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New research is changing our ideas about the history of Mars's moons and we might have found the most active region of space.

Host: Hank Green

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Mars Moons:

[ ♪ Intro ].

Sometimes, it can be easy to forget that Mars has moons. Unless you are like me and you think about Mars all the time.

But it totally does. They are Phobos and Deimos, and they’re a little mysterious. They’re so small that gravity can’t pull them into spheres, so they’re kind of lumpy and potato-shaped.

And scientists have spent decades trying to unravel where these tiny space potatoes came from. Now, a paper, published last week in the journal Science Advances, has gotten us one step closer to the full picture. Its authors say that the popular origin story of a titanic, apocalyptic collision a few billion years ago isn’t quite accurate.

Instead, there might’ve been just a mini-apocalypse. The surfaces of Phobos and Deimos look a lot like those of many asteroids. And since the asteroid belt sits right outside Mars’s orbit, scientists thought for a long time that the Martian moons were asteroids that had been caught by the planet’s gravity.

But being gravitationally captured would’ve put huge stresses on both bodies. Detailed studies of Phobos have shown that it, at least, would’ve probably broken apart under that much stress. Plus, both moons also orbit more or less in line with Mars’s equator, which would be a big coincidence if they came from random directions like captured asteroids.

It would make perfect sense if they formed around Mars itself, though. That led to today’s most popular idea: that, billions of years ago, Mars collided with a protoplanet -- a huge, rocky precursor to the stuff in today’s solar system. That collision launched lots of rock into space, and some of it eventually became Mars’s moons.

That’s pretty much how our moon formed, so there’s some precedent for this. There’s also lots of evidence on Mars itself for a gigantic collision. Like, almost all of the northern hemisphere is at a lower elevation than the southern one, which could be explained by a huge impact.

But this new paper says there’s a problem with these ideas, too. Based on earlier calculations and simulations, scientists had settled on an impactor that was about a third Mars’s size and about two percent its mass. But these authors simulated possible collisions in more detail than ever before.

And their work showed that such a big impactor would’ve launched way too much rock into space. It would’ve formed long-lasting moons that look a lot different from Phobos and Deimos. They found that, to get the right conditions for the current moons, you’d need to hit.

Mars with something between ten and a hundred times lighter. So far, this is the best model we have of how these moons formed, but the debate isn’t over yet. In 2029, it and the other impact models will be tested once and for all when the Japanese.

MMX mission brings a piece of Phobos to Earth. A huge impact would’ve dried out the rocks escaping from Mars, so the sample would still be dry today if that’s how they formed. But if the moons started as asteroids, they wouldn’t have been baked as effectively, so the sample would be a lot wetter.

We’re just going to have to wait and see. Meanwhile, twelve and a half billion years ago, there was no Mars what a sad universe that was. But according to a paper published in this week’s issue of Nature, there was a small group of galaxies called SPT2349-56.

I think that’s what they called it back then at least. Well that might be the most active region of space we’ve ever seen. This group, which we’ll call 2349 for short, was discovered using the South Pole Telescope.

Shockingly, this is a telescope near the South Pole, where the atmosphere is clearer than almost anywhere on Earth. It’s thanks to those near-perfect conditions that the team found 2349 — although they did examine it with a bunch of other telescopes later. Based on how light changes as it travels through space, the team estimates that we’re seeing this object as it was just 1.4 billion years after the Big Bang, when the universe was about a tenth its current age.

That’s only a few hundred million years after the very first galaxies formed, so there weren’t the kinds of gigantic clusters of hundreds or thousands of galaxies we see today. Instead, 2349 is officially what’s called a protocluster: an early seed of today’s much larger groups. Objects like this tell astronomers that galaxies clumped together from the very beginning, and that there wasn’t a big gap between the first galaxies and the first groups of them. 2349 is pretty dense for a protocluster — only about 425,000 light-years across — but it’s really busy in there.

It contains at least fourteen galaxies, and they’re all forming new stars somewhere between fifty and a thousand times faster than the modern Milky Way. Combining all these stats with models for protocluster evolution, the paper’s authors think 2349 eventually formed one of the largest clusters in the modern universe. Of course, we can’t really know what it looks like today, because the idea of “today” doesn’t really work in astronomy.

The light from its earliest stages is just reaching us, so we’ll have to wait for like billions of years for the light from later stages to get here. For now, all we can say is that it’s one of the densest and brightest protoclusters we’ve ever seen. And there are some signs that it’s even bigger, too.

There are five additional galaxies that might also be part of this group, although the paper’s authors couldn’t quite tell. But if they are, it would make this the most active region of space that scientists have ever discovered. Still, the universe is big, and there are a lot of places to look.

So we’ll keep you posted if anyone finds something that breaks the record. Thanks for watching this episode of SciShow Space! If you would like to stay in the loop about the latest news from all over the universe, you can go to to subscribe.

And heck, why wouldn’t you? [ ♪ Outro ].