Thanks to Cassini and other spacecraft, we’ve learned a tremendous amount about the icy worlds in the Solar System, from Jupiter’s Europa to Saturn’s Enceladus, to Pluto’s Charon. Geysers, food for bacteria, potential oceans under the ice and more. What new things have we learned about these places?
This episode was recorded on Thursday, 5/31/2018.
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Icy moons in our solar system
Icy moonsIcy Water Moons That Might Host Life (Infographic)
Saturn’s Icy Moons Are a Little Less Mysterious Thanks to Cassini’s Long Mission
Jupiter Icy Moons Orbiter – JPL – NASA
Cassini finds molecular hydrogen in the Enceladus plume: Evidence for hydrothermal processes
Triton looks like what we thought Pluto would
Europa Clipper Mission
Reprocessed Galileo data shows Europa’s geysers
Podcast Transcription provided by GMR Transcription
Fraser: Astronomy Cast, Episode 494. Icy Moons Update. Welcome to Astronomy Cast, our weekly facts-based journey through the cosmos. We 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 Pamela, how’re you doing?
Pamela: I’m doing well. How are you doing, Fraser?
Fraser: Good. So, there’s some congratulation in order.
Pamela: I wasn’t gonna say anything.
Fraser: Oh. I know you’ve been really quiet about this, but as – for anybody who hasn’t heard, Pamela has been inducted into, as the 2018, into the Academy of Podcasters Hall of Fame. Congratulations Pamela.
Pamela: Thank you so much, Fraser. But you have an asteroid named after you, so you still win.
Fraser: Yeah. So, as the host of Astronomy Cast, you’ve been – it’s hilarious. I’m sure it’s for the slacker astronomy as opposed to this show. That’s what it’s gotta be for. It’s one of the first people – I’ve already figured it out. So, yeah, no. Congratulations.
Pamela: Thank you.
Fraser: Alright. Let’s move on to the episode.
Pamela: Wait, wait. We have an announcement from the weekly space hangout crew.
Fraser: Oh yeah like some kind of number.
Pamela: Yeah. So, the 500th episode of Astronomy Cast is only six episodes ay-way. And our beloved friends over at the weekly space hangout crew, they’ve done the math and they figured out that when you factor in our summer hiatus, 500 is gonna land in the middle of September, and they’re inviting everyone to come celebrate.
They’re inviting the fans, that’s you, the viewers, the listeners, everyone to Edwardsville, Illinois, the weekend of September 15th and 16th for a weekend of astronomy-related festivities culminating in the live recording of the 500th episode of Astronomy Cast.
We’re actually going to use the adorable little Art House Theater that’s down the street from our ASP offices, and this makes me more happy than I can describe. You’ll be meeting me, Fraser, along with a whole bunch of the folks from the weekly space hangout crew from the CosmoQuest crew, and recording live 500th episode.
And Fraser, I hear you’re also going to put together a Q&A episode for Universe Today.
Fraser: I am? Sure. That sounds good.
Pamela: And we’re gonna work on other things like some of you guys know that I paint planets in my spare time, and I’m going to teach you how to paint planets too if you want to and you come. We’re also considering outings to local attractions. We’re gonna be holding most of the events on Saturday at a brewpub, and if we can still walk Saturday night and the skies are clear, we’re gonna be doing [Inaudible] [00:03:01].
Fraser: Oh. That sounds great. How do people get involved?
Pamela: There’s going to be a registration going up Monday, June 4th on hopefully both the weekly space hangout and Astronomy Cast, but go to AstronomyCast.com and check the menus, and I’ll try to put a highlighted link in for you to find. So, that’s Monday, June 4th, which for all of you listening out in the podcast world is either today or has already happened
And for all of you watching live, just wait till Monday, and you’ll be able to sign up.
Fraser: Alright. Fantastic. And I’m sure will mention it a couple more times before we reach the end of this season.
Pamela: It’s true.
Fraser: Thanks to Cassini and other spacecraft, we’ve learned a tremendous amount about the icy worlds in the solar system from Jupiter’s Europa to Saturn’s Enceladus to Pluto’s Charon, geysers, food for bacteria, potential oceans under the ice, and more.
What new things have we learned about these places?
Pamela: Uh, they’re weird.
Fraser: They’re weird and they’re awesome, and there’s a lot of them.
Pamela: Yes. I don’t even know where to start on this.
Fraser: What was the state of our knowledge when we first tackled icy moons, which was in the ‘50s when we did our tour through the solar system and talked about Jupiter’s moons?
Pamela: At that point, we suspected that Triton, the moon of Neptune, might be a stolen Kuiper belt object. We knew that Europa appeared to have subsurface oceans, but I’m pretty sure that was even before the Cassini discovery of Enceladus’ geysers that we first discussed this, and so this whole revolution that you look at a world and it probably has a subsurface ocean. That’s entirely new since we started creating this show.
Fraser: And it feels like it’s the vast number of places where the potential for life in the universe is worlds like this. It’s not the habitable planets that we’ve always come to think of them, the rocky worlds in the habitable zone of an Earth-sized, Earth-like star. It’s the probably thousand times as many worlds, these icy worlds, across the, you know, per solar system, that there could be a thousand times as many places to look for life in the solar system as just a plain old boring rocky planet like Earth.
Pamela: Yeah. That whole Goldilocks zone around a star, that is far too limiting. We have moved on from that.
Fraser: Yeah. That is an old idea. Many more places to look for life. Really hard, complicated, difficult places to look for life. I mean, these are not easy to places to go to.
Pamela: But we seem to have found one that we shouldn’t look. Only one.
Pamela: [Laughter] So –
Fraser: [Laughter] You’re telling me there is a place we shouldn’t look? What?
Pamela: I was reviewing icy moons today in preparation for the show, and I came across as I was reading about Callista, basically, it’s this one lone, overly cratered moon, and the way you get a surface as beat up as this particular moon’s surface is you have nothing interesting going on on your insides.
So, in trying to figure out exactly why it is that Callisto is this weird oddity that it is, I did some diving, and it’s thought that Callisto may have simply accreted with small things adding up over time, and it did it in such a way that it didn’t end up with the differentiation that we see with other worlds.
It doesn’t really have a core. It doesn’t really have a liquid ocean. It’s just basically a giant snowball that has had a whole lot of stuff beaten into its surface over the eons.
Fraser: Poor Callisto.
Pamela: Poor Callisto.
Fraser: Yeah. Although we could say that kind of thing about so many of the worlds in the solar system. Just the smashing and late heavy bombardment.
Pamela: Most things that get this big tend to have subsurface oceans. We look at Enceladus. We look at Europa. We even think Ganymede probably has a subsurface ocean. Triton has ice geysers.
Fraser: Titan could potentially have its own subsurface ocean underneath its surface oceans of methane.
Pamela: And ethane.
Fraser: Oh. Yeah. And ammonia. Let’s take a look then, you know, what is sort of the picture as we understand it now for a live these icy worlds across the solar system. What would these look like, especially if you could kind of slice them open?
Pamela: For the most part, they have a roughly rocky core surrounded by who knows what dense materials, but probably a mixture of things that don’t melt readily. Then on top of this, you have a liquid ocean, and atop of this, you have frozen solids. In some cases like Pluto and Triton, we see vast, frozen nitrogen surfaces.
We see all kinds of frozen gases on the surface including frozen water ice it appears. So, the surfaces themselves basically have a mixture reminiscent of our own planet’s atmosphere. The ratios are different, but it’s basically the same kinds of stuff. And then beneath that, we think is like good old H2O water oceans that may be a different salinity, a different pH do to stuff in the water, but we think these are water oceans.
Fraser: So, then what is keeping these oceans liquid?
Pamela: Different theories for different worlds, and this is one of the new things that is so hard for us to understand and it’s amazing having problems like this. With Europa, it’s straightforward. Europa is in a resonance with the other Galilean moons as it goes round and round Jupiter, and thanks to tidal squishing and expansion that it undergoes on a regular basis, it has heat building up in its core.
It probably has subsurface volcanism just like we have underneath our own oceans, and that is insufficient to generate heat to keep it liquid all the way to the surface. Plus the surface is vacuum. So, with that combination of not a lot of heat and vacuum at the surface, you end up with these ices at the surface.
The factors that determine if something like nitrogen, something like water, is a solid, liquid, or gas is the combination of the pressure it’s under, the temperature it’s at, and at different values you get different things, and with no surface pressure, you’re pretty much limited to solid or gas that’s running away rather quickly.
Fraser: But when you think about all of the different environments, right, you’ve got places like Europa, which is within really in under the tidal influence of Jupiter, you can understand how you got that kind of flexing and when you saw what happened to Io, its volcanism and secret proto-molecule research labs, but it’s volcanism and then you look at Europa and it’s a little cooler, so the water has melted, you don’t have the volcanism, and then you can see sort of that same process happening with Ganymede and Callisto, and then similar versions of that happening with the Saturnian system.
But now the thinking is that even as you said, Triton, Pluto, Charon, Sedna, all of these worlds out there have some amount of liquid water. So, in the is it radioactivity? I mean, is it just – it hasn’t cooled down yet since the formation of the solar system?
Pamela: We’re still trying to come up with one solid theory, and it could be different models match better or worse for different worlds, but the kinds of factors that we have to take into account how much radioactive material there in the world that is still undergoing normal, everyday, radioactive decay and giving off heat in the process.
We also have to look at if the world is in an elliptical orbit that being in an elliptical orbit will cause the forces it experiences to change over time and can, to a different degree, drive internal heating.
There also may be questions about its collisional history. If you smack the bejesus out of a world and Mimas, which is the that looks most like a death star. It has cracks running through it. It almost got knocked to pieces, and in a different kind of object, massive collisions like that have the potential to generate sufficient heat to melt the interior somewhat.
Fraser: One of the things that’s pretty fascinating. This is fairly new research that came out this year or last year was that it appears that the core of Enceladus is more spongelike than sort of a solid rocky, you know, your sort of your traditional expectation is that the core of Enceladus and Europa and these places are just this sort of rocky ball surrounded by this ocean and then surrounded by this icy shell.
And now, it looks like it’s probably more spongelike and that you’ve got say these tidal forces are squeezing the water through the center of the world being it liquid and sort of allowing it to maintain potentially this liquid state for much longer.
Pamela: And each world I have, like I said, a slightly different solution, and the fact that we’re seeing this across a whole variety of masses and radiuses, tells us this isn’t going to be something simple to figure out. We look at a world like Ganymede, which is bigger than Mercury, and it’s easy to imagine that having internal tectonic activity. It’s easier to imagine that given all the tidal forces of Jupiter, maintaining liquid.
But you look at something somewhat smaller like Enceladus, and you have to start looking for new solutions.
Fraser: And so one of – probably the most exciting discoveries, and this is fairly recent as well, is this discovery of hydrogen gas, which is coming out of the geysers that are on Enceladus. And why is that so important?
Pamela: Well, hydrogen gas gives us a hint at the internal chemical structure, and this tells us that it’s possible for there to be all sorts of complex molecules, essentially the building blocks for life down inside this world now is going to be the question, do we find similar with the Europa Clipper going out in the future.
Fraser: Yeah. I mean it’s food for bacteria on earth. This gas in the oceans is what a lot of the base of the food chain, you know, at the bottoms of the earth oceans feed off of. And so to find this in Enceladus as well is to say okay, there’s water, there’s food for bacteria, there’s food for life there.
Pamela: Well, hydrogen by itself isn’t really a good food source, it’s all the complicated molecules that it’s part of the formations of, and you and I are included in the complicated things that hydrogen is part of the formation of.
Fraser: And that bacteria like to eat.
Pamela: Well, yes. That’s true as well.
Fraser: Yeah. And so, the question is just is there an actual eco-system that’s – but all the conditions are there for there to be an ecosystem?
Fraser: Even though potentially. So, let’s talk a little bit about sort of what the plans are then to try to explore some of these worlds a little better.
Pamela: Well, they’re not as rich and complicated as we might wish. Currently, there continues to be call upon call to send a mission out to Uranus or Neptune. There is some in the exploratory phases but nothing solidly on the books as we have a spacecraft with funding dedicated and instruments selected.
And that kind of omission would allow us potentially to get a better look at Triton, which is again an exceedingly large moon, and its surface is what we expected to see when we got to Pluto. It has cracks. It has cracks. It has what’s called cantaloupe textured terrain, which is the results of frozen patterns in the surface ice, and the cracks are something like the chaotic terrain that we see on Europa.
This is another world that has geysers that have been seen. We don’t really understand Uranus and Neptune, and only by sending out more missions to the outer solar system, which again is something that’s called for, something that is being explored but something that is not definitively funded.
Only by taking that step and finding it and launching something are we going to be able to make sense of what else is out there.
Now, what is funded, what does have its instruments selected is a mission to Europa. This particular mission, the Europa Clipper, it isn’t going to land. In fact, is going to do a pretty good job of keeping itself fairly far away from the surface. It’s not even going to go quite as close as was done by the Galileo space probe, but it is carrying with it the ability to carefully map out differences in densities. It’s carrying radar equipment. It is going to let us finally answer questions about is the surface ice 10 km thick? Is it 100 km thick? Does it vary from point to point? These are the things we’ll finally be able to answer.
Fraser: I’ve heard that, with this year’s budget, they are considering bolting a lander onto it again.
Pamela: Oh really?
Fraser: Yeah. So, the lander had been removed, and then I’ve heard rumors that potentially the lander might come back, which would be really exciting. I mean, close orbiter that’s gonna go and check out Europa and take pictures of the surface finally, is going to be so great, but to actually have a lander go down and maybe sample some of the material on the surface is going to be great, but just passing through those geysers that are emanating from the surface.
One really fascinating discovery, I don’t know if you would have seen this recently, was that the data for the Europa geysers was in Galileo’s data all along.
Pamela: They’ve recently reprocessed data from the magnetometer using a completely different way of looking at it. They’ve noticed back in, I guess that would have been, the late ‘90s, early 2000s, a weird spike in the magnetometer data, and it was kind of flagged as an anomaly, and in light of the Hubble observations showing geysers we believe at Europa, they went to the data again, and you can explain this anomaly as the effects of flying through moisture. It’s just awesome.
Fraser: Yeah. I mean, and I think we’ve mentioned this in the past. How many amazing discoveries are there in the data? You just don’t know what to look for. And so, it took someone saying I wonder if there’s any confirmation that the data for those guys was in the Galileo mission, because they didn’t know to look for it back when Galileo was there. Otherwise, they would have done more intense readings.
Pamela: And technology changes, and I just earlier today preparing for this mission was looking at Triton images that I hadn’t looked at and probably over a decade, and thanks to advancements and hard work of so many of the amateurs of unmanned spaceflight who might as well at this point be called professional image analyzers, we have whole new views on Triton from the same data that was taken when we were kids.
Fraser: One, I love that you referred to this episode of Astronomy Cast as a mission that you were preparing for, but two, I mean that is a bittersweet concept that what do we know that’s new about Triton, the moon of Neptune, since we did our last episode, and sadly, almost nothing, because there has been no return missions to Triton, to Neptune, to Uranus, to either of those worlds.
They’re super fascinating, and yet we’ve only seen this one flyby from the Voyager and that’s it. And there’s no more plans in the works. As you mentioned, there is no more plans in the works to go back to either of those worlds.
Pamela: And our only new data that we have for any of Jupiter’s satellites comes from Hubble.
Fraser: Yeah. That’ll thing?
Pamela: Yeah. It’s now old enough to drink.
Fraser: I believe it’s old enough to retire. No. It’s – or at least, you know, after it got its glasses, it’s been ’97 was when it got the fix?
Pamela: I don’t remember.
Fraser: Yeah. No. It’s older than that. ’95. It launched in ’95.
Pamela: No. It launched when I was in high school. It was ’91 I think.
Fraser: Oh. Now, we’re googling.
Pamela: Welcome to the land of not knowing everything, which is where he lived constantly.
Fraser: Awake. 1990. You’re right.
Pamela: I told you. ’90 or ‘91.
Fraser: There you go. ’90. So, it’s 28 years.
Pamela: Oh good lord. It’s old enough to have a PhD.
Fraser: Yeah. Yeah. Not old enough to drink. Old enough to rent a car. Old enough to have a PhD. Old enough to, you know, be buying its first home. So, we can’t wait for James Webb.
So, what are some other interesting sort of updates that you found about some of the icy moons in the solar system?
Pamela: I think the most interesting one for me is in some ways the silliest. It’s been probably confirmed, thanks to Cassini’s hard work, after so, so many years and orbits, that some of those rings are indeed being fed by geysers coming off of moon.
It is shown definitively that the gaps we see in the rings are because some moons are more effective than sheepdogs at herding, and we’re also learning that some of these moons are basically rubble piles that are loosely held together by their own gravity.
Mimas, like I said, has cracks all the way through it, and as we look through the moons, we see sponges, we see things like Callisto that are built up from snowball after snowball getting packed together and then packed with cratering over millennia.
This variation and this capability, it’s rich when we say icy moons. That covers everything from small, misshapen things like Pan, which we now know from models, was probably formed from two similarly sized objects combining in such a way that their angular momentum made a ravioli to icy moons include Titan with its rich atmosphere and surface of nothing that exists at the triple point where it is raining, it is gas, it is oceans, it is snow.
We have icy moons like all the other ones we’ve talked about in this episode.
Fraser: It almost feels like think about sort of how much of the solar system is this ice that pretty much once you make it halfway through the asteroid belt, you get to the moons of Jupiter, all the other objects, all the other moons, everything else from that point out is all variations on this kind of object.
And these objects are so far away. So, they’re really hard to study. I mean we’ve only had a chance a few of them up close and really not get a sense of how their similar, how they’re different. That there probably are many different classifications for them, but we just haven’t sampled enough and really explored enough of the outer solar system to be able to know what’s out there.
It’s so far and it’s so hard to get to that is a little scrap of information that we get is sort of – is worth its weight in gold.
Pamela: And one of the most tantalizing things to think about is the asteroids are so close to the sun that they don’t really have the opportunity anymore for most of them to have an ocean or a layer of ice on the surface.
We know that Ceres probably has internal water yet again, the [inaudible] [00:24:57] solar system, but in general, we’re looking at a continuum of objects that go from the small carbon rich, carbonaceous chondrites through to worlds like Pluto that essentially have an asteroid core surrounded by water and ice.
Now, imagine if we took all of these Kuiper belt objects and melted them down to see what’s inside, we might essentially find another asteroid belt. The question is, what exactly is the continuum? How exactly do we understand how all these objects formed in the early solar nebula, given the different temperatures that appeared at different distances?
And are the asteroids nothing more than essentially what could have been Kuiper belt objects that got flung into the inner solar system when Jupiter and Saturn were busy linking things around? It’s going to be amazing what happens in the next decades as we continue to try and understand solar systems by studying all the ones we’ve now found other than our own.
Fraser: This part of the solar system is not gonna give up its secrets easily.
Fraser: Alright. Thanks Pamela. We’ll see you next week.
Pamela: Sounds good, Fraser.
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Duration: 28 minutes