And now Cassini’s gone. Smashed up in the atmosphere of Saturn. But planetary scientists are going to be picking through all those pictures and data for decades. Let’s look back at some of the science gathered up by Cassini so far, and we can still learn from this epic journey.
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David Joseph Wesley’s Goodbye Cassini video with Fraser
NASA Cassini Mission
Cassini Science Overview
Saturn’s moon Titan
ESA Huygens Probe landing on Titan
Seasonal Changes on Titan
Saturn’s moon Enceladus
Saturn’s moon Iapetus
The other moons of Saturn
Cassini Raw Data Node
Transcription services provided by: GMR Transcription
Fraser: Astronomy Cast, Episode 458: The Science of Cassini. Welcome to Astronomy Cast, our weekly facts-based journey through the Cosmos, where we help you understand not only what we know, but how we know what we know. My name is Fraser Cain. I’m the publisher of Universe Today, and with me is 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 you doing?
Pamela: I’m doing well. How are you doing, Fraser?
Fraser: A little sick, so people are gonna hear that in my voice. I’m a little – I got a bit of a cold. So, I’ve got one little bit of self-promotional thing to do here, which is that not only has Astronomy Cast returned, but the Weekly Space Hangout has returned to grace your earballs. And we are doing this show at a different time live, but we’re also putting a lot more emphasis on the actual podcast version of the show.
Morgan, Kimberly, Paul, all doctors of their respective fields are now permanent co-hosts of the show. We’ve still got special guests. It’s a really good time. So, if you wanna hear the news portion of the space, then make sure you do a search for the Weekly Space Hangout. Join us live. We do it at 5:00 on Wednesdays now. But also, the podcast is well worth listening to, so check that out.
Pamela: And that’s 5:00 Pacific time zone on Wednesday, 8:00 Eastern.
Fraser: That’s right. And you’ve got anything self-promotional that people should be aware of –
Pamela: Not today, but the other thing that you need to promote, which actually goes a long with this show, is you worked with David Joseph Wesley to do a piece commemorating Cassini, that is truly beautiful. And can you share its URL with our audience?
Fraser: I can, but it’s gonna be like YouTube.blahblahblahblerblerbler, you know what I mean? So, yeah. But you’re exactly right. So, David Joseph Wesley, friend of the show and composer for Hollywood – he does movie trailers and video games – and he let me know that he was composing an album just for Cassini. And I said, “That’s awesome. Why don’t we edit a video together to go along with some of your music?” So, we made a deal at the even in St. Louis and then put it live. And we actually posted it the seconds after we got the last images from Cassini. Actually, Chad did all the editing work.
We made things a little different. I wrote all the text, but we put it as closed captioning so you could actually turn it off and just listen to the music and watch the video. And if you want some of the underlying explainers and science, you can turn on the closed captioning. So, it’s on my YouTube channel, and fairly recent, it’s called Farewell, Cassini: The Grand Finale and the Final Images of Saturn. Check that out. It was a lot of fun and David did an amazing job.
Pamela: And I just love working with artists who can give us such a different insight into our science, so everyone go play with your brains in a different axis.
Fraser: And now Cassini’s gone. Smashed up in the atmosphere of Saturn, but planetary scientist are gonna be picking through all those pictures and data for decades. Let’s look back at some of the science gathered up by Cassini so far and what we could still learn from this epic journey. Where should we start? So, I think the point of this episode is to just kinda look back and refresh – you’re all feeling sad about the loss of Cassini, but let’s just remind you of all of the big scientific discoveries that were made over the course of the mission, and some of the things that are even just works in progress that we can continue to understand. So, do you wanna start from a timeline standpoint and just go back to the beginning? Because, I mean, Cassini was doing science even before it got to Saturn.
Pamela: Yeah, that’s entirely true. And they didn’t just do science of Saturn – I mean, they did that, of course, but they also captured images here at Earth on a fly-by, they captured images of Jupiter with this somewhat iconic photo of Io transiting across Jupiter on January 1, 2001, so we have this mission that has studied everything from asteroid 2685 Masursky through to the most detailed facets of the rings of Saturn.
Fraser: And with Cassini, they actually didn’t pass super close to Jupiter, so it wasn’t able to take those same level of quality images it took when it did the Saturnian System, but still it was sort of one the most powerful spacecraft and most well-equipped to pass through the Jovian System on its way to Saturn. So, did it contribute some of the science about Jupiter?
Pamela: Well, one of the things that it really did was it gave us that full world view that you don’t always get when you’re orbiting up close. So, if you think about it, if you’re orbiting close to Jupiter, it’s going to fill up your entire field of view, and there’s no rational non-fish-eyed camera that’s going to let you capture all of this at once.
So, by flying at a fairly good distance, they were able to get the big picture, and with Jupiter, the big picture means you can study ring dynamics, you can study the atmosphere, and they were able to start to get really neat insights into how that atmosphere is circulating, what is causing this alteration between the dark belts – these are the ruddy-colored ones that are either orange or dark flesh-toned depending on how you want to look at them, and what filters you use, and those white or ivory colored bands. So, we studied the belts because we could, and we had a full world view back in 2000.
Fraser: And I think some of my favorite images of Jupiter come from Cassini, especially as you said, those images of some of the moons. It got this great image of Io just transiting right in front of Jupiter. I mean, I think at this point now, we’re up to 450,000 images came back from Cassini. 450 – I forget the exact number, but it’s enormous – came back from Cassini. 25,000 of them were taken in the Jupiter System. It just took an enormous number of pictures as it made that fly-by, but this is just the beginning. So, let’s talk about actually some of the science that it did as it made its way into the Saturnian System.
Pamela: So, one of the things it did that we don’t’ pay enough attention to is it studied not just Saturn itself, but it also studied gravity and the general theory of relativity, so here we have the mission and its lead up getting to Saturn. It’s looking forward, it’s looking for deviations like the ones that we saw with Viking and Voyager. It’s looking for the curvature of space, and while it didn’t help us figure out what is up with the Voyager time anomalies, which we’ve done entire episodes to, they did show, once again, relativity is right.
So, just add one more notch to relativity. And that really starts to get at the end of the non-Saturn science, so we have an asteroid, we have the moon, we have Jupiter, Jupiter’s moon, the general relativity, and then all things Saturn. All things Saturn.
Fraser: All things Saturn. And I remember, like, I really remember as Cassini was coming into the system and we got that first look at its first moon – was it Phoebe? I’m trying to remember which was that first moon that it saw as it was –
Fraser: Yeah, it was Phoebe. And I know Voyager had never gotten that close to Phoebe, so it was just amazing what those first images were. It was so much better than anything we’d ever seen before.
Pamela: And this isn’t a big moon. This is a little, tiny beat-up potato of a moon. And it’s not a beat-up potato like an asteroid is, it’s instead made up of ice as well as rock. And so, you have these places on the surface that are pure white where they’ve recently been revealed, and this was when we first started to realize – well, I mean, we’ve suspected – but it was when we were first starting to get really clear evidence that some of these moons are essentially the stuff of comets, but orbiting as moons.
Fraser: Right, and so you’ve really got this evidence that Phoebe was this captured comet or asteroid. Astercomet? Comrasteroid?
Pamela: Well, and this was where we start to get down to we’re realizing more and more every year that there is a continuum between completely rocky objects and completely icy objects, and I’m not sure we found an extreme of either yet because the harder we look at the earthy things, the more volatiles we find, and, well, we keep looking at icy things that have gravel. So, with Phoebe, what we found is a large amount of water ice that is part of an asteroidy moon, so yeah, comet asteroid.
Fraser: Of course, we mentioned at the beginning of the show that we did this video, and so all of these timeline milestones are kind of fresh in my brain is that then as it came into the system, the Huygens Probe, which was this European probe, detached and it was ready to do its mission to get to Titan. Now, do you wanna talk about the fly-by sides of Titan or do you wanna talk about the science done from Huygens first?
Pamela: Well, it all sort of tells one interweaved story. So, you have the summer of 2004 is when we get that first amazing fly-by of Titan. There was surface imagery. There was radar imagery. And this was when we were starting to see, “Oh, wow. There are lakes of methane that appear because of the difference in gravity and the difference in temperature to geologically act very similar to water lakes here on Earth.” So, we’ve said it before, we’ll say it again, water here on Earth is at that magical triple point where various points, various times, you can have water vapor, you can have icy water, you can have liquid water.
With Titan, you have liquid methane, gaseous methane clouds, fog, and then you have methane ice, and this triple point cuts away at the low gravity surface and you end up with, well, as they found that summer, lakes of methane. And six months later or so, when in January of 2005, Huygens descended through those orange thick clouds, it was off to see just how much it could image before all the things that naturally demise a spacecraft took over.
Fraser: So, let’s talk about that. One of the most powerful images from that mission was this great animation that the folks from ESA did showing Huygens going through the cloud tops and being able to come down to the surface of Titan, and you could see the mountains and the landscape as it got closer, and closer, and closer, and then pebbles on the surface as it landed with a thump on the Titan surface. And not a splash into the methane ocean, which it was prepared to do, and then it gave us 70 minutes of science before its batteries ran out.
Pamela: And what really got me was this is clearly some sort of a river delta region, and if any of you out there fly a lot, you know that when you look down over a landscape that either has rivers cutting through it or has had rivers cutting through it, you end up with these thick channels and then all the tributaries coming off of that. And the reality is the tributaries are feeding into that main waterway.
And we see the exact same tributaries and main waterways when we look down at the features that Huygens saw, and coming in for a landing, it’s this amazing landscape where you have to remember what scale size you’re dealing with because it just looks like this boulder field stretching off into the distance until you look and realize, oh. Those boulders are just 10 to 15 centimeters across, so this photo is basically the same thing as setting your camera down on the slightly rocky shore of a river and catching all those rounded river stones in your photo, and these are rounded river stones.
Fraser: Yeah, yeah, and one of the big goals of the whole mission really was that they didn’t know – because the Voyager wasn’t equipped to be able to see – didn’t have that ground-penetrating radar – wasn’t able to see through that haze, that hydrocarbon haze on Titan. There was no idea what was below this surface. There was just it’s kinda yellowy-brown thick atmosphere. That’s all we know. Maybe there’s some kind of hydrosystem going on there. Maybe not. We don’t know. And Cassini went with the right tools to be able to see both to land on the surface and to be able to see the whole world from above.
Pamela: And what’s cool about having such a thick atmosphere and such a low gravity field is Huygens didn’t exactly drop like a rock, it rather kind of – it didn’t float and flit, but it did descend nice and gradually giving us more than two and a half hours of transit time down to the surface, transmitting the entire time.
Fraser: That, and then we can come back around later a bit, but we learned because Cassini was able to make so many fly-bys and orbits of Titan, each time getting closer, and closer, and closer, it was able to make all of these amazing findings about this system, about these methane seas on Titan, so it’s just some of the science that was returned by Cassini.
Pamela: So, there was roughly 50 fly-bys of Titan over the course of Cassini’s many years out at Saturn, and it was able to discover seasonal variations. One of my favorite discoveries of all is they saw these storms billowing up as the seasons changed on Titan as the spacecraft went from its equinox mission over to its solstice mission. And with this change in seasons and this arising of storms, they saw the surface in various parts go from being all shiny as though it had been snowed upon to being normal, which kinda means it snowed and then sublimated, and that is awesome.
And in other places, it got shiny and then dark, just like it snowed and melted. And it’s really awesome to be able to look at another world and see changes in surface color that appear to correspond to the same sorts of phenomenas that we see here on Earth. The storms being seasonal, that completely makes sense, but we saw it, and we just like to see these kinds of things. And we’re still baffled by the chemistry of Titan. The chemicals don’t seem to be in the kinds of equilibriums we expected, the lakes don’t have the ratio of methane to ethane that we expected.
It’s forcing us to rethink chemistry and try and figure out what’s going on and dream of days that we can sterilize a spacecraft enough to justify sending it down and doing long-term explorations to try and understand what’s going on on this little world.
Fraser: One question that was brought up, and this is me completely rabbit-holing, which is that the death of Cassini was done because they didn’t wanna have this – we talked about this last week – they didn’t want the spacecraft to crash into one of the icy moons and potentially pollute it with life, Earth life, and yet Huygens went to the surface of Titan and sat there, and it’s possible that there’s a whole other level of subsurface ocean that isn’t icy ocean like you have on Enceladus on Titan, and that’s a bit of a – that’s a whoa, why’d that happen?
Pamela: Well, and who knows what sort of methanogens – or ethanogens? I don’t know if that’s a thing – could exist in Titan’s lakes. This basically comes down to we sent a spacecraft and realized, oh, expletive. It might be capable of sustaining life out there and yeah, mistakes – I don’t know if it was mistakes. We learned something. We learned something, and we didn’t have a robot that roved. It’s the Rovers that are more problematic. So, yes, there’s potentially one place where there’s little tardigrades going, “I’m here in my chrysalis,” waiting for the opportunity to emerge. Hopefully, there aren’t little methanogens native to Titan going, “Oh, god. We’re dying.”
Fraser: Right, because the tardigrades plant their little flags in the Titan methane sands. So, let’s go on to Enceladus then, which is sort of one of the next – and again, when many, many, many fly-bys of Enceladus and bits and pieces figured out over time, so let’s have some the highlights of what was found at Enceladus.
Pamela: It’s hard to know where to start other than that sucker’s got water. And each time they went by, once they realized there were ice geysers, more and more was looked for and more and more because we looked was discovered. This is the awesome thing about having these multiple attempts at going by is you can do refined science. So, you start with, “Oh, wow, geyser.” Okay. Go back. Try and get sample of geyser with the dust capturing instrument. Try and measure differences between the internal rotation and the surface rotation. Try and get all of these measurements that in different ways build up a picture of what is the ocean, what is it made of, and how extensive is it.
And through combining passage after passage and all the different instruments that Cassini had to offer, we were able to build up this model where it appears that Enceladus has extraordinarily salty seas that allow the surface to not be coupled to the rocky core, which implies that the oceans are expansive, allowing the surface to essentially float on top of the ocean. And it’s largely because of Enceladus that we had to kill Cassini, deorbit Cassini violently.
And one of my favorite things about the mission, and I don’t even know if this was part of the intention, but I really wanna find someone to ask is one of the last photos to come back was of Enceladus basically setting behind Saturn. So, here’s this picture of Enceladus setting that’s one of the last things to be seen by Cassini that burned up that whatever life may exist on Enceladus can continue to exist and not accidentally get killed by Cassini, so it’s just this terrible poetry of “I’m dying for you, and I stare at you as I plunge into the surface”. And I don’t think that was what was intended, but that’s where the “worked on the high school literary magazine” part of my brain went.
Fraser: We can kinda fast forward to some of the most recent observations, which was just this stunning discovery that not only does it have these geysers, but in fact there are these hydrogen molecules –
Fraser: No, but hydrogen, like food for methanogens or for various extremophiles is actually percolating around – it’s probably being released from vents at the bottom of the ocean and is making its way out into space where Cassini was able to sample it, which is the most exciting discovery for astrobiology in the solar system.
Pamela: And it’s just one of so many worlds where we know that Europa has even a greater volume, so when you start realizing this is just the first one we’ve looked at in detail, what else do we have to discover when we start going through the geysers that are associated with the hot spot on Europa with the next mission we send that direction? This is just a tantalizing start to understanding these icy worlds.
Fraser: Yeah, yeah. More spacecraft necessary. Let’s move on. So, do you know what my favorite moon is? Do you remember?
Pamela: I think it’s Iapetus.
Fraser: It is Iapetus, yeah. And I love the way Iapetus looks. It has this dark side and this light side, and it has this ridge that just looks like it’s straight out of some kind of science fiction movie.
Pamela: And it has these dark splotches along the boundary between the light side and the dark side, and the splotches – they’re so black that they kind of look like “and the great nothing ate this part of the moon”. And it’s this fabulous case of light and dark have different thermodynamics, so as this little moon goes round and round Saturn, one of its sides happens to be picking up all of the organic darkness – dark smudge, sooty, complex organics – and I don’t mean sooty as in terms of burning, but I mean it in terms of the structure of the molecules, and that material builds up on one side of Iapetus.
It heats up a little bit more than the light side does, so the volatiles evaporate away, go, and then settle down, and rebrighten the white side where it can happily freeze back, so you have this volatile cycle going on. And it’s just super cool and this equatorial ridge – that one is more mysterious. We think maybe it’s just because its where it tends to accumulate stuff and things, but that model doesn’t totally work. It’s weird.
Fraser: Right. It’s where it ate a ring. Maybe.
Pamela: Maybe, yes. We don’t fully know. It’s awesome.
Fraser: More research necessary. Time to send another mission back. But again, these are just a couple of them. We’ve got Iapetus, my favorite. You’ve got Rhea, and Dione, and Hyperion, which looks like a sponge cake.
Pamela: Yes, and then you have the ravioli, otherwise known as Pan.
Fraser: Is that your favorite? I don’t know.
Pamela: It’s the most endearing. I don’t really have a favorite of Saturn’s moons because part of me is like, “Titan. It’s so cool. The chemistry doesn’t make any sense, and chemistry doesn’t make any sense anyways.” Then there’s Hyperion, and then Mimas, of course, looks like the Death Star, so who doesn’t love Mimas. But Pan looks like a ravioli and we always record after lunch that I have skipped prepping for the show, so it always comes up.
Fraser: Right, and where you come from is known for its fried ravioli.
Pamela: It’s true. It is true.
Fraser: I’ve discovered that. There are a couple things that I thought were pretty great with Saturn as well. One is on the planet itself, in the mid part of the mission – because as we said earlier, it got a chance to see the seasons move through the planet. It got to see this amazing storm appear in the northern hemisphere in – what? 2010-ish? 2012? Sort of halfway-ish through the mission.
Pamela: This is one of the great things about how the mission kept getting extended is first it saw this fabulous moment of the sun being edge on to the rings around equinox, and then as we came out of equinox heading towards solstice past 2009 – 2009 was the most boring year to ever look at Saturn – heading into 2010 and beyond, as the seasons changed, there was this storm that was first noticed by amateur astronomers because you can see the full disk, and it slowly spread across the mid-northern latitudes and just spun up, creating a while belt of storm, and that was just awesome.
And it showed that you have this rich complex chemistry. You have thermodynamics at play. You have all of these different materials that when they change what ratio is where in the atmosphere, you get changes in color that go with the storms.
Fraser: For the Cassini mission, NASA put together this – for the grand finale, every year they would put together these Top 10 in Science discoveries, and it’s like choking on a fire hose. I mean, it’s discovery of an ocean of water on Titan, as I mentioned earlier, underneath that methane crust, underneath the ice crust. The discovery of even on Titan, this southern polar vortex that changes with the seasons. The source of the rings, which were where some of its rings came from thanks to some of the moons. Man, there’s just too much.
Pamela: The hexagon that changed colors! So, there’s this crazy – I think we did an episode on it, or at least we talked about it extensively earlier –
Fraser: One of the mysteries of –yeah.
Pamela: There’s certain things that Cassini was supposed to figure out and couldn’t, like how long a day is on Saturn. We do not know this precisely.
Fraser: Still don’t know that.
Pamela: No, because the radio source that we used with Voyager apparently moved in altitude or Saturn changed its day length by six hours, and that didn’t happen. So, yeah, Saturn is awesome and confusing and now doesn’t have an orbiter. And one of the things we haven’t brought up yet is Cassini was built on a legacy of the Mariner missions, of the Voyager missions, where this was tried, and true, and proven hardware that had been used for explorer after explorer after explorer, and it’s the last.
It’s the last of that line, and we don’t have any great explorers built on this same hardware platform that’s so tried, and true, and awesome planned for the future, and we don’t really have a lot planned for the future. So, we’re glad we have so many gigabytes of data because we don’t know we’re gonna get any again.
Fraser: And I think as its final project – we talked a bit of the death of it and how that was coming together. Earlier this year – was it back in April? – they changed its trajectory to do something very risky, that it was gonna pass in between the planet and its ring system, taking images as it went of both the planet at a closer level than it had ever seen before, but also the rings from a perspective that it had never seen before passing above and below, and above and below, and that science – I’m sure we haven’t heard the last of that part of it yet, that as its final act, it gathered all of that amazing – all of those data of the rings, and we’re gonna see that coming out over the next planetary society meetings and planetary space meetings over the next decades.
Pamela: And part of it is we still are working to build powerful enough computers to fully understand the dynamics of the rings and the density of the material, and we’re just getting to the point that we understand that just like the density of water in the atmosphere and the opacity of the atmosphere don’t always have a one to one correlation. You can look through a rainstorm easier than you look through fog, even though the fog isn’t as dense as the rain in terms of particles of water per square meter. We’re finding that the rings of Saturn, the places they’re most opaque aren’t the places where they have the highest particle density or density of material per square meter. And having that kind of information changes how we do our models.
We’re starting to have better and better understanding of where are all of the gazillion moons? They still haven’t even named all of the ones that Cassini found. There’s one lone little moon that still has a license plate instead of a name. And as we understand the origins of the rings, as we understand the shepherding moons of the rings, as we understand the densities, as we understand how gravity waves propagate through them, how you get these spoke structures, the how we get to the as we understand part is as we build bigger and better computers that can run more sophisticated computer models will eventually get the theory to match the data. But we’ve got the data.
Fraser: Yeah, we do. And all of those images, all of those data – it’s just an enormous amount of material that is back. One thing that I really love when people are like, “Where are all the real pictures from Cassini?” I can point people at the reverse dated raw archive of 450,000 pictures of Cassini that you can just take, download, process, do what you want. Go ahead. All that data – it’s all there.
Pamela: And a shoutout to the unmanned space flight community because they have people that have figured out how to do amazing, masterful processing, and in the final days of Cassini while the science team and the imaging team were all up to their eyeballs in making sure they were getting every last ounce of information out of all of the storage units and memory and downloaded to Earth, while they were working on the spacecraft, it was these folks that were sending out the last and final pictures and their full color glory to allow us to see one last time through Cassini’s — well, not eyes, but through its cameras.
Fraser: Absolutely. Alright, well, thanks, Pamela, and thanks, Cassini. We’ll see one of you next week.
Pamela: I’m hoping it’s me.
Fraser: I’m sure it’s you.
Pamela: Okay. Okay. Just checking.
Fraser: Alright. See you later.
Male announcer: 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 can email us at info@AstronomyCast.com, tweet us @AstronomyCast, 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 missed the live event, you can always catch up over at CosmoQuest.org or on our YouTube page.
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Duration: 34 minutes