Ep. 353: Seasons on Saturn

You think we’re the only place that experiences seasons? Well, think again. Anything with a tilt enjoys the changing seasons, and that includes one of the most dramatic places in the Solar System: Saturn, with its rings and collection of moons.

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Announcer: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the world’s longest-running online astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser: Astronomy Cast Episode 353: Seasons on Saturn. 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, a professor at Southern Illinois University, Edwardsville and the Director of Cosmoquest. Hey Pamela, how are you doing?
Pamela: I’m doing well. How are you doing?
Fraser: Good. So you’ve got a bit of an announcement. You’re going to be somewhere shortly?
Pamela: I am going to be down on the fine island continent of Australia from about October 2nd to October 12th, so ten days, and I’m going to be doing a dedication talk out at Siding Spring Observatory for the new iTelescope that’s going in and then doing a whole variety of events in the city of Melbourne. They have packed every moment of every day in really awesome ways and I’m hoping that some of you out there listening will have a chance to come out and hear what’s up.
Fraser: That is awesome. So yeah, lots of opportunities to hang out with Pamela. If you live in the Melbourne area this is your chance.
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Fraser: All right so you think we’re the only place that experience seasons? Well, think again. Anything with a tilt enjoys the changing seasons and that includes one of the most dramatic places in the solar system; Saturn, with its rings and collection of moons. So Pamela you’re still digesting the conference that you did in Portugal at the EPSC Conference.
Pamela: The European Planetary Sciences Conference, yeah. It was an amazing meeting and it really gave me the opportunity to sit down and take in a ton of science talks which I – often when I’m at meetings I end up bogged down in the politics of being in this business meeting and that business meeting and this planning session and at EPSC this year I was there as press and I was like dangit, I’m just going to take in absolutely everything I can while I’m there.
Fraser: Yeah.
Pamela: And I took in Saturn.
Fraser: You know what? We’re going to sort of rabbit hole for a second here but I really enjoyed the meetings of the Astronomical Societies, and this is just another one of them, and one of the reasons is because you get sort of the full, deeper news and a lot of the stuff that you learn at these meetings nobody ever gets around to making a press release for. And because the attendance of the journalists is fairly low you just don’t get – a lot of the stuff that you hear at these meetings just never makes it out to any other places. It’s a very strange thing and I quickly learned that that’s where the real news was, was in these presentations by these scientists. I learned to just ignore the press releases and seek my own news.
And actually this is really the third episode. We talked about asteroids and comets. We’ve talked about the presence of water and where we’re all finding it. And now we’re talking about some really interesting sort of updates on what’s happening at Saturn.
Pamela: And what is awesome about these meetings is quite often you get all of the mission teams at the meeting both as their planning meeting and them presenting all of their science in really nice context. So in this case everyone was these celebrating the fact that Cassini has been renewed for four more years and they’ve completed ten years of science and –
Fraser: Ten years?
Pamela: Yeah, isn’t that kind of intimidating to think about?
Fraser: Yeah. I remember being excited and reporting on Cassini arriving at Saturn.
Pamela: And in those ten years they’ve got to see it going from northern solstice, so a northern summer through into equinox in 2009 and now we’re slowly approaching the southern solstice. Over these years they’ve seen storms change, colors change, snow and rain on Titan, and one of the things that really got me is they’ve had more science papers published than gigabytes of data.
Fraser: Wow.
Pamela: So they’ve published over 3,000 papers and they’re only at half a terabyte.
Fraser: So the orbital period of Saturn is 29 years.
Pamela: Yes.
Fraser: So over the course of ten years they’ve pretty much been able to see one third of Saturn’s orbits. They’ve been able to see it go through almost half of its seasonal fluctuations.
Pamela: Yeah.
Fraser: So now with this much data can you confirm or deny Saturn has seasons?
Pamela: Saturn does indeed have seasons.
Fraser: Wow, see this is an update. When we started Astronomy Cast nobody was sure. Does Saturn have seasons or not?
Pamela: I think we felt safe saying it had seasons but I don’t think anyone would have felt comfortable predicting even that the hexagon on the South Pole was going to stay in place or that you were going to get the amazing rain and snowfall that we see on Titan. All of these changes, it’s the details that we’ve found that are truly amazing.
Fraser: Yeah, so let’s talk about some of the events that happened on Saturn that have changed over the duration while Cassini has been there and observing.
Pamela: I think the most interesting one is the winter hemisphere actually changes colors and this is because – well, if you’re suffering winter on Saturn you get a double whammy. On one hand you have the fact that when it’s winter the pole closest to you is pointed away from the sun so the amount of light you get per square meter is reduced because the sun isn’t anywhere near straight overhead. So this is the same reason we get winter and summer here on the Earth. In the summer the sun is close to straight overhead, lots of light per square meter. In the winter it’s down closer to the horizon and the light gets spread out as it comes in at that angle. Less light, less heating: winter.
Well on Saturn you have this extra problem called Saturn’s rings and those rings get tilted as well and act like a sun shield for whichever hemisphere is in winter. And those rings can block a lot of light. So you have this amazing cooling that goes on as you go into shadow and what light does get through is at a lower flux and this actually allows different chemicals to build up in the atmosphere of Saturn and not get destroyed by the UV of the sun and not get destroyed by thermal heating at the same rate that they get destroyed in the summer. And that all adds up to colors that are closer to what you see on Neptune and Uranus.
Fraser: So how much of like a temperature difference do you see?
Pamela: I do not know that detail.
Fraser: Not a specific number, but are you seeing like a really significant drop in temperatures? I mean it’s just a –
Pamela: So what you’re seeing is tens of percent change in the amount of incident light.
Fraser: Yeah, it’s kind of amazing. Like the rings are ten meters thick and yet you get this block that just covers the – that puts the shade down on the planet and so it’s really dramatic. You’re not going to see that really anywhere else in the solar system.
Pamela: And so as you end up with the tilt going from sun incident on the northern hemisphere to equatorial, in which case you get very little shadow anywhere on the surface of Saturn. Things start to heat up and that temperature gradient and that change in temperature as the heat flows from one hemisphere to the other it generated these amazing white, billowy storms that we just saw race through bands in the upper atmosphere of Saturn and it was amazing to see thermodynamics played out at such a large scale.
And with Saturn, because it is rotating so fast for such a big object and to be entirely clear we’re not entirely sure how fast Saturn is rotating. This is one of the great stupid mysteries of the solar system. But, it’s rotating on order of every ten hours and that’s pretty fast for something that Saturn’s rings would just comfortably fit between Earth and moon. And as it rotates so quickly you get these bands that confine the weather within the different turbulent layers. So we saw these beautiful spirally storms confined within a bands billowing around as the seasons changed.
Fraser: Right, but we get that situation here on Earth. As we get the seasons changing we get hurricane season.
Pamela: Yes.
Fraser: We get these temperature gradients and temperatures are moving from one hemisphere to the other and it creates various storms. But in this case the changing temperature is so dramatic that you get a more dramatic – and there’s no land to stop those storms so you get a really serious set of storms that brew up.
Pamela: And what’s kind of cool is in the cases of looking at the moons where we do have the difference between land and lake, especially with Titan. You also see the weather change and you also see the seasonal storms billowing up as you go across these 30-year seasons. So Titan is tilted very much the same way Saturn is and so it was slowly going from northern summer through equinox and now approaching southern summer. And we saw a similar – well, here it was really cold so we’re seeing what we believe was snow, cases where the surface of Titan got shinier. And then went from being shiny to being dry, which we think means that it snowed and then it went straight from ice sublimating out to being dry, which isn’t something we see here on Earth.
Then we also saw instances where it appears to have rained because the soil darkened up. You can, when you’re flying over America over the farming areas right after harvest, you can see the dark land where it’s rained and you can see the sharp delineation between someplace that rained and someplace that didn’t. Well the same dark-soiled lineations have now also been seen on Titan.
Fraser: So I mean I guess when I imagine – you mentioned the Saturnian system but the rings, the whole system, is tilted at an angel – I forget the exact amount, twenty something degrees, and then all of the moos are following that path and so they’re tilted as well. And so that’s how they’re going to get – they’re going to get that same tilt as Saturn goes around the sun. And so it’s kind of amazing that you would get those – well that Titan is right at that point where the season that it makes such a big difference that you would get the liquid methane in this case going from snowing to raining.
Pamela: And it’s not just methane, although that’s the one that we talk about most and it is the bulk issue, but at Titan you have both methane and ethane at their triple point which means that they can both, at various different places, be liquid or solid or gas. And for some reason one of the two hemispheres of Titan has significantly more lakes than the other hemisphere and all of the lakes on Titan added up are about the size of Lake Superior here on Earth, which is really impressive when you think about Titan is significantly tinier than the Earth’s moon.
So you have this little tiny world that has when you add its lakes up lakeage similar to the size of Lake Superior and that lake, as near as we can tell from looking at radar returns, is a mixture of ethane and methane, although majority methane.
Fraser: Did you just say lakeage?
Pamela: Yes.
Fraser: All right. I’m not even going to look that one up. Right, so are there any other impacts of the seasons then?
Pamela: Well it’s not always the impacts that are so interesting as the things that refuse to go away. So with Saturn I hinted earlier that the amazing hexagon that it has on the southern hemisphere also has a massive vortex in the very center of it that’s a spherical goes down several kilometers vortex through the atmosphere. Those show no sign of changing with the seasons so far. We originally saw the hexagon with the Voyager missions. We’re now decades later with Cassini. That hexagon isn’t going away with the changing seasons and so that just starts to lead okay, well why do we only have it on one hemisphere? What’s driving it?
We have models that are okay but we can’t fully get there from here yet.
Fraser: So the original expectation was that this hexagon would shift hemispheres based on the season? That it would wrap up when it was in the southern hemisphere and then show up in the northern hemisphere and go back and forth?
Pamela: Yeah.
Fraser: And that’s not happening?
Pamela: Yes, that’s –
Fraser: Even when the southern hemisphere is what’s directly pointed at the sun as opposed to the northern hemisphere.
Pamela: Yes.
Fraser: Weird.
Pamela: Yes.
Fraser: But we added that to our whole mystery show so –
Pamela: And then you see the things that aren’t effected that much like little tiny Enceladus that has its southern hemisphere ocean and why is that only on the southern hemisphere that we’re seeing this cryovolcanism? Why don’t we see that on the northern hemisphere as well? Is that going to change? So far, it doesn’t look like it’s going to and when you look at the surface features the surface features don’t indicate that there was some past great north expanse of cryovolcanism. And so there’s lots of little enigmas.
Fraser: And so again, with Enceladus with these tiger stripes and the cryovolcanism I guess the expectation would be that whichever hemisphere was in the light, was experiencing summer on Enceladus that’s where the [inaudible volcians] would be happening?
Pamela: Well there is actually no real anticipation that we are going to find cryovolcanism on Enceladus. That was just one of those “whoa, where did that come from” moments of how is it something that tiny ends up having that much tidal heating? We weren’t quite prepared for that and now we’re trying to make sense of what we see and it’s kind of awesome to have puzzles.
Fraser: Yeah. More data needed.
Pamela: Yes. And as we move into the next few years with Cassini it is going to plunge through one of those erupting jets and capture material out of it. And so far we’ve been able to have 132 close flybys of Saturn’s moons, which over ten years doesn’t seem like that many but it’s orbiting – Cassini is orbiting with an orbit not too different from the moon’s orbit around Earth. So you have to think on the moon goes around once a month and so Cassini has only completed a few over 200 orbits in those ten years.
So figuring out how to do 132 close flybys across 200 orbits is some pretty amazing orbital calculating.
Fraser: And the fact that it’s been able to sip its fuel to be able to make those little burns from time to time to get close pass.
Pamela: Yeah, it’s really amazing what they’ve pulled off with that spacecraft.
Fraser: Now – I mean this is a total distraction but Cassini is not equipped to deal with – like it was never meant to collect particles of ice from –
Pamela: Actually, it was.
Fraser: Really?
Pamela: Yeah. So the anticipation was that it would be collecting ring particles when it flew near the rings, it would be collecting interstellar particles. And so it actually does have particle collectors on it. So what’s cool was they had anticipated that they’d essentially get to be sampling a lunar sea, in this case Enceladus, because it has what appears to be a subsurface ocean, cryovolcanism. We’re getting water samples from Enceladus without having to land there.
Fraser: But it won’t be able to detect life?
Pamela: No, but it will be able to detect molecules.
Fraser: Yeah.
Pamela: Saltiness.
Fraser: What about the rings? Do the rings have any change over the seasons?
Pamela: We haven’t so much seen seasonal changes to the rings as started to get an understanding of how – I don’t know how to say this without punning so I’m going to go for it. The impact of meteors hitting the rings was more than we expected. So you actually get as a meteor or comet – we can’t identify what it was that hit the rings. As something hits the rings it will break apart and over the course of multiple orbits of that stuff in the rings it generates ringlets and wave formations and so you have the stuff that was impacting the rings changing the local dynamics of the rings for quite some time and that’s just really cool to watch.
Fraser: Now how long is Cassini going to be orbiting for? When’s it done?
Pamela: So we have about four more years. Three more solid years of science and then it starts this amazing set of plunging orbits until eventually it just plunges into the surface of Saturn.
Fraser: So what, 2016? 2017?
Pamela: Cassini is going to keep going until 2017, and during that final year it’s going to enter a more and more elliptical orbit going out to greater distances and then working to plunge its way through the rings, working to get very close to the surface. And one of the frustrating things to a lot of people is that it would be kind of amazing if, as it goes through these final – basically orbit 250 through death, which will be around 260 something, probably 265, could go as high as 270s. As it approaches these end of life orbits it’s going to eventually start potentially getting damaged more and more.
So they’re going to turn it to have its big antennae dish in front of it as it plunges through the rings. They’re going to do everything they can to keep the spacecraft going, but as it hits 293 it’s going to be getting down in its orbit to 50,000-60,000 miles above the surface and it’s going to get turbulent and probably going to start tumbling. With the high speeds and the tumbling that is potentially going to go on it won’t be able to send back radio data.
So there’s this huge, amazing potential for it to collect awesome data, but it can’t send it back and so that’s one of those bittersweet things.
Fraser: I’m sure they’ll figure that out.
Pamela: Well it’s actually a problem of time speeds. We forget how long ago they had to start building Cassini. So it has one of the first ever solid state drives and it doesn’t hold that much data. Its antenna is good but it can’t send back information that quickly. And if over ten years we’ve only gotten back half a terabyte in those few days it can’t send back all the data we might want it to collect.
Fraser: It’s too bad that it won’t make it for a full – I guess will it be pretty close? 14 years? I guess by the time it’s done it will have almost made it halfway through Saturn’s full seasonal orbit?
Pamela: And that was the goal, to keep it going as long as they could and do the last final run by the inner moons as they head towards death. So they’ll go by Janus, and Pan, and Pandora, and Epimetheus, I’m not quite sure how to say that, all as it plunges in towards death. They did everything they could to get as much science out of it as they could, but they just couldn’t guarantee the spacecraft much beyond that and with so much potential for life we can’t risk accidentally crashing Cassini onto one of these moons and potentially bringing a plague to what life might be there.
Fraser: And it’s also too bad that there isn’t something there to pick up where Cassini left off, to watch the other half of this orbit because this first half you make all of these measurements and all of these calculations and you gather all this data and then use that to make your models and then you want to verify those models against reality as Saturn essentially goes through the next set of its seasons. You’ll see we should expect Enceladus to do this, we should expect Titan to do that, we should expect the storms on Saturn to do this. And you’re not going to have those close-up observations to actually watch these things unfold the way we gathered them in the first place.
Pamela: And this a multifaceted problem. We brought up some of this last week where we have issues of we don’t have the money to build a new spacecraft. We’re not actively in the process of building the spacecraft and it takes years to get out that far. And even if we did have the funding and the will we don’t have the plutonium to fuel a radio thermal generator, and out at that distance you can’t use solar power very effectively.
Fraser: Yeah. All right, well thanks Pamela.
Pamela: It’s been my pleasure.
Fraser: Thanks 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 at astronomycast.com. You can email us at info@astronomycast.com. Tweet us @AstronomyCast, like us on Facebook or circle us on Google+.
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