January 10, 2018
The sixth planet from the Sun, Saturn is a gas giant that’s best known for its striking icy rings. How old are the rings, and how are they changing over time? And what’s up with Saturn’s mysterious hexagon-shaped storm in the north polar region? In this episode of Gravity Assist, NASA’s Planetary Science Director Jim Green talks with Cassini Project Scientist Linda Spilker about the ringed planet, its fascinating moons and propeller-like features, and the most startling discoveries from NASA’s Cassini mission.
Jim Green: Our solar system is a wondrous place with a single star, our Sun, and everything that orbits around it - planets, moons, asteroids and comets - what do we know about this beautiful solar system we call home? It's part of an even larger cosmos with billions of other solar systems.
Hi, I'm Jim Green, Director of Planetary Science at NASA, and this is Gravity Assist.
With me today is Dr. Linda Spilker from NASA's Jet Propulsion Laboratory. She's the project scientist for our Cassini Mission, which as everyone knows, had a recent spectacular finale at Saturn.
So, Linda, have you caught up on your sleep, and what's happening next to all the Cassini data?
Jim Green: What in your mind were some of the most startling discoveries from Cassini?
Linda Spilker: Well, with Cassini, probably one of the most startling discoveries was to find actual geysers coming out of four fractures at the south pole of this tiny moon Enceladus, only 300 miles across. We thought it should be frozen solid, and yet we had tantalizing clues from Voyager, this bright icy moon, not many craters, right in the middle of this ring called the E ring, what could be going on, and then from the geysers, figuring out that there's a global ocean of liquid water underneath Enceladus' icy crust, that that ocean is salty and has organics and other key ingredients that might make it possible to have life in that ocean on Enceladus, even hydrothermal vents on the sea floor. We found tiny nanosilica grains that could only grow in very hot water and excessive hydrogen, all of these clues pointing to these hydrothermal vents.
And on Earth, you have teeming life around these vents deep in the Earth's sea floor--far away from sunlight. So, we wonder could there be life possibly on Enceladus?
Then of course, Titan, giant Titan: the size of the planet Mercury with a thick nitrogen-rich atmosphere and landing a probe on the surface of Titan and unveiling this world that was covered with this photochemical haze, like Los Angeles on a smoggy day, and we couldn't see through to the surface with Voyager. But, with Huygens, we could see the surface, river channels, icy rounded pebbles. And it turns out that liquid methane--that methane plays the role on Titan that water plays on the Earth.
You can have methane clouds, methane rain, methane flowing through river channels, filling lakes and seas and, hey, you know, could there be something very unusual going on in those methane seas on Titan?
And Titan has a global ocean of its own of liquid water underneath its icy crust, so two ocean worlds at Saturn--what a remarkable discovery.
Jim Green: You know, it really was. Those two bodies alone really started a whole new way of thinking in planetary science about having habitable areas around giant plants. You know, who would have ever thought that prior to those kind of discoveries?
I mean, Europa was kind of an anomaly, but it had, you know, the other Galilean moons tugging and pulling at it. And so, it had some different circumstances. But, boy, Titan is just such a fabulous body.
Linda Spilker: Yeah, so many interesting moons at Saturn.
Jim Green: Yeah. What are some of your favorite moons?
Linda Spilker: Well, I like, of course, Enceladus because it's so interesting with its global ocean, but also there's Iapetus that has one bright and one dark side, and over the course of the Cassini mission, finding out that the dark material is actually Phoebe dust, this moon captured by Saturn far away from the planet, bombarded by micrometeorites and that the dust comes in and gives Enceladus a dark side.
And then how about Mimas with that giant crater, and actually, if you look at the temperature data, what looks like a Pac-Man on Mimas. Or places where electrons hit the surface of Mimas and cause the icy particles to fuse together so it heats up and cools off more slowly, and you get a very interesting temperature map of Mimas, this lens shape where the electrons hit and it looks like a Pac-Man, ready to gobble up something else as it goes along.
Jim Green: You know, you can't see that kind of stuff from Earth's telescopes. You really need to go there and spend some time orbiting those large planets. And Saturn has just been so spectacular. And of course, its rings are just such an attraction for us. What have we learned new about the rings from Cassini?
Linda Spilker: Well, in the Voyager days, I worked on Voyager, and we had this idea that the rings were individual particles gently colliding into one and other and that there were these waves in the rings created by interactions or residences with the moons. But, with Cassini, we've just unveiled the rings in a totally different way, that many of these particles stick together and form long linear clumps in the rings, that these clumps form and break apart and form again, and that most of the rings are made up of these very interesting clumps, and with Cassini equinox to see the three dimensional structures, that these rings that are 30 feet thick, perhaps even three feet thick in places, as seen by Cassini, that these larger objects may be half mile or a mile across stick up above and below the rings like giant icebergs. And we saw their shadows when the Sun was edge on to the rings, just saw incredibly fascinating to watch this in action.
Then these propeller objects--these large particles that are not quite big enough to open up a gap in the rings--we have two gaps in the rings that are created by moons, but these are not quite big enough, so it looks like two arms of an old fashioned propeller. Or on the outer edge of the A Ring this bright feature that pointed to an object that was large enough got a nickname Peggy, that perhaps Peggy was forming and might break free of the rings and become a moonlet in her own right.
Jim Green: You know, one of the really great images that I remember so well, Saturn, of course, is the eclipse where Saturn itself passes between the spacecraft and the Sun. You know, we just had an eclipse here in the United States, and so many, many people understand that basic concept. But, when we're out in space and we have objects like Saturn moving in front of us, gobbling up the whole Sun, the rings just pop out. And we discovered some new things. What did we find?
Linda Spilker: That's one of my very favorite pictures, Jim, because you can see all of Saturn's rings in that particular image, and since the Sun was covered up, we could take our cameras and spectrometers and look very close to Saturn, very high phase angles, we discovered that many of the tiny moons have rings in their own right. And so, if you have a tiny moon, particles get thrown off and create rings. So, we discovered a number of new rings in that particular image, as well.
Jim Green: What I really loved about the proximal orbits when we went below the rings is that there doesn't seem to be a lot of material falling out of the rings, but yet the rings have got to be dissipating somehow or another. What do you think is happening?
Linda Spilker: Well, Jim, one of the most amazing surprises, and it was good for Cassini, is that in that gap between the rings and Saturn, the particles are just tiny nano-grains - in other words, very safe for Cassini to dive through that gap. But, we were surprised. We were expecting that the ring particles of all sizes from the nano-grains to microns on up to perhaps even, you know, a half a centimeter might all be in that gap.
And so, what happened to those particles? What processes grinding them up were removing a lot of the water ice to leave just these little dust-size or smoke-size grains inside of that gap, and then how they fall into the planet? And these nano-grains seem to be distributed above and below that equatorial plane of the rings. We've been mapping out their distribution, but that's a puzzle. What happened to the big particles?
Jim Green: Well, does that tell us that we're in store for having the rings last a very long time around Saturn?
Linda Spilker: The rings might last for a longer time, but there's still some process because those ring particles we know gradually, like pouring water on a table, they're sort of flowing towards Saturn. There's nothing to stop that. But, what is grinding them up or what process perhaps is holding the bigger ones in place even? And if that's true, then perhaps the rings could last longer than a mere 100 million years.
Jim Green: Yeah, it's just absolutely spectacular because several of those (images) actually have Earth and Mars and Venus just on the other side of the solar system where the Sun is illuminating, and we can see them in images by Cassini—really beautiful images
Linda Spilker: Yeah, and one of those pictures where you could see Mars, Venus and the Earth and the moon, I love it because we asked all the people here on Earth to go out, wave at Saturn in that 20-minute window so that the photons from their waves could be captured by the Cassini cameras and then asking people to send us their selfies, take a picture of yourself waving at Saturn. We took all those pictures and recreated that mosaic out of people pictures.
Jim Green: Well, you know I was one of them. I was out there waving.
Linda Spilker: I was, too.
Jim Green: Pretty spectacular.
Well, you know, the recent observations of the rings from a new perspective, and that's the proximal orbits where we flew just below the rings and above the atmosphere, gave us some new information about that and in particular about the mass of the rings. What can you tell us?
Linda Spilker: Well, we've been studying--in that gap between the rings and Saturn, we could make very precise measurements of Saturn's gravity and of its mass. Then we had information from outside the rings in Saturn. So, we could take the difference between the two and get the mass of the rings.
And it's turning out to be trickier than we thought that Saturn's gravity and its mass distribution is not at all what our models predicted, very different and that Saturn is very different from Jupiter in this regard.
So, we're trying to figure out Saturn's gravity and mass first. We have a mass for the rings. It looks like it's a little bit less than we expected, which means the rings might be a little bit younger. But, the error bars are huge. So, we still don't have a very precise number, and we're trying to work hard to calibrate the data, get the best orbits to fit all of it together and understand Saturn first, take out Saturn and then get the mass of the rings.
Jim Green: You know, Saturn in itself is such a beautiful planet, but it's unlike Jupiter in many ways where we can see these banded clouds, and they're really distinct, and at Saturn, it's not so obvious. But, Saturn has some really spectacular features, and in particular, that hexagon in the north polar region, that's just absolutely astounding. What do we know about that?
Linda Spilker: Well, that giant hexagon at the north pole, Jim, it's a six-sided jet stream, could put two Earths across it, so it's huge in size. And what keeps those six sides in place, we're not quite sure. But, we got some great images in these grand finale orbits, and we're working hard to model what could keep that hexagon in place.
And Saturn's very beautiful and very interesting how the seasons change. When it was winter in the northern hemisphere when we first arrived, the northern hemisphere of Saturn looked blue--almost a Neptune kind of a blue--because those haze particles had basically disappeared. The haze particles need sunlight to grow. And so, we could see deeper into the atmosphere beneath the long shadows cast by the rings.
And as the seasons changed, slowly, the northern hemisphere became more and more golden as the haze came back, and now the southern hemisphere is turning bluer and bluer as it becomes--as it's winter now in the southern hemisphere.
Jim Green: You know, one of the spectacular atmospheric features that Saturn displayed to us was the huge storm that occurred. I mean, it was absolutely unbelievable. I know the Waves instrument, which was measuring the electromagnetic spectrum from lightning, and they were seeing so many lightning strikes for long periods of time. What do we think is happening at Saturn during these big huge storms?
Linda Spilker: Well, you're right, Jim. Those lightning strikes were 10 times or even more stronger than the lightning strikes on the Earth. And these storms only happen about once every 30 years. And this one came a little bit early in the northern hemisphere, and so Cassini was right there with a chance to watch over nine months as that storm grew, completely circled around the planet, the head and the tail of the storm met, and then it slowly started to disappear. So, we think Saturn sort of builds up energy over a period of time, and about once every Saturn orbit, which is 30 years, there's this tremendous release of energy. Some people even call it like a burp where the energy comes out and creates a giant storm. And then the planet becomes quieter again with just the usual small storms.
Jim Green: You know, it's really phenomenal when you think about it - a thunderstorm that lasts nine months, wow. You know--.
Linda Spilker: --Incredible--.
Jim Green: --Only these giant planets can do giant things like that.
You know, another thing that always fascinated me about Saturn--and as you know, I'm a magnetospheric physicist--is really about its magnetic field and the fact that, you know, we've been measuring it now for 13 years being in orbit, and we're finding out all kinds of spectacular things. Can you give us an update on what's happening in that area?
Linda Spilker: Well, it's so fascinating because we think that, to sustain a magnetic field, the planet needs to have a tipped magnetic axis - in other words, take the spin axis of Saturn, the magnetic field axis should be tipped over. If you look at the Earth, if you look at Jupiter, Uranus and Neptune, all of those have tilted magnetic fields. But, Saturn is unusual. It looks like to within about .06 degrees, those two rotation axes are perfectly lined up. And that's a puzzle because we think that you need that tilt to maintain the currents inside the metallic hydrogen deep inside Saturn.
And so, maybe, just maybe, we're watching a magnetic field become slowly extinct on Saturn, or maybe something that we haven't thought of yet is hiding or masking the magnetic field and what we see just looks like a very well lined up dipole. So, we're not sure. We're continuing to study and tease it out that those two axes appear to be getting closer and closer aligned as we go deeper into the data.
Jim Green: You know, one of the things that these magnetic fields produce in magnetospheres, of course, are a lot of radio emissions, and it's from them that we actually can get an idea as to what a day is like on Saturn. But, with the orientation of that field being so closely aligned to the axis, that's been hard to do. What's the latest on that?
Linda Spilker: That’s right, Jim. With those two axes so closely aligned, usually, we can take the tilt from the axis and look at the wobble in the field to get the rotation for the interior of the planet. That's what we did at Jupiter. And at Saturn, we found this radiation, Saturn kilometric radiation, and it had a period, and we thought that was the internal rotation rate of Saturn. But, as Cassini studied in more detail, we found two periods, one in the north and one in the south, and realized that these periods were quite different from what Voyager found and that they weren't tied to the interior of the planet but to something else on the outside, maybe to the auroral regions.
And so, now we're still looking for even a tiny offset and a tiny wobble, and it'll take just a little bit more time to see if we can tease that out.
Jim Green: You know, when you think about it, that's so important for us to be able to create a coordinate system on Jupiter that rotates and allows us to identify certain features and then determine how they move within that coordinate system. So, by not having an accurate day, that must make the data analysis really hard.
Linda Spilker: Well, it makes it hard, too, for the atmospheric scientists because they don't know exactly how fast the winds are rotating. At different latitudes, the winds rotate at different speeds, and trying to understand how Saturn works has gotten a little bit harder.
Jim Green: I'm Jim Green, and I'm here with Linda Spilker, and we're talking about that fantastic planet Saturn and the Cassini Mission.
One of the things that we just got done, Linda, you and I is really worked on the e-book. Can you tell us about that?
Linda Spilker: The e-book is beautiful, Jim. It's all of these pictures from Cassini. It's like a walk down memory lane now that the mission is over and a chance to experience not only the incredible science from Cassini but the beauty in the pictures that Cassini sent back.
Jim Green: Where can we get the e-book?
Linda Spilker: The Cassini e-book can be downloaded from NASA.gov/ebooks, and it can be downloaded in any format.
Jim Green: Every one of my guests, I ask a very special question, and it's really all about how we as individual scientists get into the field. So, Linda, what was your gravity assist that drew you on to this path to become a scientist?
Linda Spilker: Well, Jim, my gravity assist really happened all the way back when I was working on my bachelor's degree. I was a physics major, and there was one woman professor in the department. Her name was Dotty Woolum. And she asked me, “Would you like to work with me over the summer and do some research on meteorites?" She was trying to understand lead and bismuth distribution and carbonaceous chondrites, and so I said, sure, I'll help out. And actually, I got to work and go up to Caltech and stay in the dorms for a couple of summers, and I loved that research and learning more about how our solar system might have formed.
And so, when I graduated and it came a chance to get a job at JPL, and they asked me do you want to work on Voyager to look at the outer planets, I said, sure, sign me up. But, my gravity assist was really with Dotty Woolum in physics teaching me about meteorites and about the solar system.
Jim Green: Well, Linda, thanks so very much. This has just been fascinating. Cassini, just really love this mission, and of course, Saturn is such an iconic planet.
Join us next time as we continue our virtual tour of the solar system. I'm Jim Green, and this is your Gravity Assist.