic tomography, and it can tell us how dense different areas are and if they’re a liquid or solid. It’s how we know about all the layers of the Earth that you learned about in school, the crust, mantle, and core. We can see those layers even though we’ve never been able to dig deeper than the crust.

Scientists have given those two blobs the super imaginative name Large Low Velocity Provinces, or LLVPs . And that is because the seismic waves that travel through them travel at about half a percent slower

Since these blobs were discovered in the 1980s, everyone has been trying to figure out what they are and where they came from. And there's no shortage of ideas.

The simplest is that they are just hotter than the rest of the mantle around them. But when scientists make models of giant megaplumes of hot material, they end up being thin and narrow, not wide and spread out like the LLVPs. And there’s debate about whether a massive plume like this could be stable for a long time.

So another idea is that they could be clusters of smaller plumes, and maybe the seismic wave picture we’re looking at blurs these together. Or maybe the LLVPs are actually made of something different than  the rest of the mantle. They have really abrupt boundaries at their edges, so that’s good evidence for this.

It could be that we are  seeing remnants of crust from very early in Earth’s history. Like a first draft of Earth’s crust. Or the LLVPs could have shown up more recently.

A leading theory is that they are  graveyards of old oceanic plates that were once ancient seafloors. These would have subducted and sunk down long ago but still haven’t fully mixed  in with the rest of the mantle. But a 2023 study has suggested an explanation for the LLVPs that is literally out of this world.

What if these things in Earth’s mantle were originally part of Theia’s mantle? It’s a weirdly simple explanation, but it makes sense that when the proto-Earth and Theia collided, some of Theia would have stuck. Previous studies have found  the force of the impact was so strong it essentially scrambled the two planets together.

Most of the iron from the cores of the planet would have fused together inside Earth’s core. And the mantles of the two  planets would have mixed, too. Some of this mixture stayed on Earth and some was thrown into orbit.

That means the moon today is made of some Earth’s original mantle, but mostly Theia’s. We know the Earth’s mantle is around 8%  iron oxide and the Moon’s is around 10%, so the researchers concluded that Theia’s mantle would have been somewhere between 13 and 18%. A lot denser than the Earth’s.

Next they created a mathematical model to simulate how the material left on Earth would have mixed after the collision. Their model revealed that the impact  separated Earth’s mantle i nto an upper melted and  lower solid layer. Initially, mantle material from both planets was mixed in the upper layer.

But over time, the denser, more iron rich material from Theia solidified and sank into the solid layer of Earth’s mantle. There, it accumulated into big blobs just above the core. And then, it just… stayed there.

The model showed these blobs could be stable over billions of years. This means the LLVPs we see in seismic waves today could actually be chunks of  a disembodied ancient planet! This theory even explains another weird observation geologists have made.

Trace gas ratios in rocks erupted from hot spot volcanoes are oddly similar to those found on the moon. Which makes perfect sense if plumes of magma are rising up from the LLVPs before erupting on the surface. If you think about it, that kinda makes these eruptions like the dead planet’s zombie hand reaching up from the lower mantle and out of the ground. we might still be feeling the effects of an event that happened more than four billion years ago So this study gives plausibility to the Theia theory explaining LLVPs.

It’s important to remember that it doesn’t disprove any of the other ideas, so the debate will likely continue. But it does have the appealing feature of killing two long-standing mysteries with one very big Mars-sized stone. Thanks for watching this SciShow video!

To support the channel and get some cool merch, you can buy February’s pin of the month at DFTBA.com/SciShow. It’s a representation of this video’s main character: The zombie remains of Theia. Next month, we’ll release an entirely new limited edition pin to celebrate even more of the awesome things in space.

Unlike the real Theia, potentially hanging around for billions of years this pin will only hang around for one month at DFTBA.com/SciShow. So stop by the online store to get yourself one. thanks as always for your support [ OUTRO ]