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Few things in astronomy are as strange or captivating as black holes. And part of their appeal might be that they're hard to figure out.

Like, even after hundreds of years, we still aren't sure what sizes they come in. We know about stellar-mass black holes, which form from collapsing stars and have masses up to a few dozen times our Sun's. And we know about supermassive black holes, which can anchor galaxies with masses millions or even billions of times greater than the Sun.

But then… there are all the sizes in-between. It might seem obvious that they should exist, but historically, these intermediate-mass black holes have only been seen in hints and glimpses, and they've long been one of astronomy's great mysteries. Although... maybe not anymore.

In a pair of papers published last week, astronomers described the best evidence yet for an intermediate-mass black hole. And they didn't just see it as it is now: They watched it form. The team calculated that this black hole formed from the collision of two smaller ones.

And it all went down some seven billion years ago, at a place that's currently more than 17 billion light-years from Earth. To study this, the researchers used LIGO and VIRGO, a pair of gravitational wave observatories. Gravitational waves are ripples in the fabric of space itself, and an observatory like LIGO detects them using a pair of lasers arranged at right angles.

As a wave washes over the detector, space literally gets bigger in one dimension, so one laser beam ends up traveling farther than the other.i And things like how much farther it traveled can tell us what the wave is like. Since LIGO's first successful observation in 2015, this technique has revolutionized how we study the collision of massive objects like black holes. And it came in handy with these new ones, too.

In this new discovery, one black hole was around 85 solar masses and the other was about 66. And when they collided, they left behind a black hole coming in at around 142 solar masses. If you're good at math, you might notice that 85 plus 66 does not equal 142.

For the record, the missing mass went into gravitational waves as energy. Either way, that mass puts this thing squarely in the lower end of the range for intermediate black holes. Which is exciting news!

This might not tell us anything about the frequency of black holes a thousand or ten thousand times the Sun's mass. But it is proof that black holes can exist outside the two categories we've seen so far. And the collision raised a new mystery, too.

Like we mentioned, the larger of the original black holes had a mass around 85 times bigger than the Sun. But our best understanding of how stellar-mass black holes form suggests that it's not possible to directly make one larger than about 65 solar masses. Above that limit, stars collapse so violently that they blow apart too fast for a black hole to form.

So this supersized black hole opens up two possibilities. One option is that we've got our physics wrong about what limits the size of a forming black hole. Or—and this might be equally compelling—this 85-solar-mass black hole had already been through at least one prior collision before blundering its way into another.

Either way, astronomers are seeing something unexpected. So, you know… we might have gotten some clarity on one mystery, but there's always another. In other news, the Moon seems to be a little rusty.

Which, on one hand, sure—the Moon is billions of years old, so maybe we shouldn't be surprised that it's a little run down. But also… rust on the Moon shouldn't be a thing, considering that you need water and air to get rust, and the Moon is pretty dry and airless. This news came from a paper published last week in the journal Science Advances.

And according to the authors, the way that rust gets there might be a bit of a wild ride. In the paper, a team of planetary scientists analyzed data collected from NASA's Moon. Mineralogy Mapper, which orbited the Moon in 2008 and 2009 as part of India's Chandrayaan-1 mission.

The mapper… well, it made maps, of the water ice hiding deep in craters near the Moon's poles. But in this study, the research team noticed something else strange about the polar regions: there seems to be hematite mixed into the soil. Hematite is a type of iron oxide, which is the scientific term for rust, and it usually forms in the presence of water and oxygen.

Mars is covered in the stuff, which was actually one of the early signs that the Red Planet was once awash in oceans. But seeing it on the Moon is stranger. I mean, we do know that the Moon has a little water, in the form of all that polar ice.

There's also the occasional water molecule elsewhere on the surface. But the Moon doesn't really have oxygen. What's worse, the lunar surface is blasted by hydrogen from the Sun's solar wind—the rush of particles constantly flying off its surface.

And hydrogen basically blocks the chemical reaction that leads to rust. So, what's going on here? Well, the key actually seems to be Earth's magnetic field.

The crushing force of the solar wind squishes the magnetic field and stretches it out behind. Earth, kind of like a tail. So when the Moon is also behind the Earth, the magnetic field is kind of pointing its way.

Then, as bits of oxygen escape Earth's upper atmosphere, they can get funneled by this field right towards the Moon. At the same time, our magnetic field can also help shield the lunar surface from getting hit by rust-blocking hydrogen atoms. So if our oxygen can wind up in areas that have both water molecules and iron-containing rocks, the chemical reaction creating rust can happen, and hematite can form!

It's probably an incredibly slow process, but, hey, the Moon has had billions of years to make it happen. Still, I'd keep an eye on it. We've already got a red planet; we probably don't also need a red moon!

Thanks for watching SciShow Space, which is produced by Complexly. If you're a US citizen and want to keep imagining the world complexly with us and also have a voice in your government, check out How To Vote In Every State. This year, we also put together a guide for first-time voters with help from our friends at MediaWise, to help you navigate everything from your news diet, to political ads, to money in politics.

Check it out at youtube.com/howtovoteineverystate. {♫Outro♫}.