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From what we know about Titan, it seems like its atmosphere should have disappeared millions of years ago. So, why hasn’t it?

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[♪ INTRO].

When it comes to atmospheres, the planets  in our solar system seem to have a monopoly. Like, of the 150 or so moons out there, only one has a meaningful  atmosphere: Saturn’s moon Titan.

But, it holds its own. More than one and a half times  the density of Earth’s atmosphere,. Titan’s is so thick with fog that visible-light  telescopes can’t see through to the surface.

Except from what we know for sure about Titan, it seems like that atmosphere should  have disappeared millions of years ago. So, why hasn’t it? Scientists  are still learning more, but new insight might be coming from a  surprising source: slowly baking comets.

In general, atmospheres aren’t easy to come by. Big planets like Jupiter and  Saturn had enough gravity to collect gas from the cloud of material  around them, back when they first formed. But smaller planets had to grow their own.

Like, Earth’s and Venus’s  primitive atmospheres were belched out by gassy volcanoes and  supplemented by molecules from asteroids. But on Titan, the situation is even trickier. See, Titan’s atmosphere is made  almost entirely of nitrogen, with about five percent  methane added into the mix.

And the haziness comes from chemical reactions  between sunlight and the top of the atmosphere, which produce organic molecules  that create that golden fog. But here’s the thing: Because light is constantly  breaking down the methane to make this fog, scientists calculate that it’d be completely  gone within about 20 million years. So something must be replenishing it, or it  probably wouldn’t be around for us to see.

Also, even though Titan is a similar size  to Jupiter’s moons Ganymede and Callisto, it’s unique in having an atmosphere. So any explanation needs to account for  not only why Titan has an atmosphere, but also why the solar  system’s other big moons don’t. One explanation scientists have proposed is that  Titan’s atmosphere is related to how it formed.

Specifically, one way that Titan  could have gotten its nitrogen is via water ice delivered to  the moon during its formation. After all, lots of different compounds  can get trapped in bubbles inside ice and hitch a ride around the solar system. So, some scientists in the 1990s suggested  that Titan may have been bombarded by comets, full of ice loaded with ammonia,  a compound that contains nitrogen.

Over time, the ammonia would  be slowly released from the ice and converted into nitrogen  to fill the atmosphere. As a nice bonus, this can also help  explain why Titan has an atmosphere while Ganymede and Callisto don’t. For one, Jupiter is a lot bigger  than Saturn, so it was hotter, essentially causing the water  on those moons to boil away.

But also, its greater gravity would have  accelerated comets into its moons faster, blasting the ice away from their surface. That said, this hypothesis can’t really  explain the origin of Titan’s methane. To be fair, comets can contain methane, too!

But the comets we’ve studied up  close, including Halley’s comet, seem to contain a different type of hydrogen  than what’s found in Titan’s methane. At first, this was a big  blow for the comet hypothesis. The chemical fingerprints of cometary  ice and Titan’s atmosphere didn’t match, so this probably wasn’t the source of Titan’s air.

But then came the European  Space Agency’s Rosetta mission. When Rosetta arrived at the comet  67P/Churyumov-Gerasimenko in 2014, it brought comets back into the picture. See, data from Rosetta revealed  that the comet was half ice, around a quarter rock, and about  a quarter organic material.

The fundamental building blocks of  organic compounds are carbon and hydrogen, the same elements in methane, but  nitrogen is also a common element. So, in a 2019 study, researchers  presented a new hypothesis, in which this organic material could be  the thing supplying Titan’s atmosphere. They suggest that if Titan  formed from comets like 67P, then it probably has a significant amount  of organic material trapped deep inside it.

If Titan’s core still has heat  leftover from its formation, then some of that material  would kind of be “baked”, heated up so that it releases  compounds like nitrogen and methane. Their models suggest that at  least half of the nitrogen, and all of the methane in Titan’s atmosphere, could come from cooked organics  in the moon’s toasty interior. The main catch?

If it’s  being produced in the core, all that gas has to make its way out somehow. On Earth, that often happens thanks to volcanoes, and some icy bodies may have something  similar, called cryovolcanism. Cryovolcanism works kind of  like terrestrial volcanism, except instead of molten rock, the  “lava” is made up of liquid water.

In both versions, though, eruptions  produce billowing plumes of gas. And these cryovolcanic gases could be  the thing to feed Titan’s atmosphere. Only scientists haven’t found definitive  evidence of active cryovolcanism on Titan yet.

It’s possible that the gases  are being slowly released from underground reservoirs hidden  beneath the moon’s unique lakes. But with so little data about Titan’s  subsurface, it’s hard to say for sure. Ironically, the moon’s thick, hazy atmosphere makes it tricky to answer  questions about how it formed.

NASA scientists hope to  finally address these questions with measurements of the surface and  atmosphere made by the Dragonfly mission, which is due to launch in 2027. Until then, though, trying to understand  the mystery of Titan’s atmosphere could help us understand more  about how all atmospheres are born, and how they evolve over time. And who knows?

The comet  hypothesis was almost ruled out because we just didn’t know enough  about different kinds of comets. So as we learn more about Titan, maybe we’ll find a new way that  those gases could escape, too. Thanks for watching this episode of SciShow Space!

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