Magnetic Field of Saturn

Physicist Michele Dougherty on the planetary dynamo, Cassini’s flybys of Saturn and why the nature of the magnetic field of Saturn remains unknown

videos | May 12, 2020

I build instruments that measure the magnetic field in the environment of a spacecraft. I was involved in the Cassini spacecraft mission, which orbited Saturn for 13 years and recently ended its life in the atmosphere of Saturn. The main science objective of the instrument was to try and understand the magnetic field that’s generated in the interior of Saturn. On the Earth, we have a magnetic field: if you stand on the surface of the Earth and you’ve got a compass, the compass needle will point to the North Pole of the magnetic field. You have something very similar at Saturn, although you can’t stand on the surface of Saturn: Saturn isn’t a solid planet; it’s a gas giant, so you’d just fall through.

Based on the observations we’ve made on the Earth, we thought we understood how the magnetic field is generated in the interior. You need an environment in the deep interior where electrical currents can flow, and if those currents flow, they generate a magnetic field which you can measure outside. The way in which it is generated is via a process called the planetary dynamo, so you have a convective overturning motion, a bit like heat bubbling on a pot of porridge on a stove. So you’ve got this overturning convective motion, heat being given off, but you also have an internal rotation as well, and those processes all combine to form a planetary dynamo that generates the magnetic field that you can measure outside.

One of the things that planetary dynamo theory tells you is you can only generate a magnetic field if the rotation axis of the planet and the magnetic axis of the planet have got a tilt between them.

On the Earth, it’s a 20° tilt; on Jupiter, it’s a 9.6° tilt, and so our understanding has always been that unless you have that tilt, you can’t continue to generate the field. Saturn, though, doesn’t seem to have a tilt. In fact, the Pioneer and the Voyager spacecraft flew past Saturn in the late 70s-early 80s, and the instruments on those spacecraft measured the tilt to be less than one degree. We didn’t understand how that could be, so we thought, let’s go there with Cassini and let’s go into orbit. It’s easy; we’ll get it sorted. The first year, we’ll get it all sorted.

So we go into orbit, and we find that there isn’t a tilt. When we have a look at the data, there are these strange signatures in the data which look as if they’re at the planetary rotation rate, which is about 10,5 hours, but depending on whether you look in the northern or the southern hemisphere and depending what season you’re looking at, they change. So clearly, it’s not coming from the deep interior. What we think is happening is there are some processes going on in the atmosphere which, depending on how much sun they’re getting, are going to change the waves that are generated.

Physicist Michele Dougherty on Cassini and Voyager missions and the possibility of life on Enceladus
So we spent 12 years orbiting around Saturn, and we couldn’t find the tilt. We had decreased the number to less than 0.06°, and we thought that maybe there was something going on in the rings of Saturn, which was masking the effect that was coming from the interior. We knew that the Cassini, the spacecraft, was going to run out of fuel; we didn’t want it to crash land on one of the moons like Enceladus because if it did and we ever found life on those moons, there would always be the question did the spacecraft take a few bacteria to the moon. So we wanted to end the mission by essentially diving into the atmosphere of Saturn and burning up, but in doing that, we wanted to get the best science that we could out of the mission.

So, for the last year, we planned flybys of Saturn. We used Titan, which orbits around Saturn, to get up out of the equatorial plane, and we had these close flybys just beyond the edge of the rings for six months, and then we moved inside of the rings, in the gap between the rings and the atmosphere of Saturn, and we did 22 of these dives. And we thought, we’re really close, we’re away from the rings, we’ll be able to measure the tilt – and still couldn’t find a tilt. We’ve now actually measured the tilt to be so small it’s less than 0.0095°, which effectively means it’s not there. In fact, one of my team members said that maybe we shouldn’t be measuring it in degrees; maybe we should be measuring it in arc seconds, and it’s about 32″; you can hardly see it.

If this is correct, we need to rethink how magnetic fields are generated on planets. If it’s by the planetary dynamo, why aren’t we seeing a tilt? One of the things that we were thinking was maybe the magnetic field at Saturn is dying, and slowly but surely, it’s decaying away. If that’s the case, you would expect the tilt to get smaller and smaller, but then when we got really close, we saw that there were higher-order moments of the magnetic field, which says it’s not dying. So maybe there’s a region above the dynamo region that’s actually masking the effect of the field that we see? But we can’t say it doesn’t have a tilt just yet; what we’re doing is we’re looking very closely at our data, trying to understand what we’re seeing. There might be some signals in the data that are coming from the deep interior, and so I’ve actually got postdocs and PhD students working on the data trying to understand that. It’s a very strange position to be in, thinking that you understand the theory.

For the last 150 years people have always assumed that the planetary dynamo is what generates the field. But Saturn is telling us that maybe we don’t quite understand it.

We know there’s a magnetic field there because we can measure it, but the question is how it’s been generated, and that’s what we really need to focus on now. Do we understand how magnetic fields are generated, and if we don’t, how come we don’t?

All we know is that there are electrical currents flowing in the deep interior, which are generating the field. What we think is that there might be a small solid core, and then above that solid core is the dynamo region; we think in Saturn, it’s probably something like fluid metallic hydrogen. So, if you strip the electrons off the hydrogen, they become metallic and that then allows the currents to flow. So we’re hoping by the time we’re finished analyzing all of the data, we’re going to be able to tell not only how deep the dynamo region is but also what it’s made of, and that will help feed into whether we can understand the dynamo.

Physicist David Southwood on the history of the Cassini mission, landing on Titan and why does humanity need to explore space
But in some ways, it’s a nice problem to have. When they designed the end of the mission for Cassini, the spacecraft wasn’t designed to do what it did; the instrument certainly wasn’t designed to do what it did. I was expecting us to survive for a single close in orbit; we survived for all 22. And so now we can use that data to try and understand what’s going on. I think a moral to take away from this is never to be sure about what you’re going to find. I really thought that during the first year in orbit at Saturn, we would measure the tilt, and that would be it. Thirteen years later, we’re still trying to find it, and I don’t think it’s there. What we now need to try and understand is why it’s not there. It’s really frustrating because I want to know what the answer is, but it’s not going to be easy. Every time someone asks me how quickly we’ll solve it, I say six months, but it’s been 18 months since we got the data, so it might take a bit longer than that.

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Professor of Space Physics, Imperial College London
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