Gravity Assist Podcast: Saturn, with Linda Spilker, Part 2

The Gravity Assist Podcast is hosted by NASA’s Director of Planetary Science, Jim Green, who each week talks to some of the greatest planetary scientists on the planet, giving a guided tour through the Solar System and beyond in the process. This week, he is joined by Cassini Project Scientist Linda Spilker, who talks about the ringed planet Saturn, which is the sixth planet from the Sun, as well as its fascinating moons and the most startling discoveries from NASA’s Cassini mission.

You can listen to the full podcast here, or read the transcript below. The first part of the transcript can be found here .

A backlit Saturn as the planet eclipsed the Sun as seen from Cassini’s vantage point. The illumination from behind revealed the planet’s rings in eerie glory. Image credit: NASA/JPL–Caltech/SSI.

Jim Green: The recent observations of Saturn’s rings from a new perspective, during the proximal orbits where Cassini flew just below the rings and above the atmosphere, gave us some new information about the mass of the rings. What can you tell us?

Linda Spilker: Well, 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. It’s turning out to be trickier than we thought because Saturn’s gravity and its mass distribution is not at all what our models predicted, they’re very different and 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.

The hexagonal jet-streams at Saturn’s north pole. Image credit: NASA/JPL–Caltech/SSI.

Jim Green: Saturn has some really spectacular features, in particular the hexagon in the north polar region, which is just absolutely astounding. What do we know about that?

Linda Spilker: Well, that giant hexagon at the north pole is a six-sided jet stream. You could put two Earths across it, so it’s huge in size. We’re not quite sure what keeps those six sides in place. But, we have got some great images [of the hexagon] in these grand finale orbits and we’re working hard to model what could keep that hexagon in place.

Saturn’s very beautiful and it’s very interesting how the seasons change there. 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 [in the atmosphere] 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.

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’s winter now there.

Jim Green: One of the spectacular atmospheric features that Saturn displayed to us was the huge storm that occurred. Cassini’s ‘Waves’ instrument, which was measuring the electromagnetic radiation from lightning, saw many lightning strikes over long periods of time from the storm. What do we think is happening at Saturn during these huge storms?

Linda Spilker: Those lightning strikes were 10 times or more stronger than the lightning strikes on the Earth.  And these storms only happen about once every 30 years. 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 builds up energy over a period of time, and about once every Saturn orbit, which is 29.5 years, there’s this tremendous release of energy. Some people even call it a ‘burp’ where the energy comes out and creates a giant storm. After which the planet becomes quieter again with just the usual small storms.

An artist’s impression of Saturn’s magnetosphere, with Cassini in the foreground. The blue region is the shock wave of the solar wind impacting the magnetosphere. The solar wind then causes the magnetosphere to stretch away in a tail-like shape. Image credit: ESA.

Jim Green: Another thing that always fascinated me about Saturn – and as you know, I’m a magnetospheric physicist – is its magnetic field and the fact that we’ve measured it for 13 years in orbit, and we’ve found out all kinds of spectacular things. Can you give us an update on what’s happening in that area?

Linda Spilker: 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, the magnetic field axis should be tipped over [relative to the spin axis]. 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 it is to within about 0.6 degrees, those two rotation axes are perfectly lined up. That’s a puzzle because we think that you need that tilt to maintain the currents inside the metallic hydrogen deep inside Saturn [where the magnetic field is generated].

So maybe, just maybe, we’re watching a magnetic field slowly become 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 it and tease out that those two axes appear to be getting closer and closer aligned as we go deeper into the data.

Jim Green: One of the things that these magnetic fields produce is a lot of radio emission, and it’s from this radio emission hat we can get an idea as to the length of a day on Saturn. 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: 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, called Saturn kilometric radiation [the radio emissions] and it had a period, and we thought that was the internal rotation rate of Saturn. But, as Cassini studied it 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: When you think about it, i’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: 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.

The mighty ‘great storm’ of 2010–2011, which covered a fifth of Saturn’s northern hemisphere, wrapping itself around the planet and lasting many months. Image credit: NASA/JPL–Caltech/SSI.

Jim Green: One of the things that you and I recently worked on is an e-book.  Can you tell us about that?

Linda Spilker: The e-book is beautiful. 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: For every one of my guests, I ask a very special question, and it’s really all about how we as individual scientists got into the field. So Linda, what was your gravity assist that drew you onto 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. I actually got to 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.

So when I graduated, there came a chance to get a job at JPL, and they asked me whether I wanted to work on Voyager and 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.