What’s so cool about Triton? How did it end up in orbit around Neptune, and what can we learn from it? Could we send a mission there one day? I discuss these questions and more in today’s Ask a Spaceman!
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Episode Transcription (Auto Generated)
Sometimes astronomy is less like a science and more like a a criminal forensics investigation. You have a crime scene, blood signs of a fight, an open window, and of course a dead body still warm. Something happened, but we can't just rerun the past. There are no hidden cameras, no eyewitnesses, no wire tapped phones, just the scene itself full of clues. and it is from those clues that we have to put together a plausible interpretation of the past and what series of events led to that specific circumstance, namely a dead body on the ground. Of course, there might be multiple plausible scenarios that we can concoct, especially if the evidence is meager or open into interpretation. So it's our job to lay out all the lines of evidence, offer plausible scenarios and well, I guess that's where the analogy ends, because in a real police investigation, the goal is to find the killer and bring that person to justice, whereas in astronomy, we can just keep arguing about it in conferences for the rest of our lives.
And the scene of our crime today is found in the outer solar system, specifically the region around Neptune, which sits roughly 30 times further away from the sun than the earth does at that distance. The sun is still recognizable as the sun and not just a slightly brighter than average star, but even a hot summer day only reaches a balmy negative 391 degrees Fahrenheit. For you metric folks, that's negative 235 degrees Celsius, but really at that temperature does the conversion even matter? It's just, it's just really cold. The outer solar system is a lonely place, aside from Neptune and its moons. There's just, well, well not much of anything beyond the orbit of Neptunes. It's the Coer belt, which is a ring of icy minor planets and rock's, not even big enough to be called minor planets, let alone little league planets, which should totally be a classification. But I digress. Neptune is the out and out boss of the outer solar system, even though as giant planets go, it's not that big.
I mean, Jupiter is over 18 times more massive than Neptune. Jupiter is so far away that all that mass of Jupiter is essentially meaningless for comparison. The distance between us and Jupiter Earth and Jupiter is four au four times the distance from the earth to the sun. Astronomical units four au, but the distance between Jupiter and Neptune is 25 au. So the distance between Jupiter and Neptune is over six times greater than the earth Jupiter distance. There's nothing out here except for Neptune. Neptune really is a big fish in a small pond, and it's calling all the shots with this gravity. And speaking of fish, there's something fishy about Neptune in its moons. Neptune has 14 known moons and probably many more, but it Neptune is really far away, and so the moons are hard to find. All those moons are very small, so small that they're not even round, they don't have enough self gravity to pull themselves into an even roughly spherical shape.
Consider, for example, Proteus, one of the largest moons of Neptune, and it's an amazing 400 kilometers across these moons come into general flavors, regular and irregular. The regular ones have nice tight orbits close to the planet and the irregular ones well, well they're irregular. They're generally further away from Neptune with all sorts of crazy orbits. But all in all, they're just a bunch of random space junk and really aren't all that remarkable except for one, except for the object that seems to have been the victim of a crime, an object that still bears the scars of some terrible act done to it long in the past, so long ago that there is a little trace of that event left behind today. An object that has completely unlike any other moon of Neptune, completely unlike anything within the entire volume of Neptune's orbit and truly completely unlike any other moon in the entire solar system, Triton Triton was discovered by English merchant and amateur astronomer.
And when we're talking about old timey amateur astronomers, we're talking people who are somewhat, moderately, extremely wealthy and not very busy working a day job. In this case, our person of interest is William Lasell who spotted in a mere 17 days after the discovery of Neptune itself. but it wasn't until August 25th, 1989 that we got our first closeup. Look at the world of Triton when the Voyager two Pro flew by on the last leg of its historic grand tour of the outer solar system, the Voyager mission spent most of its time photographing and taking measurements of Neptune, but it did get some scans of Triton and it was this flyby, you know, it was not a stop and chat for a while kind of gig. So we only have a brief survey of the surface and environment of Triton comprising something like 40% of its surface. And that's it. It's been 34 years since that brief encounter and that's it.
But still those spare measurements and the quick scans of the surface features revealed that something very wrong happened to Triton and that it's living in a place where it shouldn't be. Let's collect the evidence. First off, Tritton is big and I'm, I mean, it's not super big by planetary or satellite considerations. It's the seventh largest moon in the solar system, which is like getting seventh place in the Olympics. You're not aiming for the podium, just a personal best and a chance to be a part of the festivities and hey, but it is bigger than Pluto, which is pretty cool. But among the moons of Neptune specifically, it's huge. All the other moons of Neptune add up to less than 0.5. The mass of Triton combined. That means Triton outweighs all the other moons of Neptune combined by a factor of 20 among the outer planets. It's the largest moon relative to the planet that it orbits.
And among the whole solar system, it takes second place. The only one that beats it is the earth moon system, which takes gold here. Triton is big, interesting piece of evidence, number one for all you crime. Scene s slce out there, Triton isn't just big, it's weirdly big, but that's not all. Triton isn't a regular moon, and I mean that in a literal sense, it's, it's an irregular moon. It does not have a sane orbit. For one, it orbits backwards and the technical word here in astronomy is retrograde, meaning that it orbits in the opposite direction of Neptune spin. So Neptune spins one way, Triton orbits the other way, and it's super tilted. It's orbited as it has an angle of of 67 degrees that's an orbit closer to perpendicular than it is to parallel now. Now we've encountered irregular moons before all the giant planets have them including Neptune, but Triton has got this something extra about it. The next largest irregular moon of any planet is Phoebe around Saturn, and Phoebe weighs just 0.03% of the massive Triton.
Triton is big, an interesting piece of evidence. Number two, it has a really janky orbit. There is no other large moon in the solar system that has an irregular orbit like this. Speaking of that orbit, there is something oddly regular about it. It's almost perfectly circular. Now, in astronomy, we measure the elasticity or circularity of an orbit through a number called the eccentricity. If you have an eccentricity of zero, it means the orbit is a perfect circle, and then any number bigger than that means the eccentricity stretches out into a bigger and bigger ellipse. Essentially, it's a measure of how not a circle your orbit is. Tritons eccentricity is 0.0 0 0 0 1 6. For comparison, our own moon's eccentricity with its orbit around the earth is 0.0549, which is a thousand times more elliptical.
Triton has a nearly perfect circle orbit. It is as perfect as you can get in nature. It deviates from a circle to like one part in a million. And the problem with that super circular orbit is that there hasn't been enough time in the history of the solar system to naturally make it so circular. See, Triton is tiddly locked. One face of Triton always faces detune, just like the moon is tiddly locked to the earth. We always see the same face of the moon in the sky no matter what. If a moon starts out in an elliptical orbit where sometimes it's closer to planet, sometimes it's further away and it's locked, then the fact that it is in an ellipse will stretch that planet. There'll be tidal interactions on that moon that will stretch it out. And these tidal interactions circularize the orbit of the moon, they, they distort the moon so much, and then that distortion influences in the orbit and these feedback off of each other.
And eventually you get a circle out of it. But that takes eons and the amount of time it would take for Triton to reach this absurd level of circularity, this near perfectness of its circle is is billion, like 10, 20 billion years And. we know the solar system is not that old. So something must have happened to Triton early on to make it circular. Interesting piece of evidence. Number three, something dramatic happened to Triton early in its life. We're already three pieces of weird evidence in, and we've only talked about the basics, its size and its orbit. We haven't even taken a look at the thing itself. We've just examined the circumstances of the body. Now we need to examine the body itself. So let's take a look at the thing. And what we see is a cantaloupe. A cantaloupe that hit every branch of the ugly tree on its way down, a cantaloupe that's seen better days, a cantaloupe that that honestly doesn't look very tasty.
In fact, the surface of tritton looks so much like a cantaloupe that astronomers called large regions of the surface of tritton cantaloupe terrain. Next time you see a cantaloupe and you observe its distinct network of bumps and wriggles and ridges and tiny pockmarks, imagine stretching that fruit to over a thousand kilometers across. And you get the basic idea of cantaloupe terrain. But this cantaloupe terrain doesn't cover the whole surface. Just about a third of it, well, a third of what we can see, which is in total about 40%. So who knows how, how big this region actually is. The cantaloupe terrain is concentrated near the equator in the mid-latitudes on the western hemisphere, on the eastern hemisphere near the equator in mid-latitudes. So just traveling east, once you get out of the cantaloupe terrain and you're heading east, the terrain there is totally different. Instead of cantaloupe terrain with bumps and wriggles and ridges, we see broad flat featureless planes and really, really minor differences in height, like less than a kilometer difference between the the lowest low and the highest high broad flat nothingness.
And then here and there scattered are a few dots of what look like volcano, calders of the broad flat featureless part and the cantaloupe part, sports several colors. If by several you mean a couple, and by colors you mean variations of kind of red and kind of green. The red comes from thos, which are organic molecules that have been baked by ultraviolet radiation from the sun. We see them everywhere in the outer solar system because you need cold temperatures, UV radiation, and lots of time to generate thos a a term coined by Carl Sagan. By the way, if you see thos, that indicates that the surface there is probably slightly older because it takes time for the thos to accumulate. We see this red thos mostly in the cantaloupe terrain in the west and in the greenish colors come from nitrogen and water ices and is presumably younger because it had less time to collect the solans, so it's probably newer.
So the eastern side with the broad features, the flat plains, the volcanic cals that looks newer than the western side with the cantaloupe terrain. And then the south of the moon is totally different. Again, it's not cantaloupe at all. It's also not smooth at all, and in fact it bears no resemblance to any fruit whatsoever, so it doesn't get a whimsical nickname. Instead, it just looks like planet, generic planet, just another dead airless, frozen rock out wandering the lonely depths of the outer solar system. In fact, it appears to be a giant polar ice cap covering a good fraction of the south pole of the moon, which isn't too surprising given the fact that it's like 35 degrees above absolute zero out there. And so this is actually an ice cap made of frozen nitrogen and carbon dioxide and some water. But what's really fascinating about the south polar cap is that a, it's large, but what's really fascinating about the south polar cap is that it's dotted with dozens and dozens of cryo volcanoes.
Now it's time for a brief break to mention that this show is sponsored by BetterHelp. We've all been in those kinds of situations when we know what we're supposed to do, but our brain just gets in the way. And, we end up doing the thing we're not supposed to do. Like there is that slice of cheese and you've already had a bunch of cheese and you've had enough and you know you shouldn't have it. And wait a minute, it's already in my mouth and I'm already chewing this. How is this possible? Sometimes our brains get in our own way, and one way to help navigate that is not to eat more cheese, but to talk to a therapist. I have benefited from therapy for many years and I strongly recommend it. If you are interested in giving it a shot, I want you to try BetterHelp. It's entirely online, convenient, flexible, and suited to your schedule. How good is that? Make your brain your friend and make she your friend too. But make your brain your friend with BetterHelp Visit BetterHelp dot com slash Spaceman today to get 10% off your first month.
That's BetterHelp, H e l p.com/ Spaceman. Now, if you've listened to the show enough, you know that cryo volcanoes is one of my favorite words. And if there is some way to tie these things to magnetic fields, then that would just convince me that the Universe is designed solely for the delight of physicists. But alas, magnetic fields are not appearing in this episode, but cryo volcanoes are, cryo volcanoes are like volcanoes only colder. So if you think of a volcano on earth, a volcano is made of rock. Inside of it is liquid rock, a k, a magma, and sometimes that liquid rock comes spewing out and then it cools off and turns into normal rock. A cryo volcano is the exact same thing, but with ice instead of rock, you have solid ice making up the body of the volcano. Inside you have liquid ice, a k, a water, and sometimes that water comes squirting out onto the surface and then it res solidifies back into solid water, a k a ice.
And the south polar ice cap of Triton has dozens of these cryo volcanoes that are active. We can see the plumes coming out of the cryo volcanoes. It's wild. Before we leave our tour of the surface, like a good detective at a crime scene, we of course have to pay attention to what we see, but we also have to pay attention to what we don't see. Is there anything about this scene that is missing something that should be there or is usually there but isn't? And in the case of Triton, it is. I mean, it isn't. I mean, something is missing craters. Triton has barely any craters on its surface at all. Not in any of the three regions, not in the cantaloupe train, not in the broad flat features list part, not in the South polar ice cap. I mean, sure there are ridges and mountains and weird lumpy things, but there are very few craters.
And the only way that a planet or a moon can have that few craters is by contributing to Patreon. That's Patreon dot com slash pm Sutter. If you want fewer craters, if you want that fresh look, that young look with fewer craters impacting you, then contribute to Patreon. That's Patreon dot com slash pm Sutter, it is how you Support the show, and I truly do appreciate it. Actually, in order to have few craters, you need to resurface. You need to be alive, you need to be active. The earth has very few craters because our surface is constantly restructuring itself with wind and water erosion, with tectonic plate action, with subduction and all that, we're constantly erasing our craters. The moon has a bunch of craters because it does not have any of these activities. The more craters you have, generally the older the surface is, and Triton has barely any craters.
So it appears to be young, or at least the surface appears to be young. Here's another piece of evidence pointing to its youth or the youth of its surface. It's super shiny, has an albedo of 0.7 that it means it reflects 70% of the light that hits it. It is one of the shiniest, most reflective objects in the entire solar system because it isn't just covered with ice. By the way, the crust of that cantaloupe terrain in the broad flat featureless terrain is it's all nitrogen and water ices. That's what makes up the crust of Triton. but it's not just any ice. It's pristine ice. They haven't been dirtied up by tho ends at random dust and dirt over the ends. Yeah, it's pretty empty out there in the outer solar system, but when you have billions of years to play with, you're gonna get dirty no matter what. Except for Triton, it's resurfacing itself. It's nice and shiny. So interesting piece of evidence. Number four, the surface has three very distinct kind of patterns is active and it looks very young.
One last bit of crime scene investigation here has to do with the insides of Triton and autopsy. Now, unfortunately, we don't have any moon sized scalpels on board Voyager two, so to understand its interior, we have to do a little bit of guesswork. We have to measure what we can see on the surface. Hint, it's a lot of frozen nitrogen. Observe carefully how Triton orbits around Neptune and how it rotates on its own axis. And then take a few swags, which are scientific wild aimless guesses, and throw everything into computer models and see what shakes out. Because ultimately, the details of its orbit, of how it rotates on its own axis, how it orbits around Neptune, how it interacts with the other moons does depend on the interior. If it's denser on the inside, if you have high concentrations of very heavy elements in its core, or if they're more, more dilute in, spread out, this affects the detailed relationship between Triton and everything else.
And so we can make these detailed observations. We can make total guesses about the inside and then see what matches. See what shakes out and what shakes out is actually a liquid water subsurface ocean. No joke. I'm serious about this. You know, I've talked all the time about Europa and Enceladus, these moons in the outer solar system that are covered in ice and then have liquid water oceans in their interiors. It seems likely that Triton has a subsurface ocean too. And the reason it does, the reason we believe it does, is one, from computer modeling of how Triton orbits around Neptune and how it spins on its axis, there appears to be a liquidy layer there that affects its rotation rate and its interaction with the other moons. and it kind of makes sense. All these objects in the outer solar system were born with a bunch of radioactive elements. They were just in the nebula that formed our solar system. The earth also formed with a bunch of radioactive elements.
It's what largely responsible for what keeps the interior of our planet warm is that radioactive decay. And it's no different out here in the outer solar system. So there's enough radioactive elements to keep its core warm, and that's enough to liquefy the ices underneath the surface and make a liquid water oceans. It also appears that Triton has a heavy element core, so like a nice big rocky core, then a mantle made of water, ice, and then a liquid water ocean above that. And then a thin glazing of nitrogen covering most of the surface, although there's also water, ice and carbon dioxide, ice, a K, a dry ice, some ammonia, ice to spice things up a bit. So interesting piece of evidence. Number five, Triton has a differentiated core. All the elements inside of it are not all mixed together. The heavy elements sink to the center, the lighter elements rose to the surface.
This takes heat. The earth has a differentiated core. The heaviest elements that we're born with are in our core are in the center. They had. The only way to get there is if the whole planet spends some time being molten so that all the heavy stuff can sink and sink down to the core. And all the light stuff can bubble and percolate up to the surface. You can, you can't do that if you're solid. So Triton has a differentiated core, which means it was a lot warmer in the past, warm enough to allow all those different elements to settle into that onion like structure. And so our crime scene investigation is complete. We have a world Triton, a moon of Neptune that is way bigger than it should be, has a completely wonky orbit formed in a non-traditional way, has a young dynamic and active surface today, and was a lot warmer in the past. What's the story behind this?
These are the clues. Now we need a suspect. We need to paint a plausible portrait of the past to understand the Triton of the present. How did this world form? How did it get here? And why is it the way it is? Putting all the pieces of evidence together, we can make a pretty compelling case that Triton doesn't belong to Neptune, that it didn't form with Neptune in the mini Neptune part of the proto-planetary nebula, the dawn of the solar system. This is the crime that Neptune kidnapped Triton. That Triton came from somewhere else. In fact, now that I think about it, Triton looks a lot like Pluto or Aris or or any of the other large Coer belt objects. And if you know what, if we had first discovered Triton out in the Coer belt, we wouldn't have given it a second thought. Here's one thing that large coer belt objects share in common.
They're warm, they're big enough that they contain a lot of radioactive elements, but small enough for that radioactive heat to make a difference. Just just look at what's going on with Pluto. Pluto has that giant beautiful heart, which is a glacier field of nitrogen that is cold. Trust me, it's very cold, but it's not frozen solid. It's moving just like glaciers move on the earth, that heat has to come from somewhere. Definitely not getting it from the sun has to come from its interior. We know for a fact that Coi belted objects, especially large ones, are big enough to be, to be warm, to be powered by radioactive decay. So is Triton. Triton is warm heat from the core of Triton leaches outward. It melts part of the ice mantle and turns it into a liquid subsurface ocean, and it powers the cryo vulcanism and it powers the broad flat featureless planes. Those volcano alaras that we noted on the eastern side of the mid-latitudes, they are volcano aldara, but not of rocky volcanoes like on earth or Io, but volcanoes of ice.
They're cryo volcanoes in the broad flat plains, they're, this is such an amazing word. They're cryo lava Witt, which means slushy glacier ice. They're floodplains of cryo lava. I don't know. I'm to, I'm perfectly, to be perfectly honest here. I don't know why we invented the word cryo lava 'cause we already have words for like ice, but whatever. And these broad flat featureless planes, because they sports so few craters, we can date them, we can age them. They're less than a few million years old, which is super recent. And the cantaloupe terrain itself is likely formed from the upwelling of chunks of material pushing up against the Isis icy surface, causing it to wrinkle up like, like, well, a cantaloupe. The surface of Triton looks young because it is young, not Triton itself, it's old as get out. But the all the warmth constantly pouring out of the core reshapes, the surface paving over any craters and heat from the interior is able to explain the cryo volcanoes, the giant broad, flat cryo lava floodplains and the cantaloupe terrain.
But we need more than warmth to declare that Triton is really a coi rebelled object that has been kidnapped by Neptune. Thankfully, we have more evidence. We have its weird size and its weird orbit. Look at all the other moons of Neptune. They're small, they're junky. There isn't a lot of material available around Neptune to make large moons, otherwise there would be other large moons. And all the regular moons of Neptune likely form together because they follow the same rotation as Neptune itself. They all sit in relatively the same plane. They all have nice tight orbits. If Triton formed with Neptune as part of the Neptune system, you would expect it to be regular. You would expect it to be sane. But if Triton just came barreling in one day and got captured by Neptune's gravity, remember Neptune is the most important object way out here in the deep dark then, then Triton could have any old orbit it wanted.
It could have any random size it wanted 'cause it wasn't a part of the Neptune system. It formed somewhere else in a different way through different mechanics and then got captured. It's likely that the other irregular moons of Neptune are also captured Cofer belt debris and Triton just happens to be the biggest of the bunch. In fact, Triton coming barreling in to the Neptunian system could mess up the orbits of the other moons. There are some other weird interactions between the other neptunian moons and the presence of Triton, which is massive, at least compared to those moons. It can affect those. And so we think that's why we think Triton came barreling in. Also, if Triton came barreling in and got captured by Neptune or kidnapped by Neptune early on when Neptune was forming, when all the planets form, they look like mini solar systems. There's like a core in the center where the planet forms and then a ring, a disc of materials surrounding of that becomes the moons.
If Triton came in early enough and got trapped, it had to swim through all that material as the material was still coalescing to become the other moons of Neptune. And that friction, that drag is enough to act quickly enough to circularize Tritons orbit. This is how we think. Triton got such a circular orbit. 'cause it did come barreling in at a random angle, ended up in some random orbit, but then had to swim against all the other material that was still forming around Neptune. and it got slowed down and it got nice and perfectly circular. And that's our story. This is our, our theory, but this is the big one. A big question. How did Neptune capture Triton if we can't just say, well, Triton formed as a coer belted object got trapped by Neptune's gravity and got kidnapped and that's it. But how did that happen? We need to finish our story here, finish our theory.
The key here is that Triton has to lose energy to get gravitationally bound by another object. You need low enough energy because if you have too much energy, you can exceed the escape, the binding energy of that planet, the gravitational grip of that planet. And you just go on doing your own thing. Your, your path might get deflected, but you'll continue doing your own thing. You need a low enough energy to be captured. That's the key. We actually have three ways that Triton could have been captured by Neptune. One, is it just total random unlucky coincidence that Triton was wandering around in the outer solar system, got scattered a few times, got caught within the vicinity of Neptune and had just the right energy to end up in orbit. That's, that's not super plausible because it, it entails a lot of lucky coincidences and having just the right energy at just the right time.
But hey, unlucky things just happen. Another option is that Neptune was originally forming with some beefier moons, certainly bigger than Proteus. That's 400 kilometers across, you know, something serious that we could actually call a moon and Triton came in, wandered in and actually struck and collided with that. Moon and collisions do a great job at, at pulling energy out of objects. Just like if you get in a car crash, you're, you're likely to stop moving very quickly. And then from the debris, the Triton that we know of today formed. Another possibility is that Triton actually formed as a binary. We know that binary objects are very common in the ER belt for reasons that we don't fully understand, but observations suggest that, and that the binary itself wandered close to Neptune doing its thing. And then there was a gravitational interaction where Neptune ripped apart the binary, the binary companion, the Tritons twin flea escaped probably left the entire solar system and is now wandering interstellar space.
And that interaction caused Triton itself to lose enough energy to get Gravitationally bound to Neptune. None of these three options are for certain, and this is where we get to argue at conferences until the heat death of the Universe. But it's also why we need to go back. It's been over a generation since our last closeup look at Triton, and unfortunately, I hate to say it, there are no mission concepts to visit Triton that have advanced beyond the proposal stage in nasa. I, I have a soft spot for the outer solar system for Uranus, for Neptune, for the Coor belt. I believe it's under explored, it's poorly understood NASA for a while. Space agencies have for a while have been focusing on Mars. Now we're starting to focus more on Jupiter and Saturn, which I don't blame anyone because we have things like Europa and Enceladus with liquid water, oceans that are only a handful of au away.
They're like basically next door. We don't have to travel to the edge of the solar system to explore 'em. So I get it. It's easier to go to Jupiter and Saturn than it is to go to Neptune. But here we have this intriguing world, this case of planetary kidnapping, this victim of planetary kidnapping that ended up in a place where it did not call home Triton was not born around Neptune. Neptune captured it. And I believe there's a lot more to learn about Triton itself, about Neptune, about the ER belt, about the relationship between planets and moons, about the interaction between Neptune and the rest of the Kofi Belt. It's role, the role it plays in the outer solar system. I think there's so much more to learn, but sadly, and I, and I say this with a heavy heart, it seems unlikely that in my lifetime there will be a return trip to Triton, which irritates me and saddens me because right in our own backyard we have this crime scene, this apparent case of kidnapping.
And just like you, I don't like unresolved mysteries. Thank you to Pete e for the question that led to today's episode. And of course, thank you to all my top Patreon contributors. Actually, thank you to all of my Patreon contributors. You're all amazing. But my top ones this month are Justin G, Chris L, Barbara K, Duncan M Corey D Justin Z Nalia Scott M Rob H Justin Lewis m John, W Alexis Gilbert M Joshua John S Thomas D Simon G, Aren J and Valerie H. It is your contributions that make this show possible. I really, really do appreciate. I really do mean it. I can't. I can't thank you enough. I'm humbled by all of your generous contributions. Also, you do have homework. Go drop a review on iTunes or Spotify or your favorite podcasting service. It really does help the show. And please send me more questions. I love the questions. I love accumulating questions and I love picking topics every single episode.
The best place to go is the website Ask a Spaceman dot com. There's a link in there where you can send me a question or you can just email me at Ask a Spaceman at gmail.com and no matter what, I will see you next time. For more complete KNOWLEDGE, OF, TIME AND, SPACE.