Gravity Assist Podcast: Saturn, with Linda Spilker, Part 1

Categories: Feature Stories Saturn

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.

Here’s a short teaser of this week’s podcast:

You can listen to the full podcast here, or read the transcript below.

When Cassini ended its mission in September 2017, Saturn’s northern hemisphere was bathed in summer sunlight, in contrast to when Cassini arrived in 2004 and the northern hemisphere was in winter. Image credit: NASA/JPL–Caltech/SSI

Jim Green: As everyone knows, the Cassini mission recently had a spectacular finale at Saturn. So, Linda, have you caught up on your sleep, and what’s happening next to all the Cassini data?

Linda Spilker: Well, yes, I have caught up on my sleep, Jim, a little bit, and I actually took a vacation. Of course, the scientists are so busy looking at their data, because in almost every area, what we found surprised us. Our models just didn’t explain what we’re now seeing in the data. It’s incredible.

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 the tiny moon Enceladus, which is only 300 miles across [313 miles to be precise, or 504km]. We thought it should be frozen solid, and yet we had tantalizing clues from Voyager that this bright, icy moon, with not many craters [was active]. It’s right in the middle of Saturn’s E-ring, and there’s the geysers, and 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 for life to exist in that ocean. [There could] even be hydrothermal vents on the sea floor. We found tiny nano-silica grains [in the geyser plumes] that could only grow in very hot water and excessive hydrogen, and these clues point to hydrothermal vents.

On Earth, we have life teeming around these vents deep in the Earth’s sea floor, far away from sunlight. So, we wonder, could there possibly be life on Enceladus?

Enceladus, the small moon with the big geysers of water vapor and likely an underground ocean. Image credit: NASA/JPL/SSI.

Then, of course, there is giant Titan: the size of the planet Mercury with a thick nitrogen-rich atmosphere. [The European Space Agency] landed a probe called Huygens on the surface of Titan and unveiled this moon that was covered with photochemical haze, like Los Angeles on a smoggy day, [which prevented us from] seeing through to the surface with Voyager. But, with Huygens, we could see the surface, river channels, and icy rounded pebbles. It turns out that liquid methane plays the same role on Titan that water plays on the Earth. There are 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 if liquid water of its own underneath its icy crust, so there are two ocean worlds at Saturn – what a remarkable discovery.

Jim Green: What are some of your favorite moons?

Linda Spilker: Well, of course I like Enceladus because it’s so interesting with its global ocean, but also there is Iapetus, which has one bright side and one dark side, and over the course of the Cassini mission [we] found that the dark material is actually dust from Phoebe, which is a moon captured by Saturn and located far away from the planet, where it is bombarded by micrometeorites, [which produces dust] and that dust comes in and gives Iapetus a dark side.

Then how about Mimas, with that giant crater called Herschel, and if you look at where electrons hit the surface of Mimas and cause the icy particles to fuse together so it heats up and cools off more slowly, [it produces] a very interesting temperature map of Mimas, which has a lens shape where the electrons hit and it looks like a Pac-Man, ready to gobble up something else as it goes along.

Saturn’s moon Titan is covered in a opaque hydrocarbon smog, but infrared and radar instruments are able to peer through the haze and image the surface. Image credit: NASA/JPL/University of Arizona/University of Idaho.

Jim Green: Saturn has just been so spectacular, and of course its rings are just such an attraction for us. What have we learned about the rings from Cassini?

Linda Spilker: Well, in the Voyager days – I worked on Voyager – we had this idea that the rings were individual particles gently colliding with one another and that there were these waves in the rings created by interactions or resonance with the moons. But, with Cassini, we’ve unveiled the rings in a totally different way, that many of these particles stick together and form long linear clumps in the rings, and that these clumps form and break apart and form again, and that most of the rings are made up of these very interesting clumps. During Saturn’s equinox [in 2009, when the rings were edge-on to the Sun] Cassini saw three-dimensional structures in the rings, that these rings are 30 feet thick, perhaps even three feet thick in places, and that these larger [clumps] that may be half-a-mile or a mile across stick up above and below the rings like giant icebergs. We saw their shadows when the Sun was edge on to the rings, it just saw incredibly fascinating to watch this in action.

Then there are the propeller objects, which are 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 there was this bright feature that pointed to an object that was large enough to get a nickname – Peggy – and that perhaps Peggy was forming and might break free of the rings and become a moonlet in her own right.

Jim Green: One of the really great images that I remember so well is the eclipse where Saturn passes between the spacecraft and the Sun. We’ve just had a [total solar] eclipse here in the United States and so 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?

Cassini’s Project Scientist, Linda Spilker of JPL. Image: NASA.

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, at very high phase angles, and 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. We discovered a number of new rings in that particular image.

Jim Green: What I really loved about the proximal orbits, when Cassini went between the rings [and Saturn], 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, it was very safe for Cassini to dive through that gap. But we were surprised.  We were expecting that ring particles of all sizes, from the nano-grains to micron-sized to perhaps even half-a-centimeter across might all be in that gap. So what happened to those particles? What processes grind them up and remove a lot of the water-ice to leave just these little dust-sized or smoke-sized grains inside of that gap, and then how do they fall into the planet? These nano-grains seem to be distributed above and below that equatorial plane of the rings. We’ve been mapping their distribution, but that’s a puzzle. What happened to the big particles?

Jim Green: Does that tell us that the rings will last a very long time around Saturn?

Linda Spilker: The rings might last for a longer time, but there’s still some process [happening] because those ring particles we know are gradually flowing towards Saturn, like pouring water on a table. There’s nothing to stop that. But what is grinding them up, or what process is holding the bigger ones in place? If something is [holding them in place], then perhaps the rings could last longer than a mere 100 million years.

Jim Green: Several of those [images of Saturn eclipsing the Sun] actually have Earth and Mars and Venus just on the other side of the solar system where the Sun is illuminating them.

Linda Spilker: Yeah, and one of those pictures where you can see Mars, Venus, the Earth and the Moon, I love it because we asked all the people here on Earth to go out, wave at Saturn during that 20-minute window [when Cassini would be looking at Earth] so that the photons from their waves could be captured by the Cassini cameras, and then we asked people to send us their selfies of themselves waving at Saturn. We took all those pictures and recreated that mosaic out of people pictures.

You can read the second part of the transcript here.