We’re not learning that the vast majority of potentially habitable worlds out there are actually icy moons like Europa and Enceladus. Good news, there are hundreds, if not thousands of times more of them than worlds like Earth. Bad news, they’re locked in ice. What have we learned about water worlds and their potential for habitability?
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Fraser: Astronomy Cast. Episode 501: Water Worlds. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos, where we’ll help you understand not only what we know, but how we know what we know. I’m Fraser Cain, publisher of Universe Today. With me, as always, Dr. Pamela Gay, the director of Technology and Citizen Science at the Astronomical Society of the Pacific, and the director of CosmoQuest. Hey, Pam, how you doing?
Pamela: I’m doing well. How are you doing?
Fraser: Great. We’re in to the new season. I guess, Episode 500 was our official beginning of the new season, but now we’re back to our regular schedule. Of course, always looking to mix things up. You’re in Australia now and will be for this week and next week.
Pamela: Yeah, that’s true.
Fraser: But, like a complete professional and a trooper, you are recording at like five in the morning in a sleepy Bed and Breakfast in the outskirts of Melbourne.
Pamela: It’s fabulous and our show’s a teenager.
Pamela: Like, we’re 13 seasons old now. So, does this mean that we get to cover all of the bratty discussions in astronomy now?
Fraser: That’s exactly it. Yeah, now it goes to the rebellious stage, where it just tells us that it hates us, it hates us, it hates us. Not that that’s my personal experience from this at all, having two teenagers. So, I have something really important to shamelessly self-promote. And I’m probably gonna do even more for this, but I just wanna begin what it’s gonna be pretty much a straight month of shameless self-promotion and here it is: Boom! Check it out!
For the podcast listeners, of course, you can’t hear what I just showed to Pamela. It is the Universe Today: Ultimate Guide to Viewing the Cosmos, which is the new book, written by Dave Dickinson and me, featuring (and this is the part that I’m proud us about) a foreword by Dr. Pamela Gay.
Fraser: So, Pamela wrote the foreword, Dave wrote the book, I helped. And the help that I did that I’m most proud about (and I’ll bring this up again and again) is the photographs. So, we’ve got probably more than a 100 separate astrophotographs by, I think, 75 at least different photographers, and what’s amazing about it (and I’m just gonna keep holding it up) is that it’s pictures that you’ve never seen before.
Each one of these photographs was taken by an amateur astrophotographer. No Hubble Space Telescopes. Okay, fine. There’s a few Hubble Space Telescopes in the book. But in general, the vast majority of the photos that are in this book are all individuals. And this was really the goal, which was like, if we’re gonna teach you how to be an amateur astronomer, you aren’t gonna get access to your own Hubble Space Telescope. I wanna show you what’s possible as just a regular person with skill and patience and some reasonable gear.
So, a huge thank you to all of the astrophotographers. It was kind of like herding cats to get everybody involved, but in the end –
Pamela: Feral cats. You were herding feral cats.
Fraser: Exactly. But in the end, I think it worked out great, and I think everyone’s gonna be really, really pleased with what the final product looks like. It’s available for pre-order right now on Amazon’s and Barnes & Nobles and things like that. And I’m not exactly sure what’s going on, but for some reason you can pre-order it on Amazon US for $18.89, while the actual print price is $28. And so, I’m not exactly sure. Maybe that’s just the pre-order price? Someone should help me get to the bottom of this, if this is some kind of special promotion and incentive that I should be mentioning.
So, what I’m saying is that it appears that the price is extra cheap by quite a bit. $18 for a hard cover, full color book.
Pamela: This is a true coffee table book.
Fraser: Yeah, well, maybe. You know what it feels like? It feels like the player’s handbook for Dungeons & Dragons. If you wanna a sort of imagine a book that it feels like. If you’ve got one of those, this is almost exactly what it is.
Pamela: But that thing you leave on your coffee table because every person who enters your house will want to flip through it, that’s what this is.
Pamela: Now, I have one tiny shameless plug. Tiny, compared to this. You just returned from doing one of the astro tours with Dr. Paul Matt Sutter, and I have not posted my tour for next year. So, if you want to go travel through the American Southwest, which is kinda my home, it’s where I spent all of my summers growing up, you can do this. Go to astrotours.co/starstrider, and register today and we will see the great observatories and the great deserts of America. This is like nowhere else in the world. If you’re going to come to the United States, this is where I would send you.
Fraser: Agreed. Apologies, all the shameless self-promotion, but we’ve been gone for several months, so we gotta catch up. So, people, go to astrotours.co to sign up for your trip –
Pamela: Slash, starstrider.
Fraser: Sure, but it’s – they can scroll down. Right? And then they’re gonna explain how to misspell starstrider. It got very complicated. You just made it more complicated. So, go to astrotours.co, brows down until you find Pamela’s trip. Do it. And of course, if you wanna pick up our book, pre-order it now, go to Amazon, search for Universe Today, and I’m sure you’ll find it pretty quickly.
All right. Let’s get in to the actual show. We’re now learning that the vast majority of potentially habitable worlds out there are actually icy moons, like Europa and Enceladus. Good news: there are hundreds, if not thousands of times more of them, than worlds like Earth. Bad news: they’re locked in ice. What have we learned about the water worlds and their potential for habitability?
We’ve talked a bit about water in the solar system, we talked a bit about the icy moons in the solar system, we’ve mentioned a lot of these discoveries about the hydrogen gas that’s coming out of Enceladus, the geysers on Europa, and Enceladus. I think, if one, huge theme that’s is going in the background of planetary science, while we’ve been doing this show, if we talked 12 years ago, where should we be looking for life? We’d say: Mars. Or Earth-like worlds’ inhabitable zone. Now, it’s water worlds. It’s Kevin Costner paradises.
Pamela: Right. And we have two different forms of water worlds that get discussed in the literature, and it will break your brain slightly if you’re not careful doing these searches because we have these massive super Earths that are getting discovered snuggled up next to little red dwarf stars, and they appear, according to the models, to be completely covered in water, perhaps. And then our own solar system, we have all of these moons that, for reasons we can’t always explain, have oceans underneath (with Ganymede is one of the theories) and then are coated in liquid water and ice on their surface.
Water is common. It comes everywhere, and we’re finding new and ingenious ways that the universe is decided to heat it up every year that we do this show.
Fraser: And it’s not just the – you’re just talking about some of these larger ones that are huddled up to the red dwarf stars. I mean, even some of the smaller ones, the ones, like the Trappist. They’re now thinking that the Trappist planets could very well be large amounts of water. Like, 50% water, higher. Like enormous percentages of water, compared to what we see here on Earth, and we have discovered in the solar system so far.
Pamela: And what gets me is, how they’re figuring this out. We can’t exactly directly image the surface of these worlds yet because we haven’t spent the money to build those telescopes yet. I’m really hoping we’ll get there. But, right now what ends up happening is we have these worlds are getting discovered by Kepler, and there’s a whole bunch of them. Kepler-62e is one of the ones that gets the most attention. But there’s a whole bunch of these different worlds and we can observe them transiting in front of their stars.
So, we know how quickly they’re moving in their orbit, we are able to see how long it takes them to dim and so, you have your object and as it passes in front of the star, it takes time for it to go from completely beside the star, to completely in front of the star. And that amount of time that it takes it to move also gives us the width of the planet. So, with these transiting systems, we have the ability to physically measure the size of the planet, if it’s the right kind of dip. Once we know how big the world is, we know how it’s orbiting, we can start making assumptions about what its mass is, and then the modelers are free to model. And they plug in to their computers.
Well, you throw these things in, you throw these things in, assuming, okay, if the star has this composition, the Solar Nebula would’ve had this composition. And in the models it works out what elements would’ve gotten blasted to the outer solar system during planetary formation. What elements would’ve been left behind. What should this world have been made of? And it appears that to get the sizes we’re seeing, you need a world covered in water.
Fraser: Let’s tackle these two flavors of water worlds. Frozen and liquid, one after the other. For the ones that are close in, that are warmer and have been – I mean, here in the solar system, if you get too close to the Sun, you get your water blasted away because of the solar radiation.
Fraser: So, how are they thinking that maybe these planets are able to hold onto their water?
Pamela: It all comes down to what kind of star is it. I don’t know about you, but I have incandescent lights in my house in random places still because I’m a bad human being. And I know that the 100 Watt light bulbs, if I touch those suckers too soon, totally gonna burn myself. Whereas the lower Wattage bulbs, the ‘not turned up all the way on the fader bulbs’, if I want to replace those with a fluorescent bulb, I just turn them off and grab them and it’s good. Stars, it’s the same way. If they’re big, they’re hot, they’re gonna get rid of all of the water and burn that planet. You have a smaller, redder, cooler star, and water can be liquid, but not boil away.
These stars also go through a different kind of launching into existence, whereas a red dwarf, it’s going to be an angry little X-ray jettisoning object for a while. But it’s not going to go through that prolonged period of being super hot.
Fraser: The star matters, and so as you’re finding these smaller, red dwarf stars and you’re finding these planets around them, the conditions are different. You’re not gonna get that extreme temperatures and powerful – the solar winds can be bad, but you’re not gonna get the same temperatures that are going to blast away the particles of water. And it sort of says that in fact, there probably was a lot more water in the inner solar system back in the early days. It’s just the Sun was having none of it, and was just getting rid of it, and it was only what could form, or could be collected onto the Earth that we were able to hang onto it, and the rest is blasted out into deep space. Goodbye forever.
Pamela: And most of our water is actually believed to have been delivered later. It’s now thought that it probably came from meteorites hitting us that still had a lot of water trapped in their rocky matrix. So, we don’t have our own water. It was taken away from us by an angry, young Sun. These worlds didn’t have that angry star.
Fraser: Well, I know that this is still a controversy. Delivered by comets was, I think, the long-standing, original theory, and now, as you’re saying, that it’s maybe water trapped in the matrices of meteorites and asteroids and rocks instead, and even possibly it was just scooped up in situ by the Earth as it was going around the Sun in that cloud of water. So, what are these worlds look? You know, if you could visit them up close, what would you see?
Pamela: Water as far as the eye can see, clearly. But beyond that, because they are closer to their stars, these objects are often as close as Mercury is to the Sun, you would have this glowing, obvious to the eye, red, large object in your sky. They would have thick, humid atmospheres (much thicker than our own atmosphere), so you’re going to have that extraordinary, rich, blue sky.
If you’ve ever had a cloudy fish tank when you first turned the light on, the tank appears to be (as long as it’s cloudy with even scattering and not green cloudy, because it’s cloudy with algae) – If you have a cloudy fish tank, it appears to be filled with rich blue water, that rich, scattering of light in a humid atmosphere is what you’d experience from the surfaces of one of these worlds. You’re talking blue skies, large, red star, the sunsets would go into deeper shades of purple than we have here, on our own Earth. And if the atmosphere’s thick enough, you’re going to start scattering things so that it’s sunset all the time.
Fraser: And then what about habitability? I mean, obviously, if it’s just a ball of water floating, it might be tough –
Pamela: Well, they’re not balls of water.
Fraser: No, no, no. But, I’m saying, like obviously if it’s a sphere of water, that makes life difficult, but the point is that the mixes are high. Like, here on Earth, water accounts for (and I forget what the number is), but like less than a percent. Like, it’s a tiny amount of water compared to dirt. But some of these worlds could be 50%, 70%. Like, they’re really a tremendous amount of water with a rocky core underneath it. What about habitability for these places?
Pamela: As long as you have the other stuff, you’re good to go. And so, these worlds are considered to have the metal iron cores, the other atoms rich in them, just deeper down. So, you can imagine that those cores still would have the trapped heat, they would still potentially have at one point, early in their existence, been capable of seeding life deep in ocean vents. Once that life is seeded, you have that potential for life migrating all throughout the ocean depths. We don’t actually know on our own planet, did life originate near the surface, or did it originate deep, some place like the Mariana’s Trench, but when the earth was completely different set up.
Without knowing where life originates (or if it originates both places) we can hope that life could’ve originated deep down in the murky, dark depths of that ocean, and then evolved to fill all the different ecosystems, and all the different heights within the water, where different amounts of light permeate.
And one of the articles I read prepping for this actually said, now, imagine, we have flying fish here on Earth, we have things like rays that’ll jump out of the water and splat back down to scare the Jesus out of their prey. Well, imagine the leaping fish of these worlds that catch glimpses of probably clouds and not stars, as they jump out. These worlds are considered probably be mostly cloudy because they’re just that humid.
Fraser: One thing is really interesting. There was a great piece of research that was done earlier this year (actually, just a couple of months ago) about people ran some simulations for what a very deep ocean water world would be like, and so one of the big assumptions was that there’s no way for the material to get from the bottom of the ocean up to the top –
Pamela: You can’t oxygenate the bottom. Yeah.
Fraser: And now, according to their simulations, in fact, it’s probably a lot more likely. And in fact, they could. And one of the things that’s really interesting is that the world could maintain its temperature for a significantly longer period of time. Billions and billions of years around a red dwarf star because they’re so stable in the long term. And I think the other part is really interesting is that – I mean, we know that red dwarf stars are so volatile in their youth, but the water is protective. So, suddenly you’ve got the perfect protection, a lot of protons in the water, matched with the perfect, long-term temperature stability of a red dwarf star. So, now these places are looking pretty intriguing.
Pamela: And this causes us to then go down the more science fiction pathway of, how do you detect and how would civilizations in these worlds exist? And it’s one of those firm reminders that our own ocean-dwelling mammals may have advanced language. We’re pretty sure the whales have language, we just don’t know how advanced and how conceptual it is yet, but because you’re under water where they’re not going to be dealing with electricity beyond shocking one-another, like moray eels, they’re not gonna develop the kinds of technologies that we’ve had to create here on the surface of the world.
So, you can end up with advanced societies that have arts, have language, have song, but may not have any form of technology at all, other than tool-usage. And we do see all sorts of tool usage in the ocean.
Fraser: They wouldn’t be able to do very well beyond perhaps a kilometer of that top water. So, what kinds of tools would they even be get access to? It would be a very strange kind of society. It’s kinda sad. It’s clearly a science fiction story that someone needs to write.
Now, I’d like to go – shift gears now and talk about the kinds of worlds that we see now in the outer solar system places because potentially, there are hundreds of these worlds just in our own solar system. Not the ones around the red, but they – things like Europa and Enceladus. As you said, they think there’s Ganymede, they think there’s Titan, perhaps even Pluto, Sharon, Sedna.
Pamela: I’ll give you tens. I’m not sure about hundreds. Tens, I’ll give you.
Fraser: Some of the research that I saw was not only hundreds, but potentially thousands of times in the Milky Way, more of these icy worlds.
Pamela: Oh, Milky Way. I thought you were saying in our solar system. Okay.
Fraser: That’s why I went to hundreds in our solar system. So, it’s thousands in the Milky Way, I went with just hundreds in the solar system.
Pamela: So, I’m gonna still nope that and go with tens.
Fraser: Sure. Feel free. I’ll send you the paper if you want.
Pamela: So, within our solar system, we see worlds that have liquid beneath their surface that are heated through a variety of different means we do and don’t understand. Ceres is the largest of the asteroids and it was discovered by the Dawn Mission to have what appears to be cryovolcanism that has been ongoing with a hundreds of candidate volcanoes having been discovered through imaging where they looked to find the hills that are properly sloped to represent extinct volcanoes. And they’re able to trace back how essentially these cryovolcanoes appear to have turned on and turned off across the decades, and centuries, and millennia.
And now we’re able to see (at least one) recently active volcano on its surface currently. And exactly what would trigger this pressure build up of icy liquid beneath its surface is something that we’re still trying to figure out. Here, on Earth, we know that part of the reason that our planet’s just warm on the inside isn’t just leftover heat from the formation of the planet, but there’s actually ongoing radioactive decay. If you live in New England with granite, you know Radon builds up from these nuclear decays. And it could be that – well, Ganymede has one expletive of a Radon problem because of the built-up heat from nuclear decay processes within the world.
So, that’s one way that you can get subsurface oceans, perhaps. Going further out into the solar system, you have our quintessential Europa and Enceladus, which are these fairly large (in the case of Europa), very large icy moons that have a rocky core deep down, have a – argued about how deep liquid ocean above that, and then an argued about how thick icy crust on top. And with Europa, it’s very clearly heated through tidal stretching and compression from its interactions with the other moons and through its orbit around Jupiter.
With Enceladus, we’re still working on the dynamics because we didn’t think the tidal force, as it dealth with was enough to trigger the heating necessary to keep water liquid. Then when we look at Pluto, it’s just like WTF, Pluto? How – how? How? So, there, I’ve even heard ideas that it got re-heated through a massive collision that created that hearth that it has and that lead to liquefaction of sub-surface ices.
Fraser: But there’s even potential evidence of cryovolcanism of Tritan, which is the moon of Neptune. And so, the thinking is that all of these icy worlds, to some extent (even Sedna, even Eris, even the Goblin that has just been discovered) that they all have (to some extent, as you mentioned because of this leftover radioactive decay that’s going on in them) some amount of liquid water underneath an icy shell. And this is where this calculation comes from, that the vast majority of the universe – if you wanted to look for life, you would start with these, which are the more common places in the entire universe.
But the problem, of course, is that the amount of chemicals and energy that they have at their disposal is far less than what we have here on Earth, being closer to our Sun.
Pamela: And this is where it comes down to which number do you go with, is based in part on how do you figure out how low can you go, in terms of mass, and still have this liquid water trapped somewhere within it? With something as large as Pluto, as large as Sedna, as large as Eris, you can start to imagine there’s enough material to cause prolonged heating. Whereas as you start to get down to smaller and smaller objects, you’re just gonna not have enough radioactive decay to liquefy much of anything. But where that boundary is? We don’t know.
We really did think up until New Horizon’s got there that Pluto was gonna be an utterly solid, boring, cratered world that would look a lot like Ganymede and we were wrong. We were really wrong. So, I go with tens, because I think you do need to be larger object, you need to be more of a Sharon-sized object. You’re going smaller, betting more on these potato-shaped blocks of ice.
Fraser: I am just regurgitating research done by Abby Lobe and Team. So, I’ll send you his paper if you want.
Pamela: It’s all right.
Fraser: Thousands. But sure. Go ahead. No problem.
Pamela: In our solar system. A thousands in our solar system.
Fraser: I will send you the paper and you can take a look at it.
Fraser: But he is proposing that it’s a thousand in the Milky Way. Just in general –
Pamela: That, I’m totally good with.
Fraser: – for every Earth-sized world, inhabitable zone of a Sun-like star, there are 1,000 ice worlds, which have liquid water underneath a surface of ice. So, obviously, you know, tens in the solar system? Sure. But if you count up – Because of the vast majority of these red dwarf stars and who knows what’s out there. So. But the point is that if we wanna find life – But finding life would be super difficult because they’re under all this ice.
Pamela: Yes. And we actually have experience with how difficult this was. We talked about this in an episode several years ago. There was a Soviet, and then Russian project to dig into the deepest, highest altitude icy lake here, on Earth and sample the liquid water. And they went down (I believe it was over a kilometer) to get to that water, and it took them years and years and years to get there, and to get samples to look and see if there’s life in Earth’s deep lake.
And here, on Earth, we can use giant ships, giant airplanes, helicopters, big machinery. No restrictions on how much energy is required. No restrictions on the weight of what we’re using. And it still took years and years and years to get a water sample out of this lake, and it was still high risk because how do you not contaminate it? And we don’t exactly have the ability to do this on another world.
My personal opinion of best efforts so far is an idea, is a small, little instrument on a heated wire that heats its way down, and because the wire is heated, its able to keep sliding. And in this case you’re not digging a tunnel, you’re simply allowing something to slowly move down. And here it’s all about the resistance and you have something generating electricity at the surface.
Fraser: This technique is actually being used here, on Earth in Greenland. So, they’re actually – You mentioned – It was lake Vostok and they were attempting to get down to this sub-glacial lake that’s in the Antarctica and they used this drilling method to do it, and it was very difficult and very laborious. And I don’t know the actual distance they got down, but I think it was dramatically deep. But a similar technique has been done to bore out all of the ice cube observatory in the Antarctica, and they learned a lot of lessons. And they were actually able to build that telescope relatively rapidly, which is a big neutrino observatory.
But in Greenland, some new research is being done, whether actually, taking as you said, this heated probe and letting it melt its way down reeling out wire as it goes, and then the borehole freezes behind it, freezing this wire in place. And in theory (I did a whole video about this, this is why it’s so present in my brain), is it would take you one to three years, depending on the size of the probe, to get down to the sub-ice sheet layer on Europa, for example. But we don’t need to go to that work because Europa and Enceladus have been so kind to spew this material out into space for us.
Fraser: And this is true. We’ve been able to detect, but not measure anything fascinating about the water geysers on Europa, thanks to how they affected one of the – I believe it was a magnometer – that was on the Galileo mission that was found by going back and re-analyzing the data once Hubble had seen the geysers to see if we could see the effects. And then, of course, the Cassini mission, quite purposefully, flew through a plume of Enceladus and captured up the material to put through the mass spectrometer.
So, anything we throw through a mass spectrometer is probably going to die in the process, but what mixture of elements and molecules did we look for that can indicate life exists here? Because it’s one thing to find all the stuff that’s needed to eat. We find the ingredients for alcohol, we find amino acids, we find all sorts of complex molecules all across the galaxy. That’s not a question. But how do we find evidence that there’s life by analyzing the data that comes out of that mass spec?
Pamela: And so, you’ve got the Europa clipper, which is going to be on its way to Europa relatively soon. It’s gonna fly through these plumes and, as you said, how good will it be able to detect? But at least it’s a shot at it. And there’ll be all kinds of organic chemicals that in theory it’s gonna be able to see. So, it’s a fascinating time both with the exoplanet discoveries that have been made with the red dwarf stars, with the really interesting water-based worlds that around them, that suddenly the biggest threat to life has not potentially been protected by this amount of water, and then once you get outside the frost line, the vast quantities of water that’s available in these solar systems.
It is a exciting time and a time and a time to really flip the way you look at the search for life. Place your bets. Where do you think we’ll find life?
Pamela: I’m really hoping that we are able to find tiny life existing in brine on Mars so that we can just go out and directly measure with robots in our lifetime, but I suspect that the highest density of life in our solar system off the Earth is going to be found on one of these icy moons.
Fraser: I cannot wait for us to be able to make that discovery. Pamela, thank you so much.
Pamela: Thank you.
Male Speaker: Thank you for listening to Astronomy Cast, a non-profit resource, provided by Astrosphere New Media Association, Fraser Cain and Dr. Pamela Gay. You can find show notes and transcripts for every episode on astronomycast.com. You could e-mail us at email@example.com, Tweet us at Astronomy Cast, like us on Facebook, or circle us on Google+. We record our show live on YouTube every Friday at 1:30 P.M. Pacific, 4:30 P.M. Eastern, or 20:30 GMT. If you miss the live event, you could always catch up over at cosmoquest.org, or on our YouTube page.
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Duration: 31 minutes