[♪ INTRO].

Ever since the 1960s, more than two dozen spacecraft have explored Mars. They’ve explored everything from its atmosphere to its surface to its miles-deep canyons… but none had ever explored its interior, until recently.

Last week, three papers published in the journal Science revealed the first-ever map of the inside of the Red Planet. It’s also the first interior map of any other planet besides Earth. And although Mars has the same basic makeup as Earth, this map shows that the inner layers of our neighboring rocky planet hold some remarkable differences.

Now, well before these latest studies, we had a general idea of what the inside of Mars was like, because Mars formed a lot like Earth, out of a molten sphere that eventually hardened. And since it was once entirely molten, the heaviest stuff would have settled at the bottom and the lightest stuff would have floated to the top, forming layers like Earth. As for what those layers looked like, though, that was anyone’s guess.

Scientists came up with some hypotheses based on computer models and comparisons between Mars and Earth. But what they actually needed was data. The problem was, until recently, getting direct data for a map of Mars’s interior seemed like a longshot.

See, one of the most effective ways of probing the inside of a celestial body is by detecting seismic waves. Here on Earth, we generally know of seismic waves as the waves produced by earthquakes. They travel through the planet, and as they reach the boundary between one material and another, they can do one of two things:.

Seismic waves can be reflected, meaning they bounce back like light bouncing off a mirror. Or they can be refracted, meaning they pass through the boundary, but they change speed and direction. Depending on the materials they’re moving through, they reflect and refract in specific ways.

So by measuring how waves move through a planet, we can identify the boundaries between layers and say something about each layer’s composition. On Earth, this is the main way we’ve been able to map the interior. But it’s only possible because Earth has tons of seismic activity.

We’ve got all kinds of tectonic plates sliding around, crashing into each other, and generating seismic waves, even though many of them are too weak or too deep to actually feel like an earthquake. Meanwhile, up until a few years ago, planetary scientists thought. Mars was pretty quiet in terms of seismic activity.

It doesn’t seem to have any plate tectonics to speak of, which is the main driver of seismic activity on Earth. So, making a map based on seismic data seemed like it would be tricky. But this view of Mars changed a few years ago, after the InSight lander touched down on the Red Planet in 2018 and began collecting data with various instruments, including a seismometer.

In under a year, it had already recorded 174 seismic events, or marsquakes. They were gentle quakes, which suggested that they came from fairly tame events, such as rocks fracturing as the planet slowly cools. Either way, though, Mars was definitely seismically active… so, building an interior map using seismic data was not such a far-fetched idea.

The main challenge now was that Mars had a grand total of one seismic observatory: the InSight lander. So, in the absence of a bunch of global data, scientists have had to get really creative. The authors of last week’s papers used multiple methods of data processing and then compared the results to see how well they agreed.

They also worked with lots of specialists, from geochemists, to mineralogists, to sedimentologists, you name it, to wring every last drop of insight from their limited raw data. And their hard work paid off! We now have a 3D map of Mars’s insides.

And the results come with some surprises. Let’s start with the outer layer: the crust. The authors found that Mars’s crust is a few dozen kilometers thick, and it’s much less dense than they expected based on the composition of the surface.

That suggests that the deeper rock may have been physically and chemically altered by volcanic activity, which could make the ground more porous and create lighter minerals. One layer below that, there is the mantle. Like on Earth, Mars’s mantle goes from being rigid at the top to more gooey, or ductile, deeper down.

But the authors found that, unlike on Earth, the rigid part of Mars’s mantle, known as the lithosphere, goes extremely deep, some 400 to 600 kilometers below the surface. That makes it two or three times deeper than Earth’s lithosphere, even though Mars is a much smaller planet. And since so much of the mantle is rigid, it doesn’t convect well.

There is some convection, but it’s super slow, which explains why Mars has no plate tectonics. As for the Martian core, researchers found that it is liquid, as predicted, but it’s about 200 kilometers bigger than expected. And considering its size, it’s not particularly massive, suggesting that there’s a lot of light stuff in there in addition to heavy metals.

So Mars likely has a core with plenty of iron, like Earth’s, but also a lot of sulfur, some oxygen, and even some hydrogen. And that could explain why Mars’s magnetic field is so weak. Because a rotating, convecting core can set up a magnetic field if it's made of a conductive material, like iron.

But if that iron is mixed with something that doesn’t convect, like sulfur, a large magnetic field cannot form. For now, there’s still a lot of work to do confirming and expanding on these findings. But, thanks to this image, which is more detailed than any picture of Mars’s interior we’ve ever had, scientists can model and explore the Red Planet in ways that were never possible before.

Thanks for watching this episode of SciShow Space! If you would like to help support the channel, we have a Patreon, which is a place where you can support things like YouTube channels. You can check out Patreon.com/SciShowSpace if you want to learn more. [♪ OUTRO].