Moons of Saturn

Physicist Michele Dougherty on Cassini and Voyager missions and the possibility of life on Enceladus

videos | June 25, 2019

When I was a young child my father built his own telescope. This was when I was still living in South Africa, and I remember my sister and I helped build a concrete for the base of the telescope. We were so excited! And we looked through the telescope and the first thing I saw was Saturn and its rings. I thought I could see some of the moons, but I probably couldn’t because it was too far away. But then at the back of my mind I’ve always been interested in Saturn and its moons, never thinking that I would be fortunate enough to be involved in a spacecraft mission that went to Saturn and involved in an instrument that took data that helped to make a discovery about one of the moons of Saturn.

The moon in particular that I want to talk about is called Enceladus. It’s a very small moon, it’s about 500 kilometers in diameter. People have always wondered about its surface: it’s a very young surface, and if you compare it to the surface of one of the other moons close by called Mimas, Mimas is covered with lots of craters. So you would expect Enceladus too to have lots of craters on the surface but it doesn’t: the surface looks really young. The other thing about Enceladus is that it’s quite close to Saturn in an environment where there’s a lot of water vapor molecules. People have always wondered: could Enceladus somehow be the source of this water vapor?

So before the Cassini spacecraft mission reached Saturn we had lots of views of Enceladus through telescopes on the ground but it was so far away, you couldn’t really tell what was going on. The Voyager spacecraft flew past Saturn in 1979, and as it flew past Saturn it took an image of the surface of Enceladus which confirmed the fact that it’s very young, with no crater. One of the instruments on the Voyager spacecraft took an image of the surface in the infrared and it told us that it was made up mainly of water ice. That’s what we knew about Enceladus before Cassini got there.

One of the things you need when you’re involved in outer planetary missions is lots of patience. It takes years from when you first think of sending a spacecraft until at last you start getting the actual data back.

We designed and built the instruments in the early 1990s, it was launched in 1997, and it took six and a half years to get there. So we got there in 2004 and then we had our first flyby of Enceladus in 2005. What we saw in the data was strange: it was almost as if Enceladus was much bigger than we knew that it was. Saturn has a magnetic field which orbits around or rotates around at the same rate as Saturn does and if the magnetic field lines pass through a moon and the moon is a dead body the field lines of Saturn won’t see it. And we expected that. So when we flew past we saw the magnetic field lines of Saturn coming up towards Enceladus but instead of moving through Enceladus they stopped upstream almost as if it was a bigger obstacle than it should’ve been. So we had a closer look at the data. We weren’t quite sure what we were seeing. As a spacecraft flew past Enceladus it was moving very quickly so the cameras should point at Enceladus, and we were worried that maybe we weren’t resolving the movement to the spacecraft properly. So we didn’t say anything, but we thought that the data was really interesting.

A month later we had a second flyby and we saw exactly the same signature: something was stopping the magnetic field penetrating down onto the surface. In addition to that we saw in the magnetic field data lots of wave activity which if you did the calculation was telling us there’s a lot of water group ions being picked up. We tried to put a model together to describe what we thought we were seeing and one way that we could describe the observations was if Enceladus had an atmosphere. Maybe the upper regions of the atmosphere, just like on Earth, become ionized by radiation from the Sun and that stops the magnetic field from being able to penetrate? We were almost certain that’s what we were seeing. In the team we agreed this is what we were seeing. So I went down to the Jet Propulsion Lab to talk to the Cassini project because what I wanted to do was try and persuade them on a subsequent flyby to go really close to Enceladus and see if we were really seeing something interesting. I wasn’t sure if it would work but I thought it was worth a try.

And it was really interesting: we talked for three or four hours in the meeting about whether we should do this. If we did it would mean that some of the other instruments, the other teams wouldn’t be able to take data, but people were really excited at the prospect that there might be an atmosphere. If there was, that would be a real discovery because Enceladus is so small, we thought it would have long since died, any interior heat would have disappeared. So we finally agreed we would change the next flyby to go really close. Originally we planned to fly a thousand kilometers above the surface. The project agreed we would fly a 173 kilometers above the surface.

I must confess that the two or three days before that third flyby I couldn’t sleep because if we’d seen nothing at all no one would ever believe anything I said again. But what we saw in the data was spectacular. Instead an atmosphere covering the entire surface there was a plume of water vapor coming out from the South Pole. Because we went so close, all of the other instruments were able to take data as well and we found cracks at the South Pole. There was not only water vapor leaking out of these cracks but there was organic material. The implication was that there was a heat source at Enceladus which there shouldn’t have been because it should have been cold. Out of Enceladus temperatures are 170°C. How can you have liquid water there?

So we had lots more flybys on Enceladus as a result of this and in one of the flybys we measured organic material. That’s when people got really excited because one of the reasons we explore the Solar system is that we want to see if life could form elsewhere.

You need four things for life to form: you need liquid water, you need a heat source, you need organic material (and we had all three of those on Enceladus), and the last thing you need is for those first three things to be stable over a long enough period of time that something can actually happen.

So Enceladus is now one of the places in the Solar system where we think there is the potential (we don’t talk about life, we talk about habitability), the conditions might be that life could form at some stage.

So there’s now a lot of talk about sending follow on missions to Enceladus to try and go into orbit. If you think about it, to be able to understand an environment you need to spend quite a lot of time there. So there’s a lot of talk about sending future missions to Enceladus, but the problem with Enceladus is that because it’s so small, its gravitational field is very small, and so you need a huge amount of fuel to be able to slow down enough to go into orbit. That is always going to be a bit of a problem.

I think the discovery of liquid water at Enceladus has now focused people’s minds on the fact that if you’re searching for liquid water in our Solar system it doesn’t have to only be on the surface. You can have environments where you can have liquid water underneath the surfaces. You can be quite far away from the star and still have liquid water underneath the surfaces. So that also feeds into people’s understanding of exoplanets, planets beyond our Solar system. When people look at these exoplanets they always have been focusing on planets that are close to their parent star, but the Enceladus liquid water discovery has shown that you don’t have to be close to the parent star (in our case our Sun): you can be really far away but if the conditions are right you can actually have liquid water forming under the surface.

I think if I was asked what my favorite moon was at this stage I would say Enceladus just because it yields such surprises, we really didn’t expect to see this. The amount of water vapor and the amount of dust and gas that’s being given off is so big that it’s actually forming a huge ring of water vapor all the way around Saturn. When you fly through it you can actually make measurements of this increase in water vapor.

Looking back on the discovery that we made I think that the main trick was to be brave.

We weren’t sure what we were seeing but it was clearly something we weren’t expecting to see. It was something really interesting and sometimes you have to just be brave and make a change: sometimes discoveries are made like that.

Professor of Space Physics, Imperial College London
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