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Scientists have known the outside of the sun spins slower than the inside for a while, but they didn't know why until recently.
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Hank: For us Earthlings, there are few things as constant as the Sun. But the Sun is actually spinning... and weirdly, the layer of gas at its surface spins slower than the gas beneath it.

We’ve known about this for a while. Like, almost thirty years. But scientists have never had a great explanation for it. But now, a team of astronomers thinks they have finally cracked the case. They think that outside layer could be moving more leisurely because the Sun is just so dang bright. The light acts as a brake!

This all started back in the 1980s, when scientists were studying the Sun with something called helioseismology. It’s kind of like how seismologists monitor the waves moving through the Earth to learn more about earthquakes. With helioseismology, physicists can use vibrations to understand what’s going on in the Sun. Using these techniques, they could see that the Sun’s equator rotates faster than the poles -- something astronomers had known for centuries.

They also saw that the solar surface rotates slower than the gas underneath it. But here’s the thing: helioseismology is really only good at giving astronomers a really big picture -- what’s happening in chunks of the Sun like 3,000 kilometers across or deep. And the Sun’s surface is constantly moving and changing for loads of different reasons. So, scientists knew about the different-layer, different-speed mystery, but didn’t have a way to explain it.

Now, three decades later, astronomers have a new tool: NASA’s Solar Dynamics Observatory satellite. This satellite lets them watch how the gases at the very edge of the Sun’s surface move, with a resolution of 10 kilometers.

Using three and a half years of data, the team tracked changes in the Sun’s outer layer. And in a paper published in Physical Review Letters, they show that most of the slowdown happens in just the outer 70 kilometers of the solar surface, not the entire thing.

This very outer edge is where light escapes into space, and the researchers realized that that sunlight itself might be what’s slowing down the Sun’s spin. That’s because photons of light have momentum, which they takes from whatever’s emitting the light. And if whatever’s emitting the light is moving sideways, like the Sun’s surface does, light steals some of that sideways momentum, too.

Each photon steals an unimaginably small amount of momentum, but the Sun is really bright, and it’s given off a lot of photons over its lifetime. The researchers did the math, and found that sunlight slowly siphoning off momentum can just about completely explain the difference in speeds between the Sun’s layers. They still aren’t positive that this is the major cause of the slowdown, but it seems pretty likely.

And do not worry, the Sun won’t stop spinning any time soon. They checked, and a total slowdown would take far longer than the five billion years our stellar neighbor has left.

Next, let’s talk about NWA. No, not the hip-hop NWA. Not the airline -- that doesn’t exist anymore. The rock version... NWA 7635. NWA stands, here, for Northwest Africa, because that’s where the rock was found. But it’s no ordinary rock.

It’s from Mars. And a group of scientists think this meteorite could be proof that at least one Martian volcano has been continuously spewing magma for more than two billion years. That is unheard of on the Earth’s surface. Here, we have plate tectonics shifting everything around, so volcanoes would never really last for more than a few million years.

NWA 7635 is one of about a hundred Mars rocks scientists have found on Earth, expelled from the Martian surface by things crashing into it. On their way here, those rocks get exposed to high-energy cosmic rays that change some of their atoms into weird isotopes or elements that weren’t originally there. And the longer they’re floating out in space, the more weird atoms are in the rocks.

So based on what they’re made of and how long they were in space, scientists can tell roughly when they got kicked off Mars. NWA 7635 is really similar to 10 other volcanic rocks called shergottites, which are Martian meteorites made up of certain elements and minerals.

This specific group of shergottites got launched into space about a million years ago and formed about three to six hundred million years ago. But when these researchers actually checked NWA 7635's age, they found that it’s not a few hundred million years old… it’s nearly two and a half billion years old.

Because these rocks are all made of the same stuff, that means that there must have been a volcano or volcanic system making new rocks in the same spot for more than 2 billion years. In other words, it was erupting for over forty percent of Mars’s history! And that is a persistent volcano.

Planetary scientists have argued for years about whether Mars has or ever had plate tectonics... and they’ll probably keep arguing. But NWA 7635 shows that even if the surface did shift around, it probably never worked much like Earth, where the planet is significantly rearranged every couple hundred million years.

And it just goes to show that we don’t always need to jet all the way over to the Red Planet to learn about its history. The rocks just come to us! Though we should totally go to the rocks, as soon as possible.

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